A PROCEDURE FOR LABORATORY SEEDBED TESTS WITH UNDISTURBED SOIL Thesls for the Degree OI M. S. MICHIGAN STATE UNIVERSITY Nils Jakob Moller 1964 mm W / $29 4/ ”Tl/#3 I ____—-———————--1 LIBRARY Michigan State University PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 5/08 KzlProleccauPrelelRClDateDm,indd ABSTRACT A PROCEDURE FOR LABORATORY SEEDBED TESTS WITH UNDISTURBED SOIL by Nils Jakob Mdller An increasing number of people working in Agriculture realize the importance of seedbed preparation. From having been almost neglected seedbed physical factors now attract great attention. Environment control is used in many pro— fessions today, and it should be a challenge to adapt this new knowledge also the control of the physical environment in the seedbed. The literature review showed great differences be- tween laboratory germination tests and field emergence. This is interpreted as a loss in seed germination environ- ment control. From this it is obvious that more basic research in seedbed preparation is necessary. It was concluded that the drying rate of the soil in the seedbed is one of the most important factors in seed germination. A survey over physical factors affecting soil drying was presented. The present seedbed test methods are discussed and Nils Jakob Mbller two new seedbed research methods are proposed. Both the new methods provide better possibilities to carry out basic seed- bed research. A method to do seedbed tests indoors was developed. Very likely the soil structure factor can not be neglected. Therefore a technique to take undisturbed soil blocks 30 cm by 60 cm and 30 cm deep was developed. When the undisturbed soil block is brought indoors it is necessary to replace the influence from the lower soil layers. A method to supply water under tension to the soil block was tested and found satisfactory. It was found that plaster of Paris cast direct against the soil block bottom surface could be used as a transfer material for the water between the water supply system and the undisturbed soil block. Soil boxes with water supply systems utilizing plaster of Paris as transfer material maintained constant moisture contents for more than two months even under in— tensive drying conditions. Preliminary full scale planter tests were run in the soil boxes. Approved Major Professor Approved Department Chairman A PROCEDURE FOR LABORATORY SEEDBED TESTS >WITH UNDISTURBED SOIL By Nils Jakob Mbller A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Agricultural Engineering 1964 ACKNOWLEDGEMENT The author wishes to express his sincere thanks to Dr. S. Persson as major professor for his guidance and criticism during this study. Acknowledgement is extended to Professors A. E. Erickson, H. F. McColly and S. T. Dexter for their sugges— tions and comments. Special appreciation is indebted to W. K. Kellogg Foundation for the financial assistance that made these studies and research possible. The foundation also Spon— sored visits to other research centers which made signif- icant contributions to this thesis. Appreciation is expressed to the Department of Agricultural Engineering, Michigan State University and its staff for kind and valuable help throughout the research work. The author is indebted to Ingrid and Ola for their help and encouragement during our year in this country. ii TABLE OF CONTENTS ACKNOWLEDGEMENTS LIST OF TABLES LIST OF FIGURES Chapter I. INTRODUCTION Past Seedbed Research The Research Problem II. REVIEW OF LITERATURE The Purpose of Seedbed Preparation Laboratory Germination Tests and Field Emergence Seedbed Research Up to Present Time . Seed Viability and Germination Ability III. SEEDBED SOIL MOISTURE Adequacy of Soil Moisture Seed Moisture Content Necessary for. Germination Moisture Distribution in the Seedbed Soil Drying Rate The Effect The Effect Drying . The Effect Drying . The Effect Drying . The Effect Drying . The Effect of of af' 6f° if of Soil Drying Wind on Soil Drying Air Humidity on Soil Radiation on Soil. Temperature on Soil. saii éaétéré 6n°saii water.Table.Depthoon iii Page ii vi vii r> ‘h EDP ea HOUI 13 l3 13 15 16 18 18 19 19 20 21 Chapter IV. SEEDBED PREPARATION AND TILLAGE TESTS Seedbed Preparation Test Methods Conventional Methods Control of Rain Factor Greenhouse Seedbed Experiments A New Seedbed Preparation Research Method for Advanced Control of Seedbed Environment V. TESTS AND EXPERIMENTS Soil Moisture Content in the Upper Soil Layer During Spring Establishing a Bottom Boundary in the Soil Block Which Makes Possible Water Supply Under Tension Plaster of Paris as Transfer Material Mixing and Casting of Plaster of Paris Water Supply and Sealing of the .Out- side Plaster Plate Surface Testing of Plaster of Paris Plates Testing Specimens . Testing for Maximum Pore Size Testing for Average Pore Size Results from the Determination of Pore Size of the Plates Maximum Pore Size Determinations Average Pore Size Determinations Air in Water Supply System . . Procedure for Taking Undisturbed Soil Blocks . . . . . . . . Comments to the Procedure of Taking Undisturbed Soil Blocks Two Soil Boxes——One With and Another Without Water Supply iv Page 24 26 26 26 26 27 31 32 36 4O 4O 42 44 44 45 47 48 48 50 51 52 57 61 Chapter Tests in Drying Tunnel Weight Change of Soil Boxes . Water Flow Resistance in the Water Supply System Soil Boxes in an Intensive Drying Cycle Contaminations of the Supply Water Tests Using Soil Blocks . . Full Scale Planting in the Big Soil Boxes . . Test of the Water Supply System in the Big Soil Boxes . . . Another Example for Use of the Undisturbed Soil Block Technique . VI. SUMMARY AND CONCLUSIONS Summary Conclusions Literature Review . Seedbed Preparation Tests . . Water Supply to Undisturbed Soil Blocks . . . . . . . . . VII. SUGGESTIONS FOR FURTHER WORK REFERENCES Page 63 63 69 71 74 76 76 78 82 84 84 86 86 87 88 89 9O Table LIST OF TABLES Moisture content of upper soil layer. Hillsdale sandy loam. Fall plowed. No tillage during spring . Moisture content of a minimum tillage seedbed. Hillsdale sandy loam. No fall plowing. Spring plow—planted Maximum tension developed by different plaster of Paris plates . . . Average percolation rates for three plaster of Paris plates Soil moisture content with time in two soil boxes, one equipped with a water supply system and the other without water supply vi Page 32 33 48 50 62 Figure 10. ll. 12. 14. 15. 16. 17. LIST OF FIGURES Greenhouse seedbed test method Moisture content of upper soil layer during Spring Moisture content of a minimum tillage seedbed Test Specimens of plaster of Paris Cross section of soil box with water supply system Equipment for test of maximum pore size Water supply system for soil boxes providing constant tension Data from maximum pore size determinations Soil block dug free from ground Boxes forced down over the soil block Pulling of the bottom cut off sheet in under the soil boxes . . . . . Soil box turned over and ready for transport Soil box ready for casting the plaster of Paris plate . . . . . . . . . . . Inner soil box completed with plaster of Paris bottom and water supply trough Stone deforming the inner box cutting edge Bottom surface of box "broken” loose from ground vii Page 28 34 34 41 41 46 46 49 53 53 54 54 56 56 58 58 Figure Page 18. Drying tunnel experiment setup . . . . . . . . 64 19. Close up view of drying tunnel . . . . . . . . 64 20. Test of soil box in drying tunnel . . . . . . 72 21. Soil boxes in an intensive drying cycle . . . 73 22. Working soil box equipped with water supply system . . . . . . . . . . . . . . . 75 23. Soil box with CaSO4 crust on top of soil surface . . . . . . . . . . . . . . . . . 75 24. Fullscale planting in the soil boxes . . . . . 77 25. Close up view of planter unit . . . . . . . . 77 26. Close up view of soil box in the movable soil bin . . . . . . . . . . . . . . . . . . 79 27. Test of the water supply system of the big soil boxes . . . . . . . . . . . . . . . 79 28. Test of the water supply system of one of the big soil boxes . . . . . . . . . . . . 8O 29. Soil boxes for measuring seedling penetration forces . . . . . . . . . . . . . . . . . . . 82 30. Bottom view of boxes in Figure 29. . . . . . . 83 viii I. INTRODUCTION Past Seedbed Research Soil tillage is probably the world’s largest materials handling operation. Numerous field and laboratory experiments on soil tillage and planting have been made dur— ing the past. The results of these studies have not always been in agreement, and they have not produced a basic scien- tific understanding of the dynamic relationships involved in obtaining optimum conditions for seed germination and seedling field emergence. Because we are working with live material exposed to natural climatic and rheological influences many factors will make up the final results. To get accurate results it is therefore desirable to observe or control as many as possible of these factors. Most of the planting and tillage implements now in use have been developed on the basis of field experience rather than scientific studies of the relationships between soil characteristics, climatic factors, tillage implement design and seed and seedling demands. The results from tests with planters, and tillage—planting practices have often been conflicting. The explanation of a particular result has been difficult to obtain due to variation in soil conditions, lack of information or inability to control envi- ronmental conditions. Today most investigation on tillage implements is made either in the field or in the laboratory in bins with disturbed soil (natural or artificial). When tests are made outside, the results are always dependent on the weather. Inside the surrounding conditions can be controlled but the soil is not in a natural undisturbed state. The Research Problem If it were possible to carry out planting tests in— doors, with the soil in a natural undisturbed state, and in a controlled climatic environment, it ought to be possible to obtain a better understanding of the influence of the individual factors on the final result. It is possible to take big samples of undisturbed soil from the field and bring them indoors. It is also possible to produce an artificial climate indoors. But when the soil sample is cut loose from the ground it will be necessary to substitute for the influence coming from the lower soil layers. The main problem in this substitution is to find a correct way of simulating the natural internal water supply, or drain, from the soil sample. In its nat— ural place in the ground any influence horizontally, from the surrounding soil through the vertical sides of the soil sample, can be assumed to be small. The purpose of this project was to make a broad study along the lines mentioned above for a new research method in tillage and seedbed preparation. The result of the study should indicate how to perform tests regardless of the outdoor climate and therefore at any time of the year. Then any desired climatic condition or climatic pattern could be applied during the indoor tests. It might also be possible to Speed up the influence of certain climatic fac— tors. II. REVIEW OF LITERATURE The Purpose of Seedbed Preparation The objective of all cultural operations should be to provide a suitable environment for the plant during its production period. In seedbed preparation and planting the the Special purpose is (Johnson and Buchele, 1961): the establishment of a correct number of well developed plants in perfect spacing and capable of adequate yield response. In order to design a machine suitable for planting of field crops, it is necessary to have accurate information concerning the functional requirements of the machine. Al— though a great deal of research has been conducted to improve planter performance, the seedbed environment required for emergence of healthy seedlings has not yet been defined in precise terms that can be used by the engineer in designing and evaluating a planter (Morton, gt_al., 1960). Not even is information usually available to describe the commonly produced seedbed in physical terms (Morton and Buchele, 1960). Laboratory Germination Tests and Field Emergence The purpose of the standardized laboratory germina- tion test is to predict the field emergence (Brinkman, 1963; field emergence = the number of surviving seedlings relative to the number of seeds planted), Practical farming often shows an appreciable difference between conventional labora— tory germination results and field emergence. This fact was reported by Bowen (1960) in the following example. The average initial field emergence (emergence with— in 7 to 10 days of planting date) of cotton across the cot— ton belt in the United States, with the latest and best planting methods used, was approximately 25 percent. The average final field emergence of cotton (healthy plants emerged and growing after 21 days in the soil) was only about 50 percent across the entire belt. There was not a great deal of difference in these figures for the individual areas, the final stand varying from 40 to 60 percent of the planted seeds. Laboratory germination tests for this seed would have shown a viability of 80 percent or better. The 50 percent average figure for final stand means that some fields had a stand failure and others had nearly 100 percent field emergence. Holekamp, gt_gl. (1960), in a study of the soil tem- perature impact on germination of cotton seed in Texas, defined a satisfactbry stand as 40 percent of the seeds planted. Evers (1963a),working with sugarbeet planters in Germany, reported a relative field emergence (Brinkman, 1963; relative field emergence = percentage field emergence relative to percentage germination in laboratory test) of 50 to 60 percent for the best planting techniques. Brinkman (1963), also from Germany and studying Spring work in sugar- beets, Showed results from practical plantings with a rela- tive field emergence of maximum 70 percent. On the other hand when laboratory germination tests showed as low as 70 percent germination, and for severe soil conditions, the relative field emergence was lowered to 16 percent. White (1964) recommends 15 percent overplanting of corn in Michigan. This suggests a maximum field emergence for corn in this area of about 85 percent. No average field emergence figures have been found for small grain. However, it is known that big seeds, with hypogeal endospermic germination as the cereals, have higher field emergence than Small seeds (Martin and Yarnell, 1961). The reason for this is that the big seed is stronger, and it has more reserve energy available to break through the soil; The general opinion seems to be that the average field emergence for small grains is somewhat lower than for corn. Poor field emergence often means a longer time from the appearance of the first seedling until the last seedling is up. This makes timing of cultural operations, weed con— trol and harvesting more difficult. Low field emergence also means nonuniform distribution in the row which lowers the final yield. Since it is almost impossible today to reach even close to 100 percent field emergence one has to overplant. To get the correct population thinning has to be done, if field emergence is better than anticipated. This also means that the best precision planter capable of perfect Spacing of single seeds is only of limited value as long as 100 per- cent field emergence can not be produced (Brinkman, 1963). The difference in seed germination in the laboratory as compared to field emergence is interpreted as meaning that there has been a considerable loss of Seed environment control in moving from the laboratory to the field (Bowen, 1960; Evers, 1963b). Seedbed Environment The surrounding conditions prevailing in a given habitat will affect germination. In this reSpect probably not overall climatic conditions but rather the microclimatic conditions prevailing in the immediate vacinity of the seed will be the determining factors (Mayer and Poljakoff-Mayber, 1963). The seed environment consists of biological, chem— ical and physical factors. The important Seedbed physical factors are: soil moisture, soil temperature, soil aeration, Soil mechanical impedance and light (Mayer and Poljakoff— Mayber, 1963). Only the physical factors and especially the soil moisture will be dealt with here. Both the biological and chemical seed environment factors can be controlled to a much greater extent than the physical factors. The use of fertilizers, herbicides and insecticides can be made more precise than the modification of the seed physical environment (Bowen, 1964). Plant scientists have long used some form of the principle of limiting factors, which states that biological rates of growth and reSponses up to a maximum rate of response is limited by whichever factor is in most demand. Thus, if only one of the seed environment physical factors are less than Optimal it will limit the germination in spite of the fact that all of the other environmental factors are favor— able. Although there are limits to this principle it is applicable to all physiological processes (Went, 1957). The farmer has a general idea of a suitable Seedbed but he cannot describe it for instance in numbers of per- cent moisture or compactness. In fact even the scientist does not know the optimal seedbed requirements in precise terms (A.S.A.E. Seminar, 1964). The biggest factor involved which complicates the whole problem is the weather. Our possibilities to predict the weather increase, but they will probably be limited even in the future at least for local areas (Weathertrends, 1964). When one can not predict the weather of the next ten days after planting, the seedbed has to be prepared for the aver- age climate of the area. This also means that appropriate safety measures against extremes in weather conditions have to be taken into account. One of the safety measures could for instance be increased depth of seed placement to insure proper germination during dry conditions (Wiegand, 1962). However, when reliable weather trends become available the seedbed Should be prepared to give maximum germination under the forecast conditions. But in order to do this we must have the basic scientific understanding of the dynamic rela- tionship between weather conditions and seedbed preparation. Seedbed Research Upito Present Time The results of seedbed preparation and tillage opera- tion tests are usually evaluated in terms of plants per area and final crop yield. The number of plants per area can be useful if compared to the number of seeds planted and the percentage laboratory germination. The number of plants and their physical development should be determined several times to give a complete picture of the germination by time. 10 The final yield is a reSponse to the whole growing period and is practically useless as a measure of seedbed prepara— tion. Results of seedbed preparation or tillage operations have seldom been clearly related to any Specific factor (e.g., poor stand because of poor aeration) in the seedbed environ- ment. The reason for this is partly a lack of instruments but also a lack of understanding of the physical principles involved. Like other problems of the same complicated nature all five physical factors, that are known today in seedbed preparation, have to be observed simultaneously and during the whole germination process. Then each factor or combina- tions of factors have to be compared to the germination result (Bowen, 1960). Most seedbed preparation research up until now has been of an applied nature. It has been directed to the farmer helping him to choose those methods of planting which give him the best total plant reSponse in his local area and weather conditions. But these results are of little general value and give no accurate information concerning the func— tional requirements of the planting and germination processes (Beaty and Giddens, 1962). Lack of knowledge of the influence of different designs of planter details is readily Shown in operator's ll manuals for planters. Different types of openers, press- wheels, etc., are shown but very little is mentioned about how, when and where to use them (Int. Harvester, 1964). The reason for most plantings being reasonably successful lies in the seed itself. Seed Viability and Germination Ability The seed is the survival organism of the plant. It possesses remarkable complex and effective protective mech- anisms that help insure survival (Bosswell, 1961). In order that a seed can germinate, it must be placed in environmen- tal conditions favorable to this process. The requirement for these conditions varies according to the Species and variety and is determined both by the conditions which pre- vailed during seed formation and even more by hereditary factors (Mayer and Poljakoff—Mayber, 1963). In order to prevent all seeds from germinating at the same time the triggering mechanism of the germinating process of each single seed is sensitive to a certain level of its Surround- ing physical factors; The Specific triggering conditions needed for the individual seed are determined by its genetic constitution. The necessary triggering conditions for seeds from wild plants are scattered over a very wide range. When a plant is domesticated it is grown in a narrow cultural 12 pattern which gradually reduces its seeds genetic width (MUntzing, 1961). Only those seeds which germinate will produce the new seeds and all others and their genetic con- stitutions are lost. Therefore the seeds from long domes— ticated plants will only germinate when exposed to their narrow range of triggering conditions. On the other hand when the seedbed provides surrounding conditions within the triggering range most of the seeds will germinate readily and here lies the reason why the seeds germinate as well as they generally do. The environmental requirements for germination can be changed to a certain extent by plant breeding. Yogo winter wheat was bred particularly for resistance to extreme cold but it also got the capacity to germinate in a drier soil than do other varieties (Martin and Yarnell, 1961). The fact that the seeds have germinated does not necessarily mean a good stand. Strong and healthy plants come only from optimal environmental conditions. AS soon as the environmental conditions are less than optimal there is some kind of an environmental stress on the seedlings and their development is less than maximum. Also here the ruggedness of the seeds and seedlings compensate for defi— ciencies in seedbed preparation (Mayer and Poljakoff-Mayber, 1963). III. SEEDBED SOIL MOISTURE Adequagy of Soil Moisture In order to germinate the seed has to absorb mois— ture from the Surrounding soil. The adequacy of soil mois— ture for germination depends on (Wiegand, 1962): a. The moisture condition of the soil closely surrounding the seed in the seedbed b. The Soil moisture distribution in the seedbed c. The soil drying rate as a function of the seed- bed itself and its surrounding climatic condi— tions. Seed Moisture Content Necessary for Germination Hunter and Erickson (1952) found that the seed had to attain a specific moisture content before germination would start. The critical soil moisture tension above which the seeds could not absorb enough water for germination (at 250C) was 12.5, 7.9, 6.6 and 3.5 atmospheres for corn, rice, soybean and Sugar beets, reSpectively, independent of soil texture. These results indicate a considerable difference in the ability of various Species to absorb the required water for germination from the soil. The soil moisture 13 14 necessary to supply the above requirements for sugar beets was between 10.2 to 12.0 percent in Brookston sandy clay loam. The work of Stout (1955) indicated that there is an optimum moisture range for emergence of sugar beets. For Brookston sandy loam this optimum range was from 12 to 21 percent. Stout also found that the addition of one or two cm3 of water to individual seeds produced a marked increase in emergence in soils near the lower limit of moisture acquired for germination. Soil either wetter or drier than optimum resulted in reduced emergence. Hunter and Dexter (1950) found that the moisture absorption of segmented sugar beet seeds from loose soil (Brookston clay loam) containing 12 percent moisture was completed in about four hours. The seed failed to germinate in air at 100 percent relative humidity because the seed moisture content reached only 29 percent. A seed moisture content of 31 percent has been established as the minimum for germination of sugar beet seed (Hunter and Erickson, 1952). This means that some of the water necessary for germination must be supplied in the liquid form; water vapor diffusion alone is not enough. Below the critical soil mois- ture value for germination, the rate of soil moisture move- ment is too slow to supply sufficient water to the immediate environment of the seed for its germination. 15 Sugar beet seeds immersed in water reach a moisture content of above 30 percent in one-half hour (Hunter and Dexter, 1951). When planting soaked sugar beet seeds in a field test only slightly better emergence was obtained. Dexter and Niyamoto (1959) found that surface coat- ings of hydrophilic colloids accelerated water uptake from sand and hastened emergence from soil under field conditions. All these reports only propose explanations to their results. More research is necessary to explain the role of seed-soil contact on initiating the germination. A high percentage of the seed surface area in contact with the sur- rounding soil should mean increased water transport into the seed. On the other hand this also means less area for effi- cient oxygen supply. Here soil moisture content and soil granular size is important for the tranSport of water to and into the seed. Moisture Distribution in the Seedbed Most textbooks, when considering soil drying, point out that during the constant rate period of drying, the mois— ture distribution is a parabolic function of the distance from the interface of drying. Wiegand (1960) has shown that the parabolic moisture distribution occurs naturally in dry- ing soil. l6 Stanhill (1955) followed the progress of drying in the top three inches of a seedbed exposed to ”slow evaporat— ing conditions.” After 21 days a moisture decrease below field capacity was evident to a depth of three inches dis- tributed as follows: 55 percent decrease in the surface inch, 29 percent decrease in the second inch and 16 percent decrease in the third inch. Wiegand (1962) also has fol- lowed the moisture distribution in the Seedbed. He found a steep moisture gradient in the surface 5 cm. Another characteristic was the very slow rate at which the moisture content increased with depth in the moist soil below the zone from which the moisture had been depleted. Johnson and Buchele (1961) found that the slope of Newton's equation of heating and cooling plotted on semilog paper characterized the rate of drying for the seedbed. Different drying conditions as well as different soil struc— ture and compactness changed the slope of the line. Soil Drying Rate In the spring the moisture content of most of the arable soils is between saturation and field capacity. If the soil has a drainage system the soil moisture tension increases according to depth and distance to the tiles. On permeable soils the soil water tension increases further as the ground water level lowers. Most soil scientists propose 17 that the soils reach field capacity at 100 to 350 cm soil moisture tension depending on soil type (Erickson, 1963; Richards and Weaver, 1944). This soil moisture tension is still less than most seeds can produce for water uptake to start germination. This means that the drying from the ground surface and not drainage is the limiting factor in seedbed moisture content. While soil moisture content due to drainage is always high enough for germination, the surface drying soon lowers the soil moisture content in the seedbed to values insufficient for germination. It is therefore of fundamen- tal interest in seedbed preparation research to know how different factors influence the evaporation process from the soil. Transpiration from plants is negligible in seedbed preparation and is therefore not mentioned here. Also the influence of rain is not dealt with here as being a Special case where the initial moisture content of the top soil layer is momentarily increased. Fortier (1907) concluded that the factors having the greatest influence on evaporation from soils are the quan— tity of water in the top soil, the temperature of the soil, and air movement. Many observations on tranSpiration from plants and evapotranSpiration from plants and soil are reported. The number of reports on evaporation from the soil itself on the other hand is comparatively small. 18 The Effect of Wind on Soil Drying Pasquill (1943) showed that the evaporation rate from a free liquid surface and wind velocity yield a linear relation on log-log paper. This holds in a range of 0.5 to 6.0 m sec—l wind velocity. Also Harris and Robinson (1916) as well as Hung (1964) showed evaporation from soil to be an exponential function of wind velocity. Fukuda (1955) showed that variability in wind Speed or wind gustiness only Slightly effect mass movement of air within the upper portion of the soil profile. The amount of water vapor tranSported by this air movement is very small. The Effect of Air Humidity on Soil Drying The evaporation rate is a complicated function of humidity due to counteracting changes in water temperature which result from changes in evaporation caused by changes in humidity (Cummings, 1929). If the relative humidity is high or the wet-bulb depression is low the air is almost saturated with water vapor and little evaporation occurs. Humidity as all other external factors is most important at high moisture contents of the soil. 19 The Effect of Radiation on Soil Drying Radiant energy coming from the sun is the all impor- tant energy source in the soil moisture evaporation process. Soil Surface slope, soil cover, cloudiness and rotation of the earth, as well as long wave radiation of the soil sur- face itself, greatly reduce the available energy. By set- ting up an energy balance equation and using the latent heat of vaporization of water one can estimate the potential annual evaporation for any geographic location (Van Wijk, 1963). The Effect of Temperature on Soil Drying The effect of air temperature is closely related to radiation in supplying energy for water vaporization. Thus temperature and evaporation are correlated, but, Since evap— oration results in cooling, the correlation is complicated. The influence of air temperature on evaporation increases at low soil moisture contents, because at a low moisture con— tent the cooling of the soil is less (less evaporation) which raises the soil temperature and the water tranSport within the soil (Van Wijk, 1963). 20 The Effect of Soil Factors on Soil Drying Soil compaction increases unsaturated water flow but decreases water vapor flow. The influence of compaction on evaporation rate is greatest for the top layers of the soil (Vomocil and Flocker, 1960). Mulches can be anything which transports water only, or predominantly, in the vapor phase. A soil mulch isolates the lower soil from heat conduction and forms an air layer in which water vapor transfer is mainly by diffusion. Gardner (1958) showed that the rate of evaporation is in- versely proportional to the thickness of the mulch. Gardner and Fireman (1958) found that when the mulch was less than 3 mm thick, it had no effect on the rate of evaporation. However, increasing mulch thickness above this measure de- creased the evaporation rate very fast. Army and Hudspeth Jr. (1960) showed polyethylene plastic film as being an efficient mulch material. Johnson and Buchele (1961) have worked with the influence of soil granule size on the rate of seedbed soil drying. They concluded that as soil granule size increased and soil compaction decreased the over-all rate of soil dry— ing increased. By using different soil granule sizes in a stratified arrangement the drying rate could be expected to decrease. 21 The Effect of Water Table Depth on Soil Drying Only a few data are available on actual rates of evaporation from soils in which the depth to the water table has been exactly known.. However, these indicate that the depth of the water table should be an important factor in water loss from soils by evaporation. Shaw and Smith (1927) studied evaporation from soil columns in the laboratory as a function of depth to the water table and concluded that evaporation in Yolo loam waS- negligible when the water table was more than 10 feet down. Schleusener (1958) found that for water table depths less than about 12 inches the ambient variables produced approx— imately the same effect on evaporation from the soils as on evaporation from a free water surface. The data of Veihmeyer and Brooks (1954) concerning cumulative evaporation versus water table depth yield linear plots on log-log paper for water table depths of 1 to 5 feet. Philip (1957) used a diffusion equation for unsat— urated moisture flow in a homogeneous soil: bk be- t (D‘flB) +‘EFE 6 is the volumetric moisture content of the soil (cm3 water/cm3 soil) t is the time (sec) . . . 2 D is the soil moisture d1fqu1v1ty (cm /sec) 22 : £)V’ . . . D k 2?? where 9’ 15 the m01sture potent1al k is the capillary conductivity (cm/sec) z is the vertical Space coordinate. The first term on the right side of the equation is the nonlinear diffusion equation with the diffusion coeffi- cient D a function of the moisture content. The second term is the gravitational component. Philip used this equation to calculate the transfer of water from a water table at = z z to a soil surface at which the relative humidity h is W Specifically ho. He calculated the steady state flux of water to the surface E (evaporation rate) as a function of the depth to the water table 2 and the air humidity at the W 5011 surface ho: E = E (zw , ho) Philip concluded that the flux of moisture E "is virtually independent of ho, except for a very small part of the h0 range, which one might arbitrarily Specify as hO > 99 percent" (h0 = 99%rflva soil moisture suction of 13 bars). hO : 99% , E = E (2w) Thus Philip considered the moisture flow to be a function of water table depth only. 23 Gardner (1958) compared the steady—state flow equa— tion of the type: a k: (52+b) k is the capillary conductivity (cm/day) S is the soil moisture tension in cm water a and b are pressure membrane outflow data with evaporation data. Good agreement was found between theory and experimental results. He states that the evapora— tion rate is determined by climatic factors which control potential evaporation or by the maximum rate of upward move- ment of water in the soil, whichever is the lesser. It is evident from the literature review that the steady rate of evaporation from soils with a water table under natural conditions can be predicted. The agreement between prediction and experiments suggests that the unsat- urated liquid phase flow limits the rate of evaporation. IV. SEEDBED PREPARATION AND TILLAGE TESTS Tillage tests up to now have almost completely been performed outdoors. Most of these tests have been done with the prevailing weather unchanged. In the reports a certain number of measurements of the test circumstances are re— ported. However, most of these reports do not give any information about changes in the test circumstances by time. It is known (Andersson, 1961) that the soil struc- ture changes with time. The changes are especially large during Spring and seedbed preparation. It is also known that the soil builds up a certain structure by time. The final structure is dependent on many factors as soil type, climate and time (Baver, 1956). Regarding tillage tests most of the reports give enough information. But as soon as plant development is involved like in seedbed preparation the reports usually do not give enough information about changes in Soil physical factors by time. The importance of measurements by time is clear when one realizes that two seedbed tests with identi- cal initial conditions can give completely different results depending on changes in the conditions due to the weather after the planting. 24 25 Different ways are used to minimize the influence from changes in soil structure. The use of coarse textured soils with almost no structure is one way. However, this limits the tests to this type of soils. On fine textured soils changes in soil structure are important in Seedbed preparation (Johnson and Buchele, 1963). To reach a certain structure in a soil usually takes a long time (Andersson, 1961). Because of this it is common to make tillage tests outside where the soil structure has developed during natural conditions. Changes in soil struc— ture occurs as soon as the soil is dried out and rewetted (Baver, 1956). This means that in order to prevent changes in soil structure the moisture content has to be kept fairly constant. To Speed up the soil structure formation, artifi- cial processes may be used. But because these involve biological as well as chemical and physical processes which are only partly known it would be difficult to control them. The common opinion is that it is at present impossible to produce exactly the same soil conditions at different times (@111, 1964). 26 Seedbed Preparation Test Methods Conventional Methods The conventional method for Seedbed preparation tests is to use fullscale equipment in the natural field. AS mentioned before many measurements have to be done to describe the circumstances during-the experiment. This method has perfect initial conditions but the results are difficult to evaluate on account of the ever changing cir- cumstances for the tests. Only one kind of initial condi— tions can usually be studied each year. Control of Rain Factor Soil moisture content is one of the dominating fac— tors in the seedbed. The soil moisture can be regulated to some extent in outdoor seedbed preparation tests. Rain on the test plot can be prevented by covering and be added by irrigation. The control of the rain factor is of great value and does not affect the other factors very much. Greenhouse Seedbed Experiments The third method is a proposed method which has not yet been used for planting tests. The idea for the method is built upon a method for fertilizer testing used by Fredrikson (1963). 27 On a uniform subsoil, with a controlled ground water level, is placed a system of 60 cm wide, 2 m or more long and 30 cm deep boxes without bottoms. The boxes are filled with the appropriate soil in disturbed or undisturbed condi- tion. The tillage implements and planters are carried by rails at the end of the boxes. By covering the whole area with a greenhouse structure a reasonable control over the climatic conditions and soil moisture could be reached. Exact detailed information on the seedbed physical factors must be determined as in all seedbed research. The possibil— ities to carry out tests under better control and under cir— cumstances which are of Special interest seem to be increased with this method. There is some question whether the same soil could be used over and over again. The limiting factor for repeated tests is probably the soil structure (Dexter, 1964). A New Seedbed Preparation Research Method for Advanced Control of Seedbed Environment The method developed in this research program and described here goes one step further in an attempt to con- trol the testing circumstances. The basic idea is to take an undisturbed soil sample, bring it indoors and apply the desired climatic conditions to it during the test while maintaining natural soil conditions. 28 Figure 1. Greenhouse seedbed test method. Implement and planter working direction I , «K Soil blocks for I planter tests . Subsoil with controlled ground water level Greenhouse structure over test Site Tracks for ‘ \ implement and EEEEEEESLLS~I”T”’I planter test rack ,, 29 With this method the Soil structure is initially the same as in the field. Seedbed preparation could be done in— doors with a single or a few elements of a full Scale tillage implement. A scale factor can be avoided when working with full Size tillage and planter equipment. Soil samples could also be taken immediately after completed outdoor seedbed preparation and planting. These samples could then be brought inside and exposed to a de— sired controlled environment. Working indoors generally provides better opportuni- ties for instrumentation. An almost completely controlled climate as to radia— tion, precipitation, wind, air humidity and air temperature can be achieved in a growth chamber. Two things have to be mentioned about the climate. In order to avoid differences between actual outdoor seedbed climate and applied indoor seedbed climate one must know the outdoor seedbed climate exactly. Today this is not the case nor is it known how climatic histories are involved in initiating germination (Bowen, 1964). This method provides good possibilities to relate germination results to a single seedbed physical factor or combinations of seedbed factors. One advantage of climate control is that experiments can be easily repeated and performed at any time of the year. 30 .Another advantage is that different soils can be brought together in one place to ease experimental work. The influence from lower soil layers on the seed— bed effects the seedbed temperature and the moisture con— tent. Control of the temperature in the indoor seedbed soil sample would be possible. Remaining to be controlled is the seedbed moisture factor. The following part of this thesis reports on an attempt to find a suitable practice for con- trol of the soil moisture factor in the soil sample. V. TESTS AND EXPERIMENTS During the summer of 1964 work was done to get some experience from the proposed test’method with undisturbed soil blocks. Moisture determinations were made on the test field to get actual values of the moisture distribution for the upper layers of the teSt soil during spring. The work on the test method was started on two lines; arranging the water supply under tension to the soil blocks, and solving the problem of taking undisturbed soil blocks. Later on the two solutions were combined in an undisturbed soil block brought inside and equipped with water supply. The solutions to the research problem in this thesis are not supposed to be the final answer but merely an ini- tial attempt to define the problem and to point out possible solutions. 31 32 Soil Moisture Content in the Upper SoiI Layer Durfng Spfing AS shown in the literature review there are little data available on the soil moisture content distribution with time in the seedbed. In order to get some actual values of the moisture distribution in the Spring, moisture determinations were made for the soil used in this study. The soil samples were taken in a Hillsdale sandy loam which had been fall plowed, but no Spring tillage was done. They were taken in the sides of a freshly dug pit. Each sample consisted of six smaller samples. All samples were oven dried for 24 hours in 1050C temperature. Table 1. Moisture content of upper soil layer. Hillsdale sandy loam. Fall plowed. No tillage during Spring. Depth below May 1964 ground sur— Average face (cm)' 4 6 11 21 25 28 0 - 2.5 12.4 8.7 18.0 8.8 14.6 10.5 12.2 2.5 - 5.0 12.4 13.8 18.4 11.5 16.0 14.0 14.3 5 0 - 7 5 16.0 15 O 19.3 12.7 15.5 14.5 15.5 7 5 -10 0 16.1 16 8 18.7 13.5 13.4 16.3 15.8 15.0-17.5 17.8 16.4 21.1 15.0 13.7 17.1 16.9 25.0-27 5 17.4 13.3 15.2 13.8 16.0 14.2 15.0 33 The recorded soil moisture values (Table 1 and Fig- ure 1) vary considerably, probably because the samples were taken from different pits and because the soil has a natural variation in soil moisture content. During the month of May several heavy rains fell on the test site. Most of the soil moisture distribution curves have the shape of a parabola. The effects of surface drying and of rain are readily shown in the diagram. The average mois- ture content from three inches and down to 10 inches varies from 15.5 percent to 18 percent. A pronounced lower soil moisture content at 10 inches depth could be due to a plow— sole. The soil moisture distribution in a minimum tillage seedbed on the same test site is Shown in Table 2 and Fig- ure 2. Table 2. Moisture content of a minimum tillage seedbed. Hillsdale sandy loam. Not fall plowed. Spring plow-planted, May 25. Depth below ground surface (cm) May 25 May 28 June 6 0 - 2.5 14.6 3.1 1.1 2.5- 5.0 16.0 12.6 12.7 5.0- 7.5 15.5 14.8 13.8 7.5-10.0 13.4 15.0 14.5 15.0-17.5 13.7 16.5 15.4 25.0-27.5 16.0 17.3 16.7 II-II-II. o 6236 mm as: :OAHmHmmoua nonpomm mm >m2 :ON :ma on go .L oommuzm unsoum seams Epsom .m om. & .paoucoo enzymfioe Hfiom 34 ma poz .EmOH >pcmm mampmaaflm .mm >m2 .Uopnmamnson mcfluam pmSOHQ Hamw .Uonvoom mmmaaflp Eseflcfis m mo peopcoo unapmfloz .m onswfim Int-III ”N >NE 40m 1.15... mm M... IIIII I.HN m2 1. ..... Ha sax ma I In I.n.o >m2 -I- w >m2 ..oH Eu oomMHSm chonw I :m soHon anmQ III II ._ . ./ I . .. I. I .I......I-. l om ma ca SH NH OH w & .pcoucoo ouapmfioe Hfiom .UoBOHQ Adam .EmoH >Ucmm mHmUmHHM: .mcfiumm wcflusu ommaafiu oz .um>ma Hflom moans mo acouaoo onnumfioz .N muzwfim 35 Here the moisture distribution is fairly uniform immediately after the seedbed preparation. The drying out of the soil surface layer is severe. After the initial dry— ing the drying rate seems to decrease probably due to the dry surface soil which acts as a soil mulch. The planter used for this seedbed preparation was equipped with press- wheels. The compaction from these presswheels might have affected the moisture distribution below the loose top soil layer. Corn handplanted at 5 and 7.5 cmcknflfl1in this seed— bed showed approximately 80 and 90 percent field emergence, reSpectively. 36 Establishing a Bottom Boundary in the Soil Block WhiEh Makes Possible Water Supply Under Tension In order to develop a useable method for seedbed research indoors with a cut-out soil sample, it is necessary to substitute for the lower soil layers in a natural seedbed outdoors. The main influences from the lower soil layers on the seedbed are, as mentioned before, on seedbed moisture content and seedbed temperature. Exact data on SOil temper- atures in the upper 10 cm of the soil during and after plant— ing are few. Most of the available data refer to the one inch level, then to the Six inch level, and so on. In this particular field data are needed for a least every cm. Establishing certain temperatures or temperature gradients in the shallow seedbed in the laboratory seems to be fairly easy. Therefore, the temperature factor was omitted in this study and only the more difficult water relations in the seedbed were studied. The idea for establishing a water supply was based on the tension table technique. On the tension table usu- ally the transfer between the soil and the water under tension is done by filterpaper. Therefore, the first attempts to establish the wanted water conditions in the Soil were done with filterpaper as a transfer material. However, it soon became apparent that the filterpaper tech— nique was not very suitable for use with big undisturbed 37 5011 blocks. Furthermore, tensions no higher than 50 cm of water could be reached, when testing without soil, the available different qualities of filterpaper. The purpose of the transfer material and the water supply system is to substitute for the lower soil layers. The surface of the transfer material should consequently be made up of air interfaces, solid material and water inter- faces. The influence of soil air from the lower soil layers is probably small and can be neglected (Baver, 1956). The solid part of the transfer material consists of the transfer material itself. Theoretically the capillaries and the water films of the lower surface of the soil block Should be connected to the capillaries and the water films in the transfer material. The better this connection can be made the better will the water supply system work. During initial attempts to take big undisturbed Soil samples it was found that the bottom surface of the soil block was very rough (Figure 17), even when extreme care to produce a smooth surface was exercised. Therefore, for reasons mentioned earlier, some sort of a transfer material that easily adjusted its form against the soil block bottom surface seemed to be necessary. If the surfaces of the Soil block and the transfer material could not be brought close enough together no water could be supposed to rise up from the transfer material, through the interface between the 38 transfer material and the soil, and further up in the soil. This is still more apparent when one realizes that 120 cm of water tension compares to a theoretical capillary of 0.05 mm diameter (Andersson, 1960). From the literature it was clear that a fine textured material as for example Silt could be used as a sealing mate— rial. Therefore fine ground tile—powder was tested as a transfer material. In the first attempts to use the dry powder a 2.5 cm layer of the material was put into a Shallow box with a water inlet in the bottom, covered by filterpaper. Then the material was Slowly wetted from below through the re- stricted water inlet. Several problems seem to affect this method. If the water enters only a little too fast the powder will move and cavities appear. The other and more serious problem is the removal of entrapped air in the dry powder. It Seemed almost impossible to wet the material slowly enough not to trap air in the material. This possi- bly can be solved by the method used by Schmidt (1963). He replaced the air in the sample by carbondioxide. When water with a dissolved calcium salt is added the carbondioxide will dissolve in the water. The method with a dry powder which is later wetted seemed to have disadvantages and was abandoned. 39 There might be a possibility to use prewetted fine textured material. However the sealing of the lower, out— side surface of the transfer material between the box and the water distribution system will be difficult to establish. AS the next attempt to find a suitable transfer material cast plates of plaster of Paris were glued to a piece of soft-rubber and a steel plate as a sandwich design. Holes in the rubber were interconnected to the water supply tube which was inserted through the steel plate. This arrangement provided a good support for the plaster of Paris plate preventing this from cracking when tension was applied. The plaster of Paris plates used at this occasion held a tension of 3 m for several months. The success with the precast plaster of Paris plates encouraged to testing methods of casting the plaster of Paris plate directly against the bottom surface of the Soil block. The rest of this study is directed towards develop— ing this alternative of a transfer material. 4O Plaster of Paris as Transfer Material Mixing and Casting of Plaster of Paris VPlaster of Paris can be made using different amounts of water. By changing the percent of water in the mixture, different porosities can be achieved. The water holding power of blocks made with 100 percent of water (100 cm3 of water to 100 g plaster of Paris) is given by Bouyoucos (1961) as being 57 percent while those with 66 percent of water have a water holding power of 32 percent based on airdry conditions. In order to get some experience in handling plaster of Paris a number of plates were cast (Figure 4). When de— creasing the percentage water in the mixture the time from the start of mixing to the start of mixture setting decreases. In order to mix fast and well a household electric mixer was first tried. During the mixing action by this device air in the form of numerous small bubbles was forced into the mix- ture. This made the plate too porous and the air bubbles tended to move, when the mixture set, making big straight pores. The best mixing procedure seemed to be careful Slow handmixing. For samples of 200 cm3 size a mixture with 60 per— cent of water could be mixed fast enough to still be liquid when poured into the casting form. When plates for the big soil samples were cast a mixture with 80 percent of water Figure 4. 41 Test Specimens of plaster of Paris. A is the plate made first. B, C and D are plates made with 100% water in the plaster mixture, 2.5, 7.5 and 12.5 cm thick, respectively. B is a test plate with a lucite funnel. F is a plate for testing sealing material (paraffin). Figure 5. Cross section of soil box with water supply system. A is the dry top soil layer. B is the moist soil layer. C is the plaster of Paris plate with paraffin as sealing material. D is the funnel for water supply. Note the close contact between soil and plaster of Paris plate. 42 had to be used in order to make it possible to mix the whole mass before it started to set. The mechanical strength of the plates seemed to in- crease if the plates were allowed to dry and set for 24 hours. The plates broke if dried under high temperatures. No reinforcement was used for plates with a diameter of up to 20 cm. Plates 1.5 cm thick and with 20 cm diameter withstood a tension of 130 cm water without breaking. Re- inforcement was used for all the plaster plates in the big boxes (30 cm by 60 cm plate and 20 mm thick). Water Supply and Sealing of the OutSide Plaster Plate Surface To supply water to the bottom Side of the plaster of Paris plate a funnel was placed in the liquid plaster of Paris where it fastened when the plaster set (Figure 5). This made a Simple but reliable connection of the water supply to the plaster plate. Conical funnels were first used but are difficult to position correctly and they also provide a smaller interface area between the water and the plaster plate. Tin cans (Figure 4) have parallel Sides and seemed to be excellent for this purpose and were later used. The edge of the bottom of the plaster of Paris plate which was exposed to the atmOSphere had to be sealed. When the water supply was started a slight pressure of 50 cm water head was used to fill all pores of the plate with water. 43 During this moment even this low water pressure tended to press loose the sealing material from the plaster plate and after some water accumulation break the sealing. Rubber adhesive, shellac and paraffin were tried as sealing mate- rials but neither were particularly suitable. Several solutions to the sealing problem seem pos— sible. Some kind of resin could be used which soaks into the plaster surface and then hardens. This should anchor the sealing material to the plaster surface and thereby better withstand the water pressure. Other possibilities are mechanical support to the sealing material, or elimina— tion of the outside plaster surface. As soon as tension is applied to the water supply most sealing materials were sufficient because then they are pressed against the plaster surface due to the lower pressure inside the plate. 44 Testing of Plaster of Paris Plates Tests were made to find the best procedure for utilizing plaster of Paris as a transfer material. Pilot tests Showed that the evaporation from plaster of Paris plates became constant a short time after the initial wet— ting. Testing Specimens To get uniform testing Specimens a number of rings, 10 cm in diameter and 2.5 cm high, were cut from a pipe. Water supply troughs were made from one.pound tin cans equipped with copper tubes for water inlet. Rubber adhesive was used as sealing material for the plate edge. Three replicate plates with 70 percent of water in the plaster mixture and three other plates with 100 percent of water in the plaster mixture were cast. Before the mixture had set the water supply troughs were lowered to correct position in the mixture. After setting for 24 hours the sealing was applied. Six more plates were made of 100 percent water mix— ture, three of them 7.5 cm thick and three others 12.5 cm thick (Figure 4). 45 Testing for Maximum Pore Size This testing technique was based on Beskow's (1930) technique for determination of the pore size in uniform fine textured materials. He applied tension to a sample saturated with water. When slowly increasing the tension a certain value is reached at which air is pulled through the biggest pore of the material. The equipment shown in Figure 6 was used to deter— mine the maximum pore size in this study. The test plate was connected to the mercury manometer. The connecting tygon hose as well as the water supply trough was completely filled with distilled water. When evaporation takes place from the plaster plate, replacement water from the supply system is Sucked up by capillary action developed by the pores in the plate. This condition will continue until the tension in the water supply corresponds to the maximum cap— illary lifting force produced by the biggest pore in the plaster plate. At higher tension air will be pulled through the pore and the tension disappear. Since this testing technique requires a day or more for each plate, a recording device was developed (Figure 6). The mercury level in one of the manometer arms was registered by a pointer on a one week register drum. To do this the pointer was hooked up to a float, floating in the mercury. .xon HMOm on» 09 omo: Saddam noun: on“ ma 0 .coflmcop pmsmpm ow >uauume muuxo mw m .aesaoo >uauume on» mfl m .o>Hm> m ma a .oaupon oHHOMumz m ow caveman on“ mpuo>coU nofinz ens» mmmaw m mw O can m .oppousn m we < .cowmcop HangmaoU wcflpfi> Iona mmxon HMOm How Eopm>m >Haazm Soyuz 46 u muswfim .pmop on» mcfiusv Hones» wcfl ISHU map opwmnfl mm: opmam umou onH .Ezuc aofiumupmwmou map ma Q .uo upcfiom o» umoam Scum pow mqmuooccou on» mM 0 .uopmsonme >uaoume on» ma m .opmaa anon on» we < .mNfim whom ssefixme mo pmoy How pumsmflsvm .o ousmflm 47 To get exact values on the pore Size from this test the technique Should be improved by testing under more uni- form surrounding conditions, but for comparison between different plates it was considered sufficiently accurate. Testing for Average Pore Size An estimation on the average‘pore size was achieved by an ordinary permeability test. The test plate was con- nected to a Mariotte bottle. A constant head of 10 cm water was applied. The water which ran through the plate was col- lected in a graduated glass (Figure 18). Difficulties with the sealing material as mentioned before was the reason for limiting the pressure to a head of 10 cm water. 48 Results from the Determination of Pore Size of the Plates Maximum Pore Size Determinations The data from the maximum pore size determinations Seems to be reasonable. The difference between the highest and the lowest measured tension value (Figure 8) is 5 cm of mercury or 15 percent. The pointer trace clearly shows how the tension in— creases to a maximum value as water is lost by evaporation. The maximum tension value is then constant for several hours. A possible explanation to the constant tension period is, that at this value no more water can be sucked up because the limit for the capillary force is reached. This limit is determined by the Size of the biggest pore in the plate. During the constant tension period the evaporation from the plate continues. By redistribution of the moisture in the plate a uniform drying front advances into the plate. When the drying surface approaches the opposite side of the plate the tension drops. Table 3. Maximum tension developed by different plaster of Paris plates Percent water in the Thickness of Developed tension plaster mixture plate cm mercury cm 70 2.5 32.5 100 2.5 21.0 100 7.5 26.0 100 12.5 33.0 49 m OOH 6cm nmymz m mane ooem :ofimcmp >u36uoe 56 cm II It. ll mumupi umpcfloa wcfipuooom .Auomma mcfipu .86 m.» ”mmmcxoflnp mumam .Amflummpfioumwmmam Eu OOHV mumpxfie :fl amps: “Gounod OOH ”mmpmam umafiefiw mmunfi ooam ooam ooam Mao: \ - 1 \\ I l '4' I I SH muaam an mnaam ooouv cofivmcflEHopoU mNflm whom ESEfiXmE scum mme .w muawflm 50 The values Show that lower water content in the plaster mixture and thicker plates give higher tension values. Average Pore Size Determinations The permeability test was supposed to give an answer on the average pore size. Table 4. Average percolation rates for three plaster of Paris plates. A constant head of 10 cm waterwas applied. Three plates: 100 percent water in the mixture (100 cm3 water and 100 g plaster of Paris) and 7.5 cm thick plates (the same plates as in Figure 8). Plate number Percolation rate cm3 per hour 14 5 . 15 10 16 15 The values do not Seem to give a reliable answer. It is known that the permeability is proportional to the fourth power of the pore diameter (Andersson, 1960). There— fore a few bigger pores can completely cover the actual aver- age pore size. 51 Air in the Water SupplygSystem A test plate with a water supply trough made of lucite (Figure 4) was cast. When this plate was filled with water it was clearly seen how the air entrapped, in the sur— face of both the plaster plate and the lucite water supply trough, formed numerous small air bubbles. These air bub- bles later formed one big air bubble which isolated part of the plaster plate from contact with the supply water. When the principle of the Mariotte bottle was used for water supply under constant head, air dissolved in the distilled water. This air later formed air bubbles on the underside of the plaster plates. It could not be determined if any air leaked through the plates. Air bubbles in the tygon hoses did not impede the water supply. A different water supply system with valves instead of the Mariotte bottle would eliminate air entrance through the supply source. Even with some air bubbles on the under- side of the plaster-plate it is likely that the water dis- tribution within the plaster plate is uniform. 52 Procedure for Taking Undisturbed Soil Blocks In order to take undisturbed soil blocks, boxes of sheet metal 30 cm by 60 cm and 30 cm deep were made. Fig- ures 10 and 15 Show the outer and inner boxes. The main purpose of the outer box was to support the inner and weaker box (no top or bottom) as it was forced into the ground. I The 5011 block was first dug free from the ground on all sides to about 2.5 cm outside the final dimensions (Figure 9). This was done to ease the final Shaping by the inner box. The outer box, with the inner box (bolted to the outer box) inside, was then forced down over the soil block with light blows from a hammer (Figure 10). When the boxes had reached the appropriate depth, a piece of 40 mm foam rubber was placed on t0p of the soil block and the cover was bolted in place. If the soil was firm and dry, the soil block could be loosened by rocking the boxes, but if the soil was moist or loose, the bottom Sheet had to be pulled in under the boxes. In order to pull this sheet two wires were guided around two rollers and wound up on a rod on the outer box. This rod was turned by a wrench (Figure 11). After the 5011 block was cut free from the ground the boxes were turned over and brought in to the laboratory (Figure 12). ‘ ‘ “65.3," m ‘71,; - AIQ'VJ' I,“ . ' , .) Aft-1“”? .4“ ' '3'“ 4" -r. ‘ ' 1%: (fr ‘ ' I , 1"“? g. {:13}; I U . D.#Jl,".;.f“ .- ~ . ' o'r‘ """fif-J . w’?’ l i J- . 1 .1 A ‘- . . ~0 , Figure 9. Soil block dug free from ground on all sides at about 2.5 cm outSide the final dimensions. ‘ Boxes forced down over the soil block. Figure 10. . . The inner box cuts off exce551ve SO11. \. Figure 11. Pulling the bottom cut off Sheet in under the soil boxes. Two wires from the cut off sheet are guided around the lower right box corner and then wound up on the top rod, which is turned by the wrench. AJ‘é‘ ' ‘ ‘1‘ . - Figure 12. Soil box, with top cover in place, turned over and ready to transport. 55 Depending on the thickness of the plaster of Paris plate a suitable depth of soil was carved loose from the bottom of the soil box and removed. This made room for the reinforcement consisting of 1.2 mm steel wire, which was stretched over the bottom of the inner box. A 10 mm tube, reaching from the bottom of the box and to the surface of the soil block,~was placed in one corner of the box. This tube reached through the plaster of Paris plate and could therefore be used to remove the entrapped air when filling the water supply trough. After this the inner box was ready for casting the plaster plate (Figure 14). Plaster of Paris was mixed and poured into the bottom of the inner box. Immediately after filling the bottom of the box with plaster of Paris the water supply 'trough was placed in the liquid plaster. In order to give the reinforcement wires correct position and tension, weights were placed on the top of the water trough. After this the plaster of Paris was left to set for 24 hours. After setting, the inner box was taken out and the free edges of the plaster plate were covered with paraffin. At this stage the box with the soil block was ready for starting the water supply (Figure 15). Figure 14. 56 5 Soil box ready for casting the plaster of Paris plate. A is the outer soil box. B is the inner soil box. C is the reinforcement for the plaster plate. D is the air outlet tube. E is one of the guides for the wire to the cut off bottom Sheet. Figure 15. Inner soil box completed with plaster of Paris bottom and water supply trough with tygon water supply hose before turning to upright position. The bottom open surface of the plaster plate sealed with paraffin. 57 Comments to the Pr0cedure of Taking Undisturbed Soil Blocks The procedure of taking small undisturbed soil cores for soil physical examinations is well established today (Baver, 1956). The first attempts to take big undisturbed soil samples were done with the outer box Shown in Figure 10 and with an inner box Similar to the one in Figure 15 but with bottom. When taking a sample the outer box was forced into the ground and the bottom Sheet was pulled in under the box. When the top cover was in place the box was turned over and the bottom Sheet removed. Then the inner box was forCed down inside the outer box. Both forcing the inner box into the outer box and the subsequent withdrawal of it with the soil block inside was extremely difficult. Even when the boxes fitted each other well, excessive force had to be used. Furthermore, the soil Sample tended to be hanging in the inner box due to friction and adhesion to the box walls and was not resting on the soil box bottom or the intended transfer material (plaster of Paris). For these reasons inner boxes with bottoms were abandoned. A method of taking large undisturbed soil samples somewhat similar to the one described in this thesis is reported by Miscenko (1937). His boxes measured 12 cm by 24 cm and 20 cm deep, but he mentions that the size of the boxes might be increased. Fredriksson (1963) used a mobile 58 Figure 16. Stone deforming the inner box cutting edge when boxes were forced down over the soil block. I—r" A Figure 17. Bottom surface of box "broken" loose from the ground. Hillsdale sandy loam with 5 cm of old sod removed. The worm holes had to be closed before casting the plaster plate in order to prevent the plaster from penetrating up into the sample. 59 crane when taking undisturbed soil cores weighing 300 kg for a lysimeter experiment. The reason for using two boxes in the present in- vestigation was to test the possibility of making the outer box more rigid and the several inner boxes cheaper and lighter. Probably undisturbed soil blocks could be taken more easily with one single heavy Sheet metal box instead’ of with two boxes. The forcing of a box, 6f the Size used in this study, into the ground will have only Slight effects on the natural soil structure (Baver, 1956). The preceeding Shaping of the soil block further minimizes disturbances. However, some wall effect may occur due to difficulties in forcing the boxes straight down into the ground. Excessively dry soil, roots and stones will cause trouble and have to be avoided as much as possible (Figure 16). When the bottom Sheet is pulled about two—thirds of the way under the boxes, the boxes usually start to move towards the sheet instead of the sheet moving. This will give an uneven Soil bottom surface but as is already ex- plained this does not cause any problem when using cast plaster of Paris as transfer material. By using two sheets and by pulling them simultaneously towards the middle of the boxes the movement of the boxes can be avoided. 60 The use of foam rubber on top of the sample prevents changes in the soil block surface. Even loose soil struc— tures as plowed soil can probably be handled with only minor disturbances. A soil box 30 cm by 60 cm and 30 cm deep is about the maximum size which can be handled without lifting devices. 61 Two Soil BoxeS--One With and Another ‘Without Water Supply It is Shown by Veihmeyer and Brooks (1954) that the evaporation rates from wet soils decrease very fast when the top soil layer dries out. As the dry tOp soil layer becomes a few centimeter thick the evaporation rate ap- proaches only hundreds of a centimeter per day. This is true as long as the soil surface is not disturbed. If the surface is tilled one must expect that the loose top soil layer, as for instance in a seedbed, will dry out faster. 6 In order to determine the importance of a water supply system to the boxes, two soil boxes, 20 cm in diame- ter and 8 cm deep, were filled with undisturbed soil and prepared as described in ”Procedure for taking undisturbed Soil blocks." Both boxes were put on 120 cm of water ten- sion and set aside for two weeks to reach constant Soil moisture conditions. During the test one of the boxes was constantly kept on water supply, while the water supply funnel was drained and closed on the other box. The water supply was main- tained at 120 cm of water tension in the appropriate box during the test. The surrounding steady evaporation condi- tions was about 15 cm water per month. Tillage to a depth of 5 cm was done in both boxes at the start of the test. The result of the test is shown in Table 5. 62 A marked decrease in moisture content is clearly demonstrated. The size of the moisture content decrease is big enough to be significant in an indoor seedbed test. Table 5. Soil moisture content with time in two soil boxes, one equipped with a water supply system and the other without water supply. Moisture content at start Moisture content 10 days after of test, percent start, percent Decrease in Both boxes had been on a Continued No Soil moisture water supply of 120 cm of water water content in water tension supply supply box without water supply Depth below soil sur- Box Box Box Box Box face cm nr 4 nr 7 nr 4 nr 7 nr 7 0-2 5.94 7.05 5.74 4.50 2.6 2-4 10.31 11.77 9.86 8.12 3.7. 4-6 12.88 13.99 12.39 9.80 4.2 6-8 13.91 15.13 15.03 10.15 4.9 63 Tests in DryinggTunnel Weight Change of Soil Boxes Information from the literature indicated that the drying conditions in the field during spring are severe. To increase the drying conditions around the soil boxes a dry- ing tunnel was built. The drying tunnel is shown in Figures 18 and 19. The inside dimensions were 30 cm by 30 cm and there was Space for three soil boxes 20 cm in diameter. A fan was used to blow a steady air stream of about 2 m sec'l through the tunnel. Two heating cones of 600 watt each were used to heat the air. In a later experiment one 250 watt infrared lamp 40 cm above each soil box was added. AS a measure of the drying conditions in the tunnel the evaporation from a free water surface was determined. A petridish 10 cm in diameter was placed 5 cm above each. soil box (Figure 19). This dish was filled with distilled water and weighed regularly. Changes in soil box weight could be registered with a spring scale placed on the tunnel walls (Figure 19). A soil moisture tension of 120 cm water was used during the experiments. This figure was chosen from the pF curve and from the average soil moisture content for this soil as determined for the month of May, 1964. .mmxon deem one mo mcwnmaoz How oamow m mfl m .Eopm>m Saddam Hope: on“ mfi Q .Acofipfimoa Ummfimu :Mv maEmH Umumuwnfl AHA: too: on» ma 0 .cm& coflpauoam>o may mfi m .xon Hfiom on“ mfi < .Hoccsp wcfi>up on» mo Sofl> a: omoao 64 .oH onsmnm .moxon aflom ammo poem mco mum m,z .Aoooa .coHuoEV mafiapmmm :uoo m can Soupon mflumm mo umpmmaa EPA? xon HAOm HHmEm m we w .mNHw whoa ommuo>m mo “wow wow ampmm one we m mam m .anm>m Saddam nouns on“ mfl Q .GOAPAmoa woman“ :A mQEmH Umumuwcfl spas moon on“ ma 0 .xon HAOm on» mfi m .Hocczp wGASMU may mfl < .Qzuom «coEflHmQXm Hm::39 mafi>uo ‘ .wH ausmnm 65 In order to test a different way of regulating ten- sion the equipment of Figure 7 was built. Each water supply system consists of a burette arranged as a Mariotte bottle. To increase the tension, the Supply line from the burette to the soil box passes a mercury manometer. The amount of mercury necessary for a total of 120 cm tension was filled into the manometer. When the box starts to develop tension, it lifts the mercury until all mercury is found from the bottom of the manometer and up in the arm closest to the soil box. When the box in this situation continues to suck water the water passes the mercury column and from now on a constant tension is applied in the water supply system. Soil boxes 20 cm in diameter with soil in undis- turbed condition were prepared as described in "Procedure for taking undisturbed soil blocks.” The soil was a Hills- dale sandy 1oam with five centimeter of old sod removed. To start the water supply a water head of 10 cm was applied dur— ing 24 hours. After this the soil boxes were put on 80 cm tension for another 24 hours to drain. This was necessary because the burette-mercury equipment cannot function as a drainage.' After this the soil boxes were connected to the tension regulating equipment and the tension was increased to 120 cm of water. During filling of the water supply burettes with water a momentary decrease of about 50 cm water head was unavoidable. 66 A part of the results from the testing of soil boxes in the drying tunnel is shown in Figure 20. The air temperature in this test changed during the day from 210C to 270C. The average wind velocity was 2 m sec—l. The relative humidity of the air was about 30 per— cent. A better measure on the evaporation condition is given by the curve for weight loss from the evaporation pan. This curve adds up the influence from temperature, radiation, relative humidity and wind velocity. When testing the function of the soil boxes and their water supply system it is important to use severe dry- ing conditions. This must be done to prove that the water supply system will not limit the soil drying and evaporation rate during actual Spring conditions. Morten and Buchele (1960), studying emergence energy of plant seedlings, reported on the moisture loss from their soil boxes. Their highest water loss was from a soil box with 20 percent initial moisture content and 16 psi surface compaction. The data for the soil box in Figure 20 showed about the same evaporation. Johnson and Henry (1963) also working with seedbed compaction and soil drying rates used environmental condi- tions equivalent to a monthly evaporation from a free water surface of 12.5 cm. This evaporation rate is reported as 67 mm wm mm mm ‘ ‘ nonsmpamm HN cm 0% It . 00 H [\ H \O "H "[0 H P ‘ xon Hfiom uo>o can coflpmuoam>m va OOVN H503 ma 1 M u:o:\m mama soap rmuomm>o Eouw mmoa pawumz .iIIIQ Hmfipficfi 0p m>flymaou “swam: Hfiom m {I\ 1. OWI 3 l mm. s.. m omem m r I123... :8 mm \W HSEEH t.l O a It IAIN u 1 %. c‘ Maw‘ _usoa\mEo . u A l. I!!! ru 9 V . \/ I: m xoo. mo 3 W D. n. communes—m w T: E 1.1 coflpQESmcoo “opus xom 1:06 young m ._. .IIIIII I I..I|»- IIIIAvq .Hmcc:p mcfi>uo GA xon Hfiom mo pmoH .om musmfim 68 typical for the month of May for Ohio weather conditions. The evaporation rate for the open pan in the drying tunnel in this study lies around a steady rate of 30 cm water per month. Nothing is found about evaporation rates during shorter periods but it must be realized that the evaporation rate during the Spring momentarily can be much higher than the average. The curve for the box water consumption follows the curve for the open pan evaporation. The day rhythm is clear in both curves. The average evaporation rate from the soil box was 23 percent of the average evaporation rate from the open pan. Holmes and Robertson (1963) have shown that in a coarse textured soil as a sandy loam the ratio of actual evaporation to potential evaporation decreases fast with time, and much faster than for a more fine textured Soil. The actual evaporation for the soil box was 0.23 cm per day which is in agreement with data reviewed by Wiegand and Tyler (1961). However, the value is much higher than the value of 0.067 cm per day as reported by Johnson and ‘Henry (1963). The total amount of water used by the box for 21 days was 953.7 cm3. The change in totalsmfid.weight was less than 1 percent. 69 About 10 cm3 air was removed from the funnel on the 20th of September. A step in the water consumption curve is noticed but the box and the water supply system seem to continue to work. With reference to what is shown, it is probable that an undisturbed soil sample with a water supply system of the plaster of Paris plate type can be used for seedbed tests indoors, and that an equilibrium moisture condition in the soil can be maintained even at unusually high drying rates. 70 Water Flow Resistance in the Water Supply System All but one of the literature reports found deal with the use of porous plates for removal of water from a soil sample. Gardner and Fireman (1958) used porous cups to supply water under tension to soil columns in a soil evaporation study using disturbed soil. In this study the plaster of Paris plate is supposed to supply water to the soil sample. The establishment of a water movement from the water supply, through the plaster plate, and up through the plate- soil interface into the soil is dependent of its weakest point or the plaster plate-soil interface. The contact between the plaster plate and the soil must be very close. A water supply tension of 120 cm of water corresponds to a 0.05 mm capillary. If there is an open Space bigger than 0.05 mm between the plate and the soil and this cavity is in contact with the atmOSphere the plate will work as a drain but not as a water supply. It seems likely that the casting of a plaster of Paris plate against the soil bottom surface would make a contact good enough for water supply. The tests and experiments seem to verify that the close contact needed has been reached with the method described. 71 The fact that the soil sample does not extend down to a real water table must be noticed but seems to be of little disadvantage for seedbed tests. Resistance to water flow in the water Supply tube can be neglected (Gardner and Fireman, 1958). But the resistance to water flow is prob— ably higher in the porous plate, due to lower porosity, than in a correSponding soil layer. However, the total resistance to water flow can be assumed lower in the water supply System than in a soil column with the same depth as the applied tension in the water supply system. The way to adjust for the lower water flow resistance is to increase the water tension in the water supply system (Gardner and Fireman, 1958). 72 Soil Boxes in an Intensive Drying Cycle The natural seedbed is exposed to a drying cycle. The drying conditions during the day are more intensive than during the night. It was of interest to see how the soil boxes would behave in an intensive drying cycle. To intensify the dry— ing conditions a hood over the drying tunnel was made (Fig— ure 18). Three 250 watt infrared lamps were placed in the drying hood, each lamp 40 cm above a soil box. The fan and the tunnel heating devices were on for about 12 hours a day. The temperature inside the tunnel was between 200C and 250C during the nights and from 340C to 38°C during the days with the heating devices on. Figure 21 shows the re— sult from the test. Lack of time made it impossible to con- tinue the test for more than five days. The amplitude of the drying cycle, as the results from the evaporation pan weight loss curve Show, was con- siderable. However, it is evident that the soil box mois— ture content follows the cycle. There is about 4 to 6 hours lag between the curve for the evaporation pan weight loss and the curve for the Soil box weight. There is also a slight trend of total weight loss for the boxes when the drying conditions increased. 73 co mocou mcflpwmn can mQEmA ow nono»oo 4 «O (X) [\ I I oowm “pom I OH ma ON uson\m can :ofipmuoam>o Eouw mmoa pnwflos Mwmum>< m omam u pamnmz Hmnpncn.n Hones: xon Show w oaoo u unmamz Hmflpflzw m Hones: xon aflom m omkm u pawns: Haunncn H Hones: xon snow \/ pawns: Snow HaupuaH Hmflpanfi Cu m>fiumaou unmfimz Hfiom .oao>o mGASSU o>fimcmwcfi cm GA moXOD Haom .HN ousmfim 74 Contaminations of the SSupply Wdter The supply of water passes the transfer plate of plaster of Paris in the bottom of the soil sample. Plaster of Paris or CaSO4 1/2 H20 has a solubility of 0.3 g per 100 cm3 of water (200C) (Handbook of Chemistry and Physics, 1963). By placing some pieces of plaster of Paris in the water supply line the water will be nearly sat- urated with CaSO4 when reaching the plaster plate and no damage to the plaster plate is to be expected for this reason. CaSO disSolved in the supply water will form a white 4 crust on the soil surface when the water evaporates from the box (Figure 23). Whether the comparatively high concentra- tion of SO4-ions will influence the germination and growth of the young seedlings is not known. Mercury is insoluble in water (Handbook of Chemistry and Physics, 1963), why no influence is to expect from the use of mercury to increase the water supply tension. Heavy coloring from rust occurred when water supply troughs of Sheet steel were used. The iron surfaces conse- quently have to be protected. Figure 22. Working soil box equipped with water supply system. A is the soil box with undisturbed soil. B is the water supply funnel. C is corn seedlings. Figure 23. Soil box with CaSO4 crust on top soil surface. 76 Tests Using Soil Blocks Full Scale Planting in the Big Soil Boxes Three soil blocks 30 cm by 60 cm and 30 cm deep were prepared according to procedures mentioned before. The main purpose for this test was to get some experience from using a full scale planter unit with the soil blocks. Two of the Soil boxes were placed in a movable soil bin (Telischi, 1956). Long water supply hoses made it pos- sible to move the soil bin with the water supply system con— nected to the boxes. Soil from the soil bin was filled up around the soil boxes. The upper parts of the soil box end walls were removed (Figures 24 and 26). The Soil around the test soil boxes Should support the boxes during the seedbed preparation. It also made it possible to give the implements the correct Speed and approx- imate position before they moved in over the test soil boxes. With soil between the boxes it is possible to run several boxes at the same time to get replicates. No difficulties were encountered when the planter unit was run over the soil blocks. The planter seemed to work very well and in the same manner as in the field. 77 [31 Figure 24. Fullscale planting in the soil boxes A-B-C-D and E-F-G. The big soil bin is movable while planter unit is fixed to the wood bars. , 55". 4 V' 3. Figure 25. Close up View of planter unit. 78 Test of the Water Supply System in the Big Soil Boxes ‘ ' It was of interest to see if the water supply system still worked after the somewhat rough handling during the planting test. The three Soil boxes were arranged as shown in Figure 27. Each box had its own water supply which could be weighed on the small Scale. .Two of the boxes could be weighed on the big Scale. An airstream was blown over the boxes to increase the evaporation. The water Supply was started by keeping the free water surface in the Mariotte bottle 10 cm below the soil box upper surface. The tension was increased to 70 cm after' 24 hours. After initial drainage the box weight and water consumption of one of the boxes changed with time as is shown in Figure 28. It can be seen that the changes were very small indicating that the box had been in equilibrium with its surroundings. Box number two was started with the free water sur- face one cm above the plaster of Paris plate upper surface. Then the box was put on 70 cm tension as was the first box. Apparently this starting method was not sufficient since this box picked up 2 kg of water during the following month. However, after this first month the box reached the same constancy in box weight as the first box. 79 .mamum HHmEm on» :0 mmauuon Saddam Hana: man an “Bugs «an 09 .m:0upne ncou mcfl>up on» mmmmuocfl 0“ saw a mfi moxon Hoccfi 039 on“ o>on< .oamum man one so nonmfioz on panoo mmxon aflom Hoccfl 03» one .moxon HMOm man may mo Eoumsm >Ham3m Hopes may mo ummH .omon poauso ufla on» ma m .omon umHCA Hope: man we Q .xon axon on“ mo maflccfimmn on“ mfi 0 .Hamz can map mo puma po>OEou Qua: xon HHOm “mufim on“ an m on < .ann Show mana>oe an» .RN musmnm GA mmxon HMOm mo 36fi> as omoao muswflm 8O 00 ms I2 L3 3 3. 3. NH 3 on 8 wt m. m m w HoDODUO -i t - - -:--i:<- -. E 3:52! mamborbohfim, ,anmnozrxoo USED. 2. III Iloom- \/\/ II 1.. < _ I -. .l..---...ir .1. -I- . .. . . . ,. . l -Irll..«-. I--. . SOON... Hmfiwflcfi ow m>fiumamu “swam; xom 4 m soom {mo\w .mmxon Hflom mfln opp mo oco mo Eopm>m Saddam noun: may mo pmoH .wm ousmfim 81 Box number three had only one-third the reinforcement of the two first boxes. Also this box worked to satisfaction. Only about 10 cm air, at an average, was accumulated in the water supply system during two weeks. All three boxes were still working one and one—half months after the start. 82 Another Example for Use of the Undisturbed Soil Block Technique Morton and Buchele (1960) developed an instrument for measuring the energy expended during the emergence of a "mechanical seedling” (probe). They used plastic boxes 12.5 cm by 17.5 cm and 10 cm deep. Holes were drilled in the bottom of the boxes for passage of the mechanical seedling. The mechanical seedling was pushed up through the soil sample and the required energy measured. Figures 29 and 30 show one of the plastic boxes used by Morton and Buchele. The second box shown contains an undisturbed soil block of the same size as the plastic box. The box with the undisturbed soil sample was made by using the undisturbed soil block technique. The holes through the plaster of Paris plate were made of small tubes which reached up to the soil. The holes were closed with paraffin when the plaster plate was cast. Morton's and Buchele's instrument could be used with a box of this type to test samples from actual field seedbeds at different times after planting, with natural moisture con- ditions maintained, with different climates applied and with Seed actually planted in the box for comparison. 83 Figure 29. Soil boxes for measuring seedling penetra- Figure 30. tion forces (Morton, 1960). A is a soil box W1th plaster of Paris bottom for water supply under tension. B is a soil box used by Morton. «I ~ 0 .ul ! -. ” ti.“ \ . I 1" 1 ' I)‘ . . .. r7 A .2»; It \ 9 ’4 - ' Bottom view of boxes in Figure 29. A is the soil box with plaster of Paris bottom for water supply under tension. B is the soil box used by Morton. C are holes in the bottom of the soil boxes for the "mechan- ical seedling" (probe). D is the air outlet hose. E is the water inlet hose. VI. SUMMARY AND CONCLUSIONS Summary Ever since man started to grow plants he has been manipulating the soil to create a seed environment that would result in a favorable seed germination and plant growth. Still in our sophisticated society the seedbed preparation is more of an art than a science. However, in recent years agricultural scientists have realized the in— creasing importance of determining and studying soil phys- ical factors. In spite of increased research efforts, lack of basic informations concerning seedbed environment is still apparent. Physical definitions, instruments and measuring procedures have to be developed in order to put numerical values on seedbed properties and property changes. Conse— quently, no single value or group of values exists today which can adequately express the optimum conditions for seed germination and seedling emergence. The literature review showed an appreciable differ— ence between laboratory germination tests and field emergence. This means a loss in Seedbed environment control when moving from the laboratory to the field. Today our control over the 84 85 seedbed physical factors are limited which may be largely due to lack of basic information. However, by studying and summarizing present knowledge better understanding and therefore better possibilities to modify the seedbed phys- ical environment can be achieved. The literature study also gave an outline of Seedbed research today. Special attention was paid to basic re— search on seedbed physical factors. It was evident that a complex problem like this has to be broken down into its individual parts. Each physical factor has to be studied separately. But to be sure that no other factor is involved in the result, the other physical factors known to be of significance in seedbed preparation, have to be observed simultaneously. When the basic relations between soil manipulation, seedbed physical factors, germination physical factors and the climate are known one should be able to determine how to control the seedbed environment. Today scientists as well as farmers know only the general outline of these relations. With a higher probability for correct weather forecastsv these relations are needed in order to take full advantage of the weather trends. In order to get a background for the study, the soil moisture relations in a Hillsdale sandy loam during Spring were studied. The results of the moisture tests were in agreement with those in literature references. 86 A technique for maintaining the natural water rela- tions in an undisturbed soil block was developed. This in— volved a water supply system for the soil blocks were the water was supplied under tension. Several materials were tested as a transfer material for the moisture transfer between the Soil block and the water supply under tension. Plaster of Paris plates cast direct against the soil block bottom surface were tested exclusively and found to function satisfactorily. A method for taking undisturbed soil blocks 30 cm by 60 cm and 30 cm deep from the field was worked out. Soil blocks of this size were equipped with water supply systems for water supply under tension. Tests with a full scale planter unit were run in the soil blocks. These soil blocks still worked to satisfaction one and one—half months after start, when the study was ended. Conclusions Literature Review The purpose of seedbed preparation is to establish a correct number of well developed plants in perfect Spacing and capable of adequate yield response. Big differences exist between laboratory germination tests and field emergence. The lower field emergence means that some control of the seed environment has been lost when moving from the laboratory to the field. 87 During recent years scientists have been more interested in seedbed physical environment factors. All physical factors known to be of significance to seedbed environment Should be observed simultaneously to make it possible to fully explain an experiment result. The soil drying rate is one of the most important factors in the seedbed environment. There is a consider— able amount of research done on soil drying. This Should be applied to seedbed research. Seedbed Preparation Tests There is a definite need for more basic research on seedbed preparation. The possibilities to control the weather conditions above the seedbed should be used. This would.‘be the first significant improvement to today's common seedbed tests. A reasonable weather control could be achieved with a green— house structure. The proposed new seedbed preparation research method for complete control of seedbed environment is not fully developed yet. However, this method provides wide possibil- ities for basic seedbed research. 88 Water Supply to Undisturbed Soil Blocks The method for taking undisturbed soil blocks as described in this thesis seems to be usable. Soil blocks can be brought indoors and maintained at a constant moisture content. The water supply system worked for two months with 120 cm of water tension. It‘s reaction was comparatively sensitive to changes in evaporation conditions of the soil. Both at a high constant evaporation rate as well as in a drying cycle with great amplitude could the soil sample be satisfactorily supplied with water. The method provides a complete control and also registration of the seedbed moisture relations. This is due to the added possibilities of observing soil moisture move: ment to or from the seedbed through the soil below. Air accumulation in the water supply system is a minor problem which was not completely solved. The effect of the contamination of the water by CaSO4 must be kept in mind when making tests with this method. 89 VII. SUGGESTIONS FOR FURTHER WORK A technique for taking undisturbed soil samples and maintaining them in a natural state is developed in this thesis. Lack of time has prohibited actual use of the proposed methods. The practical procedure in taking undisturbed soil blocks seems to be fairly satisfactory. Single boxes made of heavy sheet metal should be tested. In order to take big samples some kind of a lifting and tranSporting device will be necessary. ' The transfer material in the bottom of the soil sample is very important for the water supply technique. More tests with the plaster of Paris plates are necessary. Plaster of Paris is cheap and simple to use but the con— trol over the final porosity was not complete. The use of an indifferent fine textured powder as a transfer material should be tested. When the different phases of the proposed technique work well actual seedbed studies could be started by placing samples in a growth chamber and taking data. REFERENCES Andersson, S. (1960). Kapillaritet. Institutionen fdr Agronomisk Hydroteknik, . Kungl. Lantbrukshbgskolan, Uppsala, Sweden. ‘ . (1961). Markfysikaliska undersUkningar i odlad jord XII. Om matjordens struktur ock struktur-fdrandringar. Grundfbrbattring, 1961 (4):201-214. Sweden. Army, T.J., and E.B. Hudspeth, Jr. (1960). Altenation of the microclimate of the seed zone. Agronomy Journal, 52:17—22. ASAE: American Society of Agricultural Engineers Cultural Practices Equipment Committee: Seminar on Minimum Tillage and Research Methods, 1964. Ohio Agricultural Experiment Station, Wooster, Ohio, 27 pp. Baver, L.D. (1956). Soil Physics. John Wiley and Sons, Inc., New York, 489 pp. Beaty, E.R., and J. Giddens. (1962). Effect of various methods of seedbed preparation and cultivation on yields of corn grown following rye; Agronomy Journal, 54:431—432. Beskow, G., 1929. (1930). Om jordarternas kapillaritet. En ny metod fUr bestamning av kapillarkraften, Sveriges Geologiska undersbkningar, Series C, Number 356. . Boswell, V.R. (1961). What seeds are and do. An introduction, pp. 1-10 in The Yearbook of Agriculture. The United States Government Printing Office, Washington, D.C., 591 pp. Bouyoucos, G.J. (1961). Soil moisture measurement improved. Agricultural Engineering, 42(3):l36-l38. 9O 91 Bowen, H.D. (1960). Some physical impedance and aeration effects on planted seeds. Paper presented at the 1960 annual winter meeting, American Society of Agricultural Engineers, Memphis, Tennessee, 18 pp. (1964). The development of planter functional Specifications from measurements of the soil physical environment. American Society of Agricultural Engineers Cultural Practices Equipment Committee: Seminar on Minimum Tillage and Research Methods, 1964, pp. 6-8. Ohio Agricultural Experiment Station, Wooster, Ohio. Brinkman, W. (1963). Vereinzelungsloser Zuckerrflbenanbaua Landtechnische Forschung, MUnchen, Germany, 13(5):128-135. Cummings, N.W. (1929). Relation between evaporation and humidity as deduced quantitatively from rational equations based on thermodynamic and molecular theory. National Research Council Bulletin, 68:47-56. Dexter, S.T.,'and T. Miyamoto. (1959). Accelerationcfi water uptake and germination of sugar beet seedballs by surface coatings of hydrophilic colloids. Agronomy Journal, 51:388-389. Dexter, S.T. (1964). Professor of Crop Science, Michigan State University, East Lansing, Michigan. Personal communication, November. EriCkson, A.E. (1963). Professor of Soil Science, Michigan State University, East Lansing. Personal communication, October. Evers, P—N. (1963a). 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