m: ammo: sm'pmmson THE. A _, MECHANICS or momma owan-mos ' ' * Thesis for tho 099m 0‘ Ph- D MECHTGAN STATE UNIVERSITY ‘ A Leland Overbe Drew ‘ ‘ T963 THESIS (5.9; This is to certify that the thesis entitled THE EFFECT OF SOIL PARAMETERS ON THE MECHANICS 0F EMERGING SEEDLINGS presented bg Leland O . Drew has been accepted towards fulfillment of the requirements for Ph.D. degree in Agricultural Engineering %/W< Date bruar 2 1 6 0-169 Lilith? “a? THE EFFECT OI: SOIL PARAMETERS ON THE MECHANICS ()1: EMERGING SEEDLINGS By LelammltjverbeW'IDrew .-\N .-\BSTR;\C'1‘ ()1: ;\ THESIS Submitted to the School fur AdVanced Graduate Studies of Michigan State Fniversity of Agriculture and Applied Science in partial fulfillment of the requirements for the degree uf DOC'IK R 01" PHILOSOPHY Department uf Agricultural Lngineerine INNS MAM. ” 1”, - AMA/L _% r/ /”é' ffl _\ ABSTRACT THE EEI‘ECT ()1: SOIL PARAMETERS ON THE MECHANICS 0E EMERGING SLEDLINGS By Lelamui<3verbe§'l)rew The difficulty of planting seeds to secure a uniform stand is one of the major deterrents to complete mechani— zation of a number of our row crops today. Variations in seedling emergence are known to be due more to seedling en- vironmental factors than to planter performance. The effects and interactions of yariables such as soil moisture. temper- ature. surface compaction. soil type. etc. create environ— ments whose precise influence on seedling emergence remains undetermined. The purpose of this study was twofold: (l) to investi- gate the effects on Soil strength of yarious soil parameters such as initial moisture content. bulk density (as repre— sented by surface compaction). and final moisture content (as represented by days of drying at TH E.); and (3) to develop a reliable means of measuring seedling thrust. Comparisons were then made between soil strength and seed— ling thrust yalues. and these were checked against the ob— served emergence. Soil strength was measured with a penetrometer instru- mented with strain gages which fed signals into an X—Y recorder. Automatic plots of force ys. displacement were (flwtaincmi as tln: prwnie cmn3rged tflirotmfli soil. contziined ill small plastic boxes. Leland Overbey Drew Seedling strength was determined by two methods which agreed closely with each other: (1) A flea mays (corn) seed- ling was grown in plexiglass blocks with a perforated bottom which was embedded in wetted vermiculite. The seedling shoot grew upward in a glass tube slightly larger than the shoot diameter. As it grew. it pushed upward against the resistance of a cantileyer beam. Strain gages mounted on the beam measured the seedling thrust. This instrument was called the seedling thrust meter. (3) Germinating seeds were placed between two layers of Soil similar to a “sandwich." The upper layer was separately supported on a small beam instrumented with strain gages so that the force transmitted between tin? shoot tip and.tln:'upper soil laiwnr\was measuredq This instrument was called the emergence force meter. The mean of 13 measurements of maximum thrust of Michigan 37b hybrid corn seedlings obtained witn the seed~ ling thrust meter was «.3u7 lbs. with a standard deviation of U,lo5 lbs, The mean of 1H yalues of maximum thrust for the same yariety obtained with the emergence force meter was U.o30 lbs. with a standard deyiation of 0.333 lbsf The two means were not significantly different. The penetrometer studies on Brookston sandy loam were made ufitdi initial scdl unwisture conttnits betuwmni 13 percent and.;Nl percent. \vith surlluxe compactiini pressurmws yarying fron2(>.3 [DST 11) s,() psi. zind ilir drw'ing tinuhs frxmi zerix to ,_ “ threu: dayws. Tknnperwtturt>\vas EH?ld tanstzuit at .é :- Tfm‘ (Lita reyealcmi that: (1) :awil strengt? i1n:reased lirnuirly Leland Overbey Drew with increasing initial moisture content under non—drying conditions. but under drying conditions there was an inter- action between initial soil moisture content and days of dry- ing; (3) soil strength increased linearly wiTh increasing compaction pressure under drying and non—drying conditions: and (3) soil strength decreased with the square of days of drying. The penetrometer studies yielded soil strength values which were consistent with seedling thrust measurements and observations of seedling emergence. In each case where the soil strength (as measured by the penetrometer) was greater than the seedling thrust (as measured by the emergence force meter) the seedling did not emerge. Conversely. when the seedling thrust was equal to or greater than the Soil strength. the seedling did emerge. TTiTE E3431XST‘(JE S()II. I%\RJEHIITIH<fl3tain a uniform stand is one of the major deterrents to complete mechanization of a number of our row crops today. The final stand of plants in a field can vary widely in spite of the fact that a farmer may use seed of known high germination and plant them with a precision planter. Thus. variations in seedling emergence are due more to seedling environmental factors than to planter performance. The effects and interactions of variables. such as soil moisture. temperature. surface compaction. soil type. etc. create environments whose pre— cise influence on seedling emergence remains undetermined. 1f sufficient information were available the farmer could accurately predict the seedling emergence and would plant at the required rate to obtain the correct stands. This would eliminate the costs of thinning or the reduced income re— sulting from insufficient stands. Regression equations can be developed from laboratory studies of the effects of the controllable variables on emergence force and growth. These equations permit one to predict the resistance of the soil to penetration by various seedlings. Similarly. laboratory measurements of the maxi- tnum thrust a seedling can develop will indicate an upper .limit for permissible soil crust strengths. The farmer will [u then strive to build a seedbed whose crust strength does not exceed that of the seedling. As farmers till the same fields year after year. Soil structure may become increasingly difficult to maintain. In fact. one might expect Soil crusts to become increasingly stronger with time in most soils; and. eventually. plant breeders may be forced to select plants with greater seed— ling strength. To use seedling strength as a criterion. the plant breeder needs a reliable method of measuring the thrust which a seedling can generate. The purpose of this study is to: (1) demonstrate that multiple regression can be useful in predicting the strength of a soil crust. using pertinent variables. and (3) develop ' a reliable means of measuring seedling strength. REVIEW OF LITERATURE The information concerning soil strength and seedling emergence is extensive. but the writer has found no reference which describes direct measurements of seedling emergence strength. A brief survey of various aSpects of the problem is presented here. Mechanical Properties of Plant Materials‘ Investigators have studied the mechanical strengths of forage materials and the rheological properties of agricul— tural produce. such as potatoes. apples. small grains. etc. McClelland and Spielrein (1057). using strain—gage equipment and an oscilloscope camera. measured the ultimate bending strengths of three common pasture plants (alfalfa. ryegrass. and Algerian oats). They developed linear regression equations expressing the ultimate bending strength as a function of the linear density (gms./cm.)-of the plant stem. They reported that the materials tested. although biological in character. obeyed established laws of mechanical behavior. Prhwe (onl) tested green and oven—dried alfalfa. timothy. arml<>ats iriljendirn; and ztlfalfa.i11 IOISltHl and (flitaincmi re; sults similar to the above findings. Mohsenin et al. (IUol. on3) investigated rheological PrWiperties. such as modulus of elasticity. the force. I J‘J deformation. and energy to yield. elastic recovery. plastic— ity. etc. for fruits and vegetables. They proposed a rheo— logical model which appears to simulate qualitatively the observed long—time effects of loading and unloading of agricultural products. such as the apple fruit. Work is being continued to determine whether or not the model will duplicate quantitatively dead load effects with reaSonable accuracy. In studies of grains. Zoerb and Hall (onO) determined some basic mechanical and rheological properties of individual bean. corn. and wheat kernels. They found that moisture content had the greatest influence on the strength properties of grain. A two—term exponential equation was obtained to express the stress—relaxation—time relationship. The mechanical properties of living plants have not been studied as much as the mechanical and rheological properties of agricultural produce and forage. Gill and Bolt (1050). reviewed the work of Pfeffer. an early German plant physiologist. and stated that he found Some plants could exert root grow h pressures of lo atmoSpheres in an axial direction and o atmospheres in a lateral direction. Gill and Miller (1050) measured the rate of growtr of seed— ling roots in small growth chambers under varying degrees of physical restraint and oxygen content. They found that roots of corn seedlings grew slower under decreased oxygen supply and with increased external resistance. While a number of retmorts state that soil impedance reduced root growth. very 'few quantitatiye measurements of seedling root or shoot thrust are given. Penetrtme:ers The use of penetrometers to measure :Uil mechanical— impedance seems to be controversial. Taylor {1v4*) stated that the resistance to penetration of a needle of a given diameter into a soil was a function of the Soil density and water content. Bayer (1050) reported the findings of several investigators. Some of these studies of Ohio Soils indicated that soil moisture was the dominant factor influencing penetrometer readings. although no simple relationship between the two could be found. In a plowed layer the zone of maximum compaction was found to be near the surface and it moved nearer the surface as the number of tillage oper— ations increased. It was concluded that even though penetro— meter readings cannot be interpreted practically in terms of Specific soil properties. the penetrometer is a useful tool as an aid in Soil diagnosis. ‘ . The simpler types of penetrometers are driven into the Soil by freely—falling weights. or the penetrometer needle itself is allowed to fall freely. using its kinetic energy to penetrate the soil. 'Stone and Williams (INSU) described an instrument of this type. Hanks and Harkness (1050) gave details of a Strain gage soil penetrometer (needle size) similar in diameter to that of a wheat seedling. Gill (105“) stated that a mechanical probe. such as a Soil penetrometer. ”can never be used satisfactorily to determine threshhold soil resistances above which roots will not penetrate. since it does not consider soil aeration." Morton (1050) devised a soil penetrometer which he called a mechanical seedling. Small (5“ x 7") plastic boxes containing soil of different moisture contents.compacted by various surface pressures were used to simulate a variety of field conditions. Each soil—box had small holes drilled in its bottom. When placed on a platform which could be lowered slowly. the probe entered through the hole in the box and moved upward through the soil and,surface crust. The probe diameter was the same as that of a seedling being studied. and the probe stem diameter was slightly smaller than the tip diameter to minimize sliding friction. The probe was mounted on a small. simply-supported beam instrumented with strain gages. DiSplacement was measured through strain gages mounted on a cantilever beam whose free end followed the movement of the platform. I'sing oscillograph equipment. Morton obtained plots of force on the probe (emergence force) vs. diSplacement. from which he could make determinations of the emergence force and energy required for his hypothetical seedliqu. [unong the concltuyhnis reached tn Nhyrton were: 1. The energy required for emergence increased directly with compaction pressure. initial soil nnyisture C(HILGHL. annount (d7 soil Egirface drying. and indirectly with moisture content zit time (Milneasurtmunit. \ 3. Under conditions of no drying. the mechan cal strength of the soil surface increased only slightly with aging. 3. Development of mechanical strength in the sur— face layer of a Brookston sandy loam is de— pendent upon compaction pressure and Soil moisture content. 4. Energy required for emergence increases with the probe diameter. Since the final depth of seeds planted initially at the same depth does not vary greatly for the normal ranges of soil moisture contents and surface compactions. we can assume that emergence forces can be inserted in th above conclusions wherever emergence energy is used. Shinaishin (10nd) studied the relationship between the performance of Morton's penetrometer and the emergence of actual plant seedlings. He found that there was a definite relation between the emergence rate and final stand of seedlings and the emergence force required to penetrate the Soil with the penetrometer probe. He stated alSo that ”the effect ofi moisture on seedling emergence was found to be more than simply the influence of water on emergence force of a mechanical seedling.” Soil Crust Strength Formation of surface crusts has been recognized for many years as a hazard to field emergence of plants. (arnes (1034) stated that the crusts appeared to be produced by the infiltration of colloids and later cementation of soil particles. He found the breaking strength of a crust. formed under a given set of conditions. to bear an inverse relation— ship to the amount of moisture in the crust at the time of breaking. In addition. he found the chemical nature of a soil affected its breaking strength and the strength of crusts (as measured by the modulus of rupture) to be greater in cotton middles than on ridges. Richards (1053) recommended the use of the modulus of rupture as an index of Soil crust strength. A rectangular beam of Soil crust (l x 3.5 x 7 cm.) is loaded as a simple supported beam with a concentrated load at its center. The maximum fiber stress is calculated and designated.as the nuxhalus of rupture. 3F 5: L) ltxi“ hhere: .: L = T cm b = 3.5 cm d = 1 cm F = breaking force in dynes Thus. S is computed as dynes per square centimeter. A milli— bar is designated as 1.00U dynes per square centimeter. and one millibar equals 0.0145 psi. Richards found that for one soil tested. an increase in crust strength from lbs to 373 Inillibars decreased the emergence of bean seedlings from 1th percent to 0 percent. O Lemos and Lutz (1037) found that natural soil crusts had a much greater bulk density. a higher percentage of particles smaller than 0.10 mm. in diameter. and a lower degree of aggregation than the underlying soil. They ascertained also that they could prepare artificial crusts or briquets that had essentially the same bulk densities as naturally occurring soil crusts. Hanks and Thorpe (loso. 1057) prepared artificial Soil crusts of known strength and combined various crust strengths witti varyiruz soil nn>istur13 contcnits. 'liey fkiund tfiuat Ilir a given crust strength the seedling emergence was lowest where the soil moisture content was lowest. At Constant soil moisture content. seedling emergence decreased with increas— ing crust strength for the plants and Soils studied. They discovered no evidence that seed Spacing and crust thickness had any effect on emergence. but Soil moisture and crust strength greatly affected emergence. However. they did find that the force required to break a l/Q—inch crust averaged 1.83 times greater than the force required to break a 3/H— inch crust (theoretically it should have been 1.77). Seed Spacings of one and two inches were compared for wheat. Drying of the surface layer of a soil seems to contri— bute more to the strength of a Soil than does aging at low nniisture*ihisses. (Till (lkhfl’) {(HHNj that \flten Elimii clay \wis dried slowly over a period of 340 days. the final soil strength was only about one—fifth that of the same soil dried It) a lver t1 peritui of tnily tJiree htHlTS. 10 Seed Environmental Factors A variety of factors affect the Speed and completeness of germination. Hagan (1053) listed temperature. moisture. oxygen. light. carbon dioxide. Soil pH. mineral elements. and activities of microorganisms. When emergence is considered also. Stout (1050) stated that in addition to the above list of factors. the following must be added: depth of planting. mechanical impedance of the soil. emergence force exerted_by the seedling. and disease. Most investigators seem to favor mechanical impedance. soil moisture content. temperature. and oxygen content of the Soil air as being of greatest import— ance in seedling emergence studies. Usually temperature is held constant. and frequently the effects of oxygen and carbon dioxide are disregarded. Stout et al. (1000). working with decorticated seed— balls of sugar beet. found that compacting a Soil did not materially affect the moisture absorption rate of the seed— balls under non—drying conditions. Bowen (lUnU). in studying the important variables af— fecting seedling emergence. used small in inch by 3H inch plots Containing all combinations of three initial soil moisture levels and three surface compactions in a balanced lattice design. After putting the plots outdoors the four variables measured were: temperature. with thermocouples: Soil moisture. with gypsum blocks: aeration (air permeability). using a permeator: and Soil physical impedance wit? inflat— able buried balloons. A iypodermic syringe with attached ,. J, ll hydraulic pressure gage was used to inflate tre buried balloons sufficiently to crack the soil surface. The follow— ing conclusions were drawn from his study: 1. Tfiie atir' pernnezfl»il.ity' of- linuny' sarwl s<>il 21ftt>r planting operations increased as the moisture content of the soil at planting time increased. a. The air permeability of this Soil decreased as ttw? surfatxa compactiini was i1u:reased. 3. Physical impedance generally increased as the Soil dried. 4. Physical impedance immediately after planting increased with the moisture content of the soil at planting. a. Physical impedance generally increased with increased surface compaction, It may be noted that these conclusions confirm the findings Morton. Hanks and Thorpe. Carnes. Lemos. and Lutz. whose work was confined mostly to laboratory studies. Soil Strength Theory Semi—empirical relationships. such as the Coulomb— Mohr equation (7': f + fl‘tan d). modified Boussinesq eqtuttitui. lierwtstt?in'55 sirfl forhnila. .1nd I rtuelic?='s equatitNizlre used in Soil nuwfluutics. but none (d' U ese re— late soil stress to soil strain or deformation in a com lete1\ satisfactory manner. According to Vandenberg P . . . (onI). "theories of elasticity and plasticity ta\e been 12 applied to the soil and in Some cases the applications are quite successful. In general. however. neither theory repre— sents the facts closely enough to provide usable results.” He further stated that Soil mechanics. which implies deforma— tions and applied forces. can never predict plant growth. At present. only two factors which affect plant growth appear to be related to soil mechanics. First. macropore Space can probably be predicted and controlled during tillage operations by utilizing Soil mechanics and. secondly. the range of ex— pected mechanical impedance can probably be predicted in terms of Soil Strength, VandenBerg also discussed the need for a plant mechanics which relates plant growth to de— formations and strengths of soils. saying that Simultaneous consideration of both mechanics would then permit maximizing [UJHTL growth f1n*1xirticular enyfiixnunental Ctnulitions. Iiarris. \QuulenBergzcat al. (lflhil. 1053. lkhnl) applied. the theory of the mechanics of a continuous medium to attempt to find satisfactory stress—strain relationships. Harris obtained some degree of success in correlating maximum Shear Stress with changes in bulk density. He could neither accept IDrreject (at the “3} confidence level) the hypothesis that mean normal stress is related to changes in bulk density. VandenBerg suggested the hypothesis that compaction is a function of mean normal Stress plus shearing strain. Bekker (on1) suggested the use of a modification of the lhernsteirtlatrmultt.for time load \(flfSUS sfizfltage x13lation— ship wherein: 1. I) 7— f: + k¢)1i11 h 13 Where: P 2 load A = sinkage kC = cohesive modulus of deformation k¢ = frictional modulus of deformation b 2 smaller dimension of the loading area n = experimentally determined exponent He stated that this was a good practical stress-Strain re~ lation for a Soil in Sinkage problems. OBJECTIVES AND METHODS OF APPROACH TO THE PROBLEM The overall objective of this study was to establish a reliable means of measuring the emergence force or thrust which a seedling can develop, Specifically. the maximum thrust of a newly—germinated seedling was measured as it grew in an enclosed. rigid Space in the absence of nutrients. The thrust developed in this manner was then compared with the actual emergence force developed between a seedling tip and the Soil crust in which it grew. The Soil surface strength (as measured by a penetro- meter) was Studied as a function of surface compaction. initial Soil moisture content. and days of drying of the Soil. Comparisons between Soil Strength and seedling Strength were then made. 14 METHODS 01‘ PROCEDURE AND APPARATUS USED Three different methods of approach were used to deter— mine the mechanical strength of a seedling. The fact that the results of one method substantiate or corroborate the results of another approach was accepted as proof that seed— ling strength truly was being measured. Penetrometer Studies The mechanical seedling developed by Morton (lUSU) and described briefly in the review of literature is illustrated in Figure l. The mechanical features of this penetrometer have not been changed. but an X — Y recorder was used to directly plot force—diSplacement diagrams. Figure 1 shows a six—volt battery supplyirngxnwer for the X—axis and two six— volt batteries with a voltage dividing rheostat being used to supply approximately ten yolts to power the Y—axis. This arrangement permits scales of l“ = 1/3“ on the X—axis (dis— placement) and l" = 1/3 lb. on the Y—axis (force). The use of calibration resistors permits rapid checking and adjust— ment of calibration when deemed necessary. For forces in excess of one pound. the ‘i—axis calibration was usually changed to l” = 1 1b. Following the lead of Morton (lUSU). the probe diameter was taken as U.UTS inches. and the plat— form was operated at jJASnHNQUMMn Speed of about 3.3 inches 13 16 .UmmmSOH mfl wfl mm xom map mo Sobwon 0:9 nwsohnp wheycw enoum one can ..EHOdeHQ UHbmmH mo OHV 2 wk . . *p . ‘ \x .m mixq x C zo:.aa2_m & ..;,.,: N::..l\> at \:\<\~\<> Vxx.\\< «mm mm>wihz HWHW <0 =mno,o, mmoma isl . .‘4 . l 17 per minute. (Morton ran several penentrometer tests at different Speeds. finding no differences among probe forces attributable to speed; therefore. he concluded that operation at the maximum Speed would be satisfactory.) For the penetrometer Studies the independent variables were as follows: (1) initial soil moisture content (% D.B.). (3) surface compaction pressure (psi). and (3) days of dry— ing. The dependent variables were force on probe tip (emergence force) and seedling growth. Seedling growth was measured as vertical growth above seed and as total length of shoot. The effect of Soil impedance on seedling emergence can be noted when vertical growth is expressed as a pro— portion of the total Shoot growth. This ratio is designated here as the Emergence Permittance. Since the greater its value. the less the compaction effect and the more rapid the emergence. Examination of the data in Table VI of the Appendix Shows this effect at the higher surface compaction values. ' . The particular values used for independent variables were as follows: Soil moisture content — 133. lo}. 305 Surface compaction — 0.3. 3. 4. S psi [Days (fifcjrying -(). l. 3. .3 For a given condition of drying there are twel\e treatment combinations of pressure and moisture content. Probe and growth readings were taken over a four—day inter— val just prior to seedling emergence. Michigan 37H hybrid T’— ls corn with germination percentage greater than 05 percent was planted at a one—inch depth. The expected time of emergence had been determined from preliminary studies. Three repli— cations of probe readings for each treatment were taken daily. and eight seeds were dug up daily from each treatment to obtain an average value for growth. Although seeds were planted in one set of boxes and probe readings taken from another set. the soil used was from a common Source. the boxes received the same compaction treatments. and all were stored in a constant temperature room of TUOF, Boxes were subjected to the Specified number of days of drying immedi— ately prior to the first day of readings. Lids were kept on once readings began. and no further drying of the Soil in the boxes occurred. AS pointed out by Morton (1050) and others. aging without drying had little effect. if any. on Soil strength: and no relation between emergence force and days after planting could be noted. To illustrate the above procedure. Suppose a soil of lo percent moisture content is compacted to 4 psi surface pressure and most of the seedlings are supposed to emerge from the one—inch depth in eight days. If the soil iS to re- ceive tun>4 were the seedling thrust meters used in this Study. Emergence Force StudieS——the Soil “Sandwich” Method I lln: acttuil ctnitact llwrce luatwetni Sh(H)t tij) anti Soil 5‘ if. II (HIV. ...r. I? (\I 33 crust could be measured using the apparatus Shown in Figures 0 and T. The Soil crust (in the upper unit) could be made with any combination of soil moisture content and surface compaction. The mechanical strength of the Soil in the lower unit could be varied also. When a piece of cheesecloth was inserted between the upper and lower units as they were being filled with loose soil. and both units were compressed Simultaneously from the top surface. the natural seedling Soil environment was simulated. After the soil was compacted. the cheesecloth was bound to the sides of the upper unit with rubber bands. and the top layer of the ”sandwich” was lifted off. The germinating seed was placed between the layers and the top layer positioned vertically.until it just contacted the seed. The initial reading was then made on the Strain indicator. AS the Shoot tip moved upward its thrust effect against the soil crust was signalled through the strain gages to the Strain indicator and the readings were compared with the initial reading. The units were calibrated initi— ally for loads between 0 and 1.35 pounds. Force vs. indi— cator readings were linear plots for all units calibrated. Units 1. 3. 3. and 4 (Figure o) utilized a fairly stiff cantilever beam of stainless steel with a sensitivity as a force transducer of approximately o,o33 pounds per tHllL reading of tin: strain indicator. lkunn .on Eden waHOQQSmI>HQEfim Map :0 Umwcsoe one ADV mwmww :aduwm Amv was: ©0D#00u Hmzoa paw Amafiacmo esp so nov mwmwm :fimuwm .meMU moan: paw #HOQQSm mpsan Hmnnsu one spoHowmwmso .Amv HmSuo mozoa 05w pad Arcchxlef::o oops; oocomooE; .E_ ;;:<% 3 3 and emergence force. This conclusion for non—drying condi— tions agrees with observations of Gill (1050). Morton (1050). and Bowen (1000). The mean values of emergence force were multiplied by their correSponding conversion factors and are shown in Table 111. These values represent the Soil impedance encountered by a seedling emerging from a one— inch depth. TABLE 111. Expected seedling emergence forces obtained from converted values of probe readings. Experiment A. No soil drying. Expected Seedling Emergence IMirce Surface Compaction 13‘; Soil .\I.(7. 1035 Soil .\I.(‘., 30.) Soil .\I.C. ().3 135i ().0]_ lIDS. (I.(Ll llJS. (l.07' lIDS. 3.0 0.07 0.15 0.33 4.0 (),lo ()_31 O.—IO 8.0 0.30 0.07 0.”.3 Experiment B. 1 day of drying. Expected Seedling Emergence Force Surface Compaction 13:) Soil .‘xI.(,‘. 10% Soil .\I.(.‘. 3t)"J Soil .‘»I.(.‘. 0.5 psi 0.01 lbs. 0.07 lbs. 0.30 lbs. .3 .I) () .()'T (I ..i'7 I) .‘3 1 4.(l 0.33 (l.Sl 1.30 8.0 0.38 1.37 3.03 TABL E II I (continued) leraeiriment C. 3 days of drying. Expected Seedling Emergence Ifiirce Surface Compaction 13. Soil .\'I.(‘. 10"; Soil .\I.(‘_ 30‘} Soil .\I.C. 0.3 psi 0.01 lbs. 0.03 lbs. 0.37 lbs. .2.() 0.()5 0..33 1 .37 4.0 0.11 0.57 3.71 8.0 0.30 1.43 4.40 Experiment D. 3 days of drying. Expected Seedling Emergence Force Surface Composition 13 1 Soil M.(‘. lo“; Soil .\I.(‘. 30% Soil .\I.(‘,, 0.5 psi 0.01 lbs. 0.03 lbs. 0.35 lbs. 3.0 0.04 0.13 0.01 4.0 0.13 0.47 1.0.0 8.0 0.31 1.15 3.30 Prediction equations for seedling emergence force Two kinds of multiple regression analyses were made to IANain equations for predicting the maximum force on the pmwdrometer probe emerging from seed depth to soil surface. 0m3expression was amnher expression was allowed to dry develcwxxl for rnni-dryiju; conditixnis. and was obtained for conditions where the Soil 0. at T0 F. The data of Experiment A. Table ‘lL were analyzed to Secure a Simple multiple linear re- gressiorr eculat.ior1 or- ttne flirm: \ . _ I 1\ ‘I‘ IUle T 03X: wh e r e : 37 Y = maximum expected emergence force. lbs. X1 = initial soil moisture content. 3 D.B. x. 2 surface compaction pressure. psi 'Tlie> exquation obtained from the twelve sets of observations in Experiment A is: (a) Y : —0.5n + 0.0344 X1 + 0.0803 X) Idle coefficient of multiple correlation was 0.037 and the stcuadard error of estimate was 0 10%. Lkfllatlx)n (.1) Inay' be tisexj for predicting soil impedance (strength) for non—drying CL3nclitionS. It was used to compute the values of soil Strength 1J1 Column 7 of Table V. Its usefulness in extrapolating to prwessures. such as 10 and 30 psi. however. is questionable. In the second regression analysis the effect of drying the soil at 700I. was introduced. With this third variable. X;. introduced. the 43 sets of observations in Table III were analyzed to obtain the multiple linear regression equation: Y = —3,31 + 0.144 X1 + 0.174 X) + 0.148 X. ———— (3) I—J .— The multiple correlation coefficient was 0..3 . and the standard error of estimate was 0.55 pounds. The coefficient Id X3 did not fall in the region of significance. its t value was 1.07 while t ‘ ’ 5—1 .03 is required for Significance an the 5 percent level. From Equation (3) one must conclude (fither that days of drying was not a Significant factor af— fwming the emergence force of the probe or that the effect midrfing on the emergence force was non—linear. figures 14. 13. EUld IA) werwe pltrtted Ix) shvnv that 'the cnifect \vas TRa) one asterisk denotes Significance at the 3» level. and tmiasterisks denote signiticance at the 15 level. I38 l2 °/o INITIAL SOIL MC. :15 B 0.6 I . LL] (4) ‘5 LL 0.5 ‘3 8.0 PSI. E o 0.4.. 0: DJ 2 DJ 9 0.3 . 2! '_'I a: . m 0.2 - (D 4.0 PSI. C) m .45 5 0| 2 ' u} 353i .(3 F’SI 0. 1v x TR 4 m 0 50 5 PSI J o l 2 3 DAYS OF DRYINQ AT 70° E. Fig. 14. Curvilinear relationship observed between dry— ing and converted penetrometer force (expected seedl1ng emergence force). EXPECTED SEEDLING EMERGENCE FORCE-LBS. Fig. 15. 39 I6% INITIAL SOIL M.C. 0.5.. ° 2.0 PSI. $0.5 PSI. ' 0 I : if :3— I 2 3 DAYS OF DRYING AT 7Q°_E_L_ Curvilinear relationship observed between dry— ing and converted penetrometer force (expected seedling emergence force). 4O 20% INITIAL SOIL M.C. (0 GD .1 4.0... I uJ L) ‘5 LL. 8.0 PSI. DJ 0 3.0.- 2 uJ o 0 0: DJ E 2.0.. . 0 4.0 PSI. .2. J O Q UJ UJ . ' 00 Q to 2.0 PSI. . LIJ o S LIJ $0.5 PSI. 0- AIL— L x ~ ' fi' ‘5 O I - 2 3 DAYS OF DRYING AT 70° F. Fig. 10. Curvilinear relationship observed between dry— ing and converted penetrometer force (expected seedling emergence force). 41 curvilinear. These curves alSo indicated that an interaction existed between X1 (soil moisture) and KK (days of drying). K Since the Soils of 13 percent and 10 percent moisture content reached their peak Strengths after one day of drying. while the Soil of 30 percent moisture content reached its peak Strength after two days of drying. z\n €(1Uilt‘ltnl (If‘ tIie fIIrnI: ) Y Z .\ 'I' Illiyl ‘I' IDEA; ‘I' I33.‘\'3 'I' 134x; + I‘SXliy‘g was programmed for the Michigan State University computer. MISTIC. and the following equation was obtained: Y Z -1.31 t 0.0333 X1 t 0.174 X) — 0,371 X — ) 0.10o X37 + 0.0573 \1X3 —————————————————————— (3) The coefficient of X1 was slightly below the significant level (t = 1.31 compared with t = 3.03 at the 33 level). The Coefficient of Ky was non—significant. but the coefficient of v3, . ., . . --. . x% was Significant. The multiple Correlation coeI11C1ent was 0.S38 and the standard error of estimate was 0.33 pound. It was next decided to delete the linear term contain— ingi;3 becauseIIn‘ its low levcd Inf Significanccn lult the linear term with x1 was not deleted for fear of reducing the significance level of the multiple correlation coefficient. The final multiple regression equation obtained was: v = ~l.ol T 0.1731 \1 + v.174 x3 — II.I_‘I—I \{37 T I'.II—IDII \1\3 ————————————————————— I—I) The multiple correlation Coefficient was 0.330 . and the ) SIJIMIJFLI errnir (H- CSIIIHJIL‘IYJS I-.5.. pntnhj. 'Tfie t.o3 lbs. 0.78 0.48 0.00 (L03 0.41 0.30 0.45 . 0.40 0.70 0.51 Sum: 4.30 _ 3 77 _ Mean: 0.037 3 X1 0,554 = \) Variance: 0.03800 0.01308 7 Standard Deviation: 0.107 0.113 InSpection of the values in Table IV shows a low value of 0.40 pound and a high of 0.00 pound which indicates that considerable differences may exist among individual seed— lings. Since the instruments could sense force differences of 11955 tflian t).0l Imyund. .it is Inelie\wxl that 'Hfie ctmnvined errors of instrument and reading could not exceed a few hundredths of a pound. Therefore. the differences among seedling thrusts must be due to some internal factors. such as genetic traits Seedling emergence force determinations Units 1 through 4 and 0 through 13 were used with a variety'cm_(unnbinatdxnis of Soil.cxnujitions for lKM>tb€d and surface crust. A number of failures occurred in earl: tests because of the failure of seedling shoot tips to penetrate hard surface crusts. A shoot tip usually took the pat“ of r I 45 ' least resistance—~growing horizontally to the edge of the unit and then attempting to grow vertically. It grew either between the surface crust and the inner edge of its container or it grew completely outside the upper unit. This problem was eventually solved by inserting l/3—inch sections of glass tubing in the upper Soil layer prior to compaction or by compacting the upper layer on a mold having a vertical cone l/4—inch in diameter tapering to a point l/J—inch above the base. This latter innovation seemed to work well. It was easy to prepare the soil crusts. and in all cases where it was used the seedlings were Y'duped" into growing into the region formed by the apex of the Cone from whence they had no other choice than to push upward. Units 1 through 4 were judged to be too rigid for best sensitivity. Combined errors of reading and sensitivity C(HJld aflMHMlt t1) as nnufla as ().05 txaund. Thaits L) thrtuqyi 13 were sufficiently sensitive. but during the last test run there was a calibration shift on Unit 0. However. the new calibration curve for this unit still was linear. All of the Soil sandwich and emergence force meters were read manually three times daily. Readings were usually discontinued after seedlings had emerged. Plots of force vs. days have shapes similar initially to hose obtained with the mechanical seedling. Values of maximum emergence forces obtained are presented in Column 4 of Table V. CompariSon of the Three Methods ' Table V lists the pertinent factors involved in the 33 40 cases of seedling emergence forces studied.4 As noted. most of these tests were conducted at the medium Soil moisture level of lo percent. The soil crusts were formed by compress— ing one inch of loose soil in the first 11 cases and 1.5 inches of loose soil for the remaining cases. In examining the values in this table One should note that the mean seed— ling strength as measured by the thrust meter in the absence of soil.\was 0.00 rxnnul. ’The descrdiytions of time various column headings are as follows: [:1 Initial Soil moisture content. percent dry basis. Column 3. Surface compaction pressure. psi. 4. Maximum force transmitted between seedling shoot tip and soil crust. as measured by the emergence force meter. 3. Converted penetrometer probe values from Table III. Experiment A. 0. Direct penetrometer probe readings of . crust strength. The crusts were made up lJl the ttq) SBCLItHlS of [Wiits 1 through 4 and l‘nits 0 through 1.3 in the same.manner as when they were used withoactual seedlings. .. Computed emergence force. using Equation (1). 8, Indicates whether or not emergence occurred tor the actual seedling under test. In Table V’tnue shoulcltflaserve thal.(fl>1umn (—l) repre— scnits ttwa strwnigtli of. the EMRlelJlg. \fliilt?(foltmnis (L3). (.w), and (7) represent the strength of the Soil crust. determined in three different ways. When a value in Column <4) equals or exceeds those in Columns (3). (o). and (7). emergence 47 should be noted. This is true in most of the cases where emergence occurred. Discrepancies exist in cases 18 and 10. but there is an explanation for case 18. since a crack \ developed in the soil crust as the unit was being set up. Several smaller differences exist. but they are within the limits of experimental error. In cases 17 through 33 the seedlings seemed unable to develop their maximum thrusts. A possible reaSon for this behavior is that seedlings of approximately 3/8—inch shoot length were used: whereas. in cases 13 through lo. germinat— ing seeds were used. Perhaps the germinating seeds had sufficient time to establish their root sy’stems before they were required to develop their maximum thrust. The 3/R—inch seedlings. on the other hand. had their shoots in a rapid— growtl: stage and pushed against the soil crust before their root systems could become well established. In examining the values in Column (4). Table V. one could assume that seedlings were exerting their maximum thrusts in those cases where the seedlings did not emerge. There are 10 cases of non—emergence: the mean value for 'seedling thrust for these cases is 0.n50 pound. The standard deviation is 0.333 pound and the range of values is from (‘HJU pound [(3:1“30 pounds. Tltis mean \altm?llxr seedling tlirust 'hi acttutl Soil is H(H. signitflicantly'(lifferwnit frtmi tflte \ttlue (d ().3“7 [xiund (untaituxj witli the sumxlling titrust me 1.01” . :. N UbLaCCh my» Hr.1: mu.,: fix.x: mm.l: :.4. smzc.: 2: N» C . M 33.; #3:; my» Hm.l: mm.,: 4%.: “M11: 1:.1 smze.: :_ _w 3.7 CLLTIC :7. «2.: 3:: ii: .51: :../_ 3.5:: i E . 3.”. UruLdCCL . may” w:.h: mm.,: 02.1: r3..: :./a smzc.: ,_~ :_ C..n UDLTISL mu» m:.: ex.: it em.: :.¢H smse; 2: y. 3.4. GQLHSCL 3% m:.: fir.: I: :N.: 1:.N_ smzc,: :4 z; : . T. 13.; TI...» 1:2 m1..: fir.x: I- #w.,: .:.v_ :w:».: r:_ :3 C . .3 ULLaSCL :7 m.3.3 #7.: in :71: 3...; #5:; r; . v; . 3 . 7 332le 3/ i- mw.fi 1. mo.: _:;.v omSo; ,; 34 1. m. 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SIEDLARI' The mechanics of seedling emergence was studied using three different methods of approach. These were as follows: 1. Multiple regression studies of the effects of three independent variables: (1) initial soil moisture content. (3) surface compaction pressure. and (3) days of Soil drying at 70oF. on the de— pendent variable (expected emergence force of a seedling). The emergence force was taken as the force required for a penetrometer probe to emerge through the soil from seed depth to surface. The probe diameter was the same as that of the shoot tip of an average corn seedling. A regression study of vertical shoot growth in terms of Soil moisture content. surface compaction pressure. and days after planting was alSo conducted. Measurements of the maximum thrusts developed by placed seedlings/in rigid plaSLic blocks with the shoots growing vertically in glass tubes. The shoot tips pushed upward against the resistance of deflecting txuttilever beahnstfliica were iiuytrumenttxitvith eltctric resistance strain gages. The perforated bottoms of the plastic block containers were em— bedded in wetted \ermiculite so that LLB Seedlings Jtl ‘0 So grew in the absence of external nutrients. These instruments were called seedling thrust meters. 3. Soil “sandwich“ units constructed So that seeds or seedlings could be placed between two layers of Soil. Various combinatitwnscif Soil moisture con- tent and surface compaction pressures were used in preparing the soil crust (upper unit) and the root- bed (lower unit). The force developed between a Soil crust and a seedling shoot tip was sensed by electric resistance strain gages and recorded as the seedling emergence force. For 13 seedlings of Michigan 370 hybrid corn the mean value of maximum thrust as measured by the seedling thrust meters was 0.00 pound with a standard deviation of 0.17 pound. The range of seedling thrusts measured was from 0.40 pound to 0.00 pound. This range of values agreed well with emergence forces measured by the soil ”sandwich“ and emergence force units. hmergence occurred in those instances where the seedling emergence force equalled or exceeded the penetro— meter measurement or computed values of soil impedance. ‘ Regression studies indicated that the effect of initial Soil moisture cOntent on seedling emergence force was linear under non—drying conditions: the higher the initial moisture content. the greater the mechanical strength of the Soil. thuler diw ing ctnulititnis thewxaivas to] IDTTPFaCtltHl betufmui initizil soil nuiisture anulll INt)lSlllrc? ct>nt<3nt — siini I ICEUIL at the 5 percent level I 4| V 0. 53 ' Surface compaction pressure - significant at the 10 percent level Seedlings can grow and develop maximum thrusts in rigid plexiglass blocks and glass tubes as long as the roots have access to moisture and oxygen. The thrusts of these seedlings can be measured with the seedling thrust meter or a modification of the instrument. depending on seedling‘dimensions and shapes. The mean of 13 measurements of maximum thrust of Michigan 370 hybrid corn (using the seedling thrust meter) was 0.507 pound with a standard deviation of 0.105 pound. The range of thrust measured was .frtnn ().4() ptuincl 1t) 0."o 190tnid. The mean of 10 emergence force measurements (in soil) in which the seedlings developed their maxi— mum thrusts (no emergence occurred) was 0.o50 poutwl witli a stzuujard cheviatixni of (1.335 [xiundq The range of thrusts measured was from 0.30 pound ti) l..30 tx>uruls. There was no significant difference between the mean value of seedling thrust obtained with the seedling thrust meter and that obtained with the emergence force and/or soil ”sandwich“ unit. Since no soil is required and the preparation for a test is simpler. the seedling thrust meter is the preferred method of measuring the seedlin= thrust. 54 0. High initial soil moisture levels (within the till— able range) are conducive to increased soil strength after compaction. but the high moisture level en— courages rapid seedling growth which tends to more than offset the harmful effects of the compaction. Ideally’. Soils tfl-illgh nuxisture (unitent :fiuauld provide the most rapid seedling emergence if sur— face compaction is kept to a minimum value. say 0.5 psi. during planting of the seed. l0. Penetrometer force—diSplacement plots. when pro— perly interpreted. can be used to closely approxi— mate seedling emergence forces. cu . ~._- . O ' RECOMMENDATIONS FOR FUTURE INVESTIGATIONS More penetrometer studies are needed to verify the curvilinear relationship between the drying time of Soil and the seedling emergence force. This Should be done using the same soil depth and range of compaction pressures used in this study. The Bevameter. a ring—Shear instrument. Could be used to determine whether the shearing strength and co- hesion of the Soil decrease with the square of the drying time as does resistance to penetration of a probe. This method should be an independent Check to substantiate the relationship. Additional work with the soil “sandwich“ and emergence force meters is needed. The instrumentation Could be im— proved by powering the strain gages with high—capacity batteries and feeding bridge outputs into a recording potentiometer. As many as lo emergence force meters could be operated simultaneously with one lo—point recording potentiometer. The lateral rigidity of the emergence force meters should be increased and perhaps diaphragms with at— tached circular foil strain gages should be used as force transducers rather than the simply—supported beams. 'The seedling thrust meter should be adapted to the thrust measurements of Some other seeds. such as Soybeans. sugar beets. small grains. etc.. and its versatility and .1] 2)] J O ‘usefulness observed. Plant scientists could use the instru— Inent to test for seedling strength differences among yarieties of a given Species. Effects of light. temperature. moisture. and nutrients on the emergence force of a given variety could be determined. A mathematical model that represents actual turgor pressures in roots and shoot of a seedling should be developed. :\1nechanical model employing variable hydraulic pressures should be constructed and its performance studied in an actual or an artificial Soil. The mechanical model might be perhaps 10 or more times larger than actual seedling size. but through the principles of dimensional analysis the true seedling thrust could be predicted. An instrument for field testing of Soil crust strengths which uses an electromagnet to lift steel balls from seed depth could be devised. For a given distance between electro— magnet and steel ball. the attractive force is a function of the current through the coil of the magnet. The current magnitude Could be varied by means of a rheostat which could be calibrated to read in force units for a given size of steel ball. An alternative is to use a strong permanent magnet of some material like Alnico. for example. and vary the distance between magnet and steel ball. The force be— tween magnet and steel would vary inversely with the square of he distance separating the two and a calibration curve could be prepared. A load cell of approximately 3/4—inch diameter should ‘1 'Jl be developed. One of these could be planted directly above a seed and used to measure the thrust the seedling would generate before it grows horizontally to bypass the obstruc- tion. A load cell of this type could indicate residual stresses existing in the seedbed after the compacting agent has been removed. This would permit study of germination rate as affected by residual soil stresses. IKLFERIDKSES Baver. L. D. (1050). Soil Physics. Third ed. John Wiley and Sons. pp. 387—338. Bekker. M. G. (onl). Mechanical properties of soil and problems of compaction, Trans. ASAE. 4:3:331-334. Bowen. H. D. (lune). Some physical impedance and aeration effects on planted seeds. Presented as paper No. no— 030 at the ASAh winter meeting. Carnes. A. (1034). Soil crusts. Agr. Eng. lS:loT~lTl. Gill. \V.11. (1U5U). fi>il compactiini‘by traffic. .Mgr. Eng. 4(T:3Lh3—3ikl. (105”). The effects of drying on the mechanical strwntgth pd I;loyd (:lays. Shiil Sci.. Soc, :Mner. I?roc, 33:353—357. and G. H. Bolt (1030). Pfeffer 5 studies of the root growth pressures exerted by plants. Agron. Jour. 47:]JMi—103. and R. D, Miller (1030). A method for study of the influence of mechanical'impedance and aeration on the growth of seedling roots. Soil Sci. Soc. Amer. Proc. 30:154—157. Hagan. R. M. (1033). Soil physical conditions and plant growth, Academic Press. X. Y.. pp. fioT—SnS, Hanks. R. J. and x. A. Harkness \lUSo).. ‘oil penetrometer employs strain gages. Agr. Eng. 37:553—554. Hanks. R. J. and F. f, Thorpe (lUSn), Seedling emergence of wheat as related to the soil moisture content. bulk density. oxygen diffusion rate and crust Strength. eril Sci.. Soc. .hner. l’roc. .lt:.NIT—31(‘. (1037). Seedling emergence of wheat. Sorghum. and Soytmunts as iiH luenctmilny soil <:rust Estrengtli. and Intiisllirt* C(HTICTII. St)i1 fSci . Stu;. :MHCI‘. 1)rtu:. .21:;{37— JI ’j. Harris. W. L.. W. F. Buchele. and L. E. Malvern (10o1). The relation among mean stress. volumetric strain. and dynamic loads in Soil. Presented as paper No. oU—nHJ at the ASAE winter meeting. Lemos. P. and J. F. Lutz (1057). Soil crusting and some factors affecting it. Soil Sci. Soc. Amer. Proc. 31: 485-401. McClelland. J. H. and R. h. Spielrein (1057). An investi— gation of the ultimate bending strength of some common pasture plants. Jour. Agr. Lng. Res. lszS—jul. Mtfilsenirl. R. 31.. fl. h. (Xx)per. [4. D. ’fukey'tleOJ). ;\n. engineering approach to evaluation of textural factors in fruits and vegetables. Presented as paper No. ol— 331 at the ASAP summer meeting. Mohsenin. N. M. and,H. Goehlich tonl). Techniques for determination of mechanical properties of fruits and vegetables as related to design and development of harvesting and processing machinery. Presented as paper No. 01—137 at the ASAP summer meeting. Morton. C. T. (1050). Basic factors affecting the energy required for emergence of plant seedlings. anub— lished M.S. thesis. Michigan State University. Prince. R. P. (1001). Measurement of ultimate strength of forage stalks. Trans. ASAh 4:1:3Hs-300, Richards. L. A. (lUSJ). Modulus of rupture of soils as an index ot crusting o1 soil. Soil Sci. Soc. Amer. Proc. 17:331—333. Shinaishin. 0. A. (onU). ”he interrelation between emergence force as measured by a mechanical seedling and emergence of plant seedlings. fnpublished M.S. thesis. Michigan State Vniversity. Stone. A. A. and I. L. hilliams (1030). Measurement of soil hardness. Agr. Eng. 3H235—3n. Stout. H. A. (1050). The effect of physical factors on sugar beet seedling emergence. fnpublished Ph.D. thesis. Michigan State University. F. \V. Sruxjer. zind \v. f. liuctuale (ltuifl). 'The 01- soi l ctnnpai:titni «n1 mtiistttre aflasoriatitni lw' s \ beet seeds. Quarterly Bulletin of the Michigan Aeri— cultural preriment Station. 43:3:54\—557. L $4 0 H, r—-~ Taylor. D. W. (104*). Fundamentals of soil mechanics. John \tiley'zuuj Sons. [3. 53\. ht) VandenBerg. G. E. (onl). Requirements for a soil mechanics. xZoerb . Trans. ASAE. 4:3:334—338. (1003). 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