FIELD ADAPTATEON OF swam .mps-usgon MEASUREMENTS WITH ms PLATSNUM MiCROELECTRODE Thesis for the Degree MM. 5. MECHIGAN STAT: umvsizsxw David Miller Van Daren, Jr. 1955 '[HE'gls FIELD ADAPTATION OF OXYGEN DIFFUSLON MEASUREMENTS WITH THE PLATINUM MICROELECTRODE 3! DAVID MILLER VAN DOREN, JR. A THESIS Submitted to the School of Graduate Studies ot’lichigan State University of Agriculture and Applied Science in partial fulfillment of the requirements fer the degree of EASTER OF SCIENCE Department of Soil Science 1955 «(HF-CH5 ACKNOWLEDGMENT The author wishes to sincerely thank Dr. A. E. Erickson for his guidance and assistance given through- out this research project. Dr. Erickson gave freely of his knowledge and time and provided the necessary equipment to carry this investigation to a successful completion. Special appreciation is also extended to the author's wife, Janet, for her valuable aid in editing and typing this paper. 11 881872 FIELD ADAPTATION OF OXYGEN DIFFUSION MEASUREMENTS WITH THE PLATINUM MICROELECTRODE BY DAVID MILLER VAN DOREN, JR. AN ABSTRACT Submitted to the School of Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Soil Science 1955 Approved: m ' L - M a" ”I. .w. .a-h' . -Am' I. ... . ~ ‘1 . m t A , ‘ ~ 4 ‘.D W u n n I .I‘u< J ‘ .' . . e o . .-.- I. David M. Van Doren, Jr. ABSTRACT FIELD ADAPTATION OF OXYGEN DIFHJ SION MEASUREMENTS WITH THE PLATINUM MICROELECTRODE A method of evaluating soil aeration and subsequently soil structure by measuring oxygen diffusion with the platinum microelectrode is presented and adapted for use under field conditions. A theoretical parameter for comparing the structures of different soils is devel- oped. Laboratory experiments performed verify the use of the parameter and standardize the diffusion measure- ments. The final equation to be used for comparison of field physical conditions is: 309159}; where 1t is the diffusion current, (31's is the initial uniibrm oxygen content of the soil solution, -9— is the moisture content of the soil in percent by weight, and S is a structure factor dependent upon the spacial arrange- ment of soil solids, pore size distribution and the distribution of water in the pores. Field measuranents nude on several soil treatments in an attempt to measure structural differences between various cultural treatments show no significant differ- ences in structure. It is assumed that these differences, if present, are slight. Tillage practices are followed with good success throughout the first three months of the growing season, illustrating the effect of tillage and rainfall on soil structure. The future of this I“ In O'- \r a". v {e fj' 1“! measurement of soil physical conditions is discussed. David M. Van Doren, Jr. 1. 2. 3. 7. TABLE OF CONTENTS Page Number Introduction - - ------------ 1 Theory - ---------------- 5 Apparatus- - - -‘- - - - - - - - - - - - 15 a. Manufacture of Platinum Micro- electrodes - - - - - - - - - - - - 14 b. Manufacture of the Electrode ContmlBox------------ 1'7 c. Manufacture of the Standard Reference Electrode- - - - - - - - l? d. Assembly of Apparatus- - - - - - - 24 e. Operation of Apparatus - - - - - - 24 Laboratory--------------- 2'? a. Polarogram with the Stationary Platinum Microelectrode- ----- 27 b. Determination of Moisture Film on the Platinum.Microelectrode- - - - 30 ?ield Experiments ----- - - - - - - - 38 a. Ferden Field Experiments, Chesaning, Michigan- - - - - - - - 38 b. Horst Farm Experiments, Akron, Iichigan - - - - - ........ 42 c. Studies on Cropping Rotations - 1955 - - - - - -------- - - 44 Summary and Conclusions ------- - - 52 Bibliography - ------------- 54 iii INDEX OF FIGURES Page Number Figure 1 Diagram of Completed.Microelectrode 15 Figure 2 Microelectrodes in Various Stages of Construction - - - - - - - - - - 16 Figure 3 Microelectrode Control Box- - - - - 18 Figure 4. Wiring Diagram for Microelectrode Control Box - - - - - - - - - - - - 20 Figure 5 Standard Silver-Silver Chloride Reference Electrode - - - - - - - - 22 Fugume 6 Sample Electrical Circuit for a Single licroelectrode - - - - e - - 23 Figure 7 Assembly of Apparatus for Field Measurement of Oxygen Diffusion with the Platinum Microelectrode - - - - 25 ‘Figure 8 Polarographic Determination of Oxygen Diffusion in 0.1 N Potassium Chloride with a Stationary Platinum Microelectrode— - - - - - - - - . - 29 Figure 9 Log itG-Versus Log C for Natural Corest - - - ------ - - - - - - 54a iv INDEA OF TABLES Page Number Table I Legend for Microelectrode Control Box Figures - - - - — - - - - - - 19 Table II Values of SE? Calculated for Several Natural Cores at Various Moisture Tensions - - - - - - — - 52 Table III Initial Readings (C) in Microamps for Several Natural Cores at Various Moisture Tensions - - - - 34 Table IV Slopes for Curves of Log ite-Versus L080 OOOOOO nun-c--- 55 Table V Values of 1:9 Calculated for cI.3 Several Natural Cores at Various Moisture Tensions - - - - - - - - 56 Table VI Results of Oxygen Diffusion Mea- surements on Soil Conditioner Plots, September 10, 1954 - - - - 40 Table VII Results of Oxygen Diffusion Mea- surements on Horst Organic Matter Plots, 1955 - ---------- 43 Table VIII Average of Structure Factor 169' 01.5 for Six Days on Cash Crop Rota- tions, 1955 - - — - ------- 45 Table IX Seaso 21 Variation of Soil Struc- ture 3 t0 as Affected by Cultu- ral P gcti es and Rainfall- - - - 48 Table X Comparison of Structure of Plow Layer and Sub-plow Layer in Cash Crop Rotations, 1955 ----- - - 50 INTRODUCTION Soil structure and physical edaphology have been extensively studied by soil scientists for many years. A thorough understanding of these soil conditions as well as other soil factors will make possible the pro- duction of larger, better quality crops from the land now available for cultivation. Many methods of measuring various phases of soil structure, in both the field and laboratory, have been proposed. Soil structure has been observed with a microscope (2). It has been characterized by certain physical properties of the soil such as aggregate stability, size distribution of aggregates, bulk density and pore size distribution. It has been characterized by functional relationships such as permeability to water and air, gaseous diffusion, and by measuring the rigidity of the soil. The different methods of mea- suring or characterizing soil structure and its relation to plant growth are much too numerous to describe in this paper. A few of the major advantages and disad- vantages of these methods will be discussed later. In selecting a type of measurement suitable for characterization of physical conditions and their relationships to plant growth in one Operation, the - 1 - 2. problem was approached from a functional point of view. What naturally occurring process is a function of soil structure and is also very much related to plant growth? Certainly soil aeration is such a process. Secondly, what naturally occurring process would best characterize soil aeration? The movement of a gas through a concen- tration gradient, preferably caused by a plant, is such a process. The diffusion of oxygen toward an actively respiring root has been chosen as the best single characterization of soil aeration and therefore soil structure. This measurement will not predict the stress the soil struc- ture will be able to withstand without being altered, the length of time that it will remain in its present state or its resistance to root elongation. However, it will give a reliable estimate of what the present structure is, not piece by piece, but integrated as it affects the plant. Instead of using a plant root as such, a small piece of platinum wire is substituted for the root. The oxygen at the surface of the platinum is removed by electrical means much as the oxygen at the surface of a similar sized root is removed by respiration. This causes a concentration gradient of oxygen between the surface of the platinum wire (plant root) and the main body of the soil atmosphere. The gas than tends 3. to diffuse from the region of greater concentration to that of lesser concentration of oxygen. The rate at which this diffusion can occur is dependent upon the concentration of oxygen in the soil atmosphere, the moisture content of the soil, and the obstruction to flow of the gas by the soil particles. This latter process is due to what is called soil structure because it is affected not only by the size distribution er these particles, but also by the special arrangement of the primary particles with each other. Therefore, the method of measuring oxygen diffusion with a platinum microelectrode as developed by Lemon (6) has been chosen to try to characterize soil physical conditions in gitg. It might be of interest to note some of the ad- vantages and disadvantages of this method as compared to other methods. As stated previously this measurement will not indicate the stability or longevity of the present structure nor will it indicate individual phases of soil structure. However, this method will give an indirect measure of particle arrangement. Unlike many other measurements it can be made in 2233, thereby obviating the need for collecting "representative" soil samples. It is a rapid method that gives a good single measure of physical edaphology. It is a measure of a natural process and is a result of the entire aspect of soil structure. The major disadvantage 4. characteristic of this measurement is the fact that a large sample (a field) is being measured by a micro method. If enough measurements are taken, however, this objection may be dispensed with. This method of measuring oxygen diffusion is not completely untried, having been used by several other investigators. Archibald (1) has used the platinum microelectrode to measure the effect of soil aeration on the germination of cats and sugar beets. Wiersma andeortland (15) used this technique to measure the effects of aeration on sugar beet growth and correlated aeration with response to peroxide fertilization. Several papers have been written and not yet published in which the platinum microelectrode was used to evalu- ate the effect of various manipulations of the soil on soil aeration and subsequent plant growth. After this brief introduction to the concept in- volved, it seems advisable to look into the actual theory of the operation, introduce the equipment invol- ved, and finally utilize this method of characterizing soil structure in the field. THEORY This method of measuring oxygen diffusion with the platinum microelectrode was developed by Lemon (6) and Lemon and Erickson (7,8). When oxygen is removed from a small volume of the soil atmosphere, oxygen will diffuse from the surrounding volume to replace the gas removed. This removal of oxygen is accomplished by the reduction of oxygen at a bright platinum wire cathode. Applying the principle of polarography, Lemon used a potential of ~0.8 volt between the platinum microe electrode and a saturated calomel reference electrode to produce the limiting diffusion current of oxygen. At this potential, assuming that no substance other than oxygen is involved in the reaction at the micro- electrode, the current flowing through the circuit is directly proportional to the quantity of oxygen diffusing to the surface of the microelectrode. However, one restriction must be made. The surface of the micro- electrode must be covered by a water film for reduction to take place. Kolthoff and Lingane (5) have applied Fisk's Law of Diffusion to this type of measurement. - 5 - (l) 1t where it nFAfir 0": : nFAD (3%))! =0, "t the current in microamps at time t. the number of electrons involved in the reaction at the microelec- trode with each oxygen molecule. Faraday (96,500 coulombs). flux at the electrode surface. area of the microelectrode in square centimeters. diffusion coefficient of oxygen in square centimeters per second. original uniform cancentration of oxygen. distance from the electrode surface in centimeters. a II N c: t: :>?hh1 7?. II II II H II Assuming that the residual current due to ionic flow is very small, as it is in the soil, the entire current is a result of the reduction of oxygen at the microelec- trode. Lemon has shown that the limiting diffusion cur- rent is reached.in three to five minutes. Therefore the value of t used in this paper is five minutes. The value of n is either two or four depending upon whether hydro- gen peroxide or water is the reduction product of oxygen at the microelectrode. This problem has not as yet been solved. At a constant potential and constant electrode size, n,‘F and A are constants. The diffusion current, it, is then proportional to D ”dB—flxwf The greater the original concentration of oxygen, the greater the concentration gradient from the body of the soil atmos- phere to the electrode surface, where the concentration of oxygen is essentially zero, and the greater the diffusion current. The diffusion coefficient of oxygen ,1! 7. depends upon the nature of the gas itself and upon the characteristics of the soil which impede the movement of the gas to the microelectrode. Since the nature of the gas is constant at a given temperature, the variation in D between locations in the soil is due to properties of the soil. These properties are essentially soil structure and soil moisture. ‘Fick's Law may be simplified to show the variables influencing the diffusion current for any situation in the soil. ‘ <2» 1.: when where K= nFA, and D, C and x have the same meaning as before. Kolthoff and Lingane analyze the factors influencing the diffusion coefficient, D: == RT (3) D r52 where R== molar gas constant. T== absolute temperature. N = Avagadro 's number. Sg= mobility of oxygen through the soil. According to Kolthoff and Lingane, the diffusion coefficient of a molecule depends upon the characteris- tics of the molecule (in this case oxygen gas), the medium in Which the movement takes place, and if the rhodium is aqueous, upon its ionic strength. For the range of salt concentrations in most soils and the accuracy with.which the oxygen diffusion measurements can be obtained, this slight dependence upon ionic 8. strength may be neglected. The factors influencing S: are just the characteristics of the molecule and the medium. The medium consists of the three phases of the soil - soil air, soil water, and soil solids. Comparison of data from various tables shows that the mobility of oxygen is much greater in air than in an aqueous solu- tion. Under a given set of soil physical conditions as the moisture content of the soil increases, it is logical to assume that the rate of diffusion decreases due to the effective reduction of the mobility of the oxygen. Therefore: (4) g, 43;... where-ed= soil moisture content in percent by weight. b=- exponent, assumed equal to one for the present. Also, the mobility of oxygen is dependent upon the physical impedence offered by the soil solids. (5) .5. as where S may be defined as a soil structure variable consisting of the mechanical impe- dence to oxygen diffusion due to the arrange- ment of soil particles and the pore size distribution and subsequent distribution of water in the pores. As the impedence to oxygen diffusion increases, S de- creases. Relying upon present conventions, as S decreases ‘the desirability of the structure decreases. Combining equations (4) and (5): <6> six—s- and (7) Si: kg where k is a proportionality constant depending upon the characteristics of the gas, oxygen, as related to its mobility through air, aqueous solutions, and across air-water interfaces, and is independent of soil physical conditions. Combining equations (5) and (7): _RT 8 (8) D r15; or (9) D== mg where K' = £31k Combining equations (2) and (9) yields: It follows from equation (10) that if a measure of-e and (fig-Leos: be obtained and K and K' evaluated fbr a particular set of conditions, the parameter of soil structure, S, can be determined by measuring the diffusion current, 1t' K can be calculated from n, F and A, but K' as yet cannot be calculated because the absolute value fbr k has not been obtained. Therefore, equation (10) can be altered to: (11) 1t 06 g (fix-mm Rearranging: (12) s J: “’9 PfifiX—‘mt (Knowing it,-03 and‘%%) different soils may be compared X:O.t vvith each other on a qualitative or comparative basis using equation (12). 10. The diffusion current, 1:, is measured in micro- amps and-9 in percent by weight. The remaining factor (ax hogas not been measured quantitatively. A method is being developed using a measure of the current flow at t-— 0 to determine the original concentration of oxygen in the soil in equilibrium with the surface of the micro- electrode. The method may be summarized in a few sen- tences. When measuring oxygen diffusion at the micro- electrode the first increment of current flow from t== 0 to t==?é?.is due to the concentration of oxygen at the surface of the microelectrode. This concentra- tion should be a measure of that throughout the rest of'the system. The initial flow is recorded as the farthest point of swing of the microammeter needle. However, this reading is affected by the surface adsorp- tion of oxygen by the platinum and the characteristics of the microammeter. These latter factqrs affecting the initial swing should be the same for each micro- electrode provided the electrodes are similar as to construction and history and the ammeter is stable and does not shift with time. Therefore, this measurement of the original oxygen concentration, which will here- after be referred to as the "initial reading", can be used in a comparative manner in equation (12). Using the initial reading as such to compare soil structure, its relationship tok%%a can be shown as X: 3,15 follows: a. b. O. Therefbre, C can be substituted for 11. When t== 0, the concentration of oxygen in the soil atmosphere is proportional to the initial reading, C. If t== 5 minutes in every case and the electrode is a constant size, x is defined. (Lemon evalu- ated x=- 5 millimeters .) Therefore, ~ C 0C initial reading x=mt mm or HQ :3 K. 3L ’ (9X X'O.t c m . . where K'c is the proportionality constant de- pendent upon the adsorption of oxygen on the platinum wire. or, (IA-H) X x=o.t=K°C where Kc== K' 5 mm C in e uation a X Xc°,t q (12) yielding equation (15). (15) s 11:9 (T where it and C are measured in microamps and -O-in percent. The parameter, S, is dimension- less and can be used to compare the structure of different soils and locations. In summary, the parameter, 8, includes the physical variables (1) arrangement of soil particles, (2) pore size distribution, and (5) the distribution of water in the pores, all of'which contribute to the physical impedance of the soil to oxygen diffusion. Equation 12. (15) is valid provided the following are experimentally held constant: A, T, t, and the applied potential. If these are constant, x and R will be constant, and n, F, R and N are always constant for soil conditions. By following carefully the prescribed procedure in the field or laboratory, these conditions can be maintained, with the exception of constant temperature. Errors introduced here are small because of the small variations in soil temperatures during the growing season. Therefore, reasonably accurate results may be obtained. APPARATUS* Use of microelectrodes to characterize the physical pnaperties of an entire field requires specially designed equipment. The microelectrodes must be small enough to simulate plant rootlets and be of such a size and shape that they will not alter the physical properties of the region in which they are inserted into the soil. They must also be strong enough to withstand a good deal of punishment, and cheap enough to manufacture if there is considerable breakage. Since the electrodes are micro in nature, they will encounter a wide variety of conditions, even in a soil with fairly uniform physical characteristics. If a large enough number of measurements are taken, however, a statistically significant average of these various locations may be calculated. This average will effective-’ ly characterize the entire system known as soil structure for the field under consideration. Fbr this purpose an electrode control box was designed to enable the operator to take ten measurements in little more time than it takes to make a single measurement. Finally, the standard reference electrode must be large enough to withstand continuous current flow *All of the apparatus described here was developed by Dr. A. E. Erickson of the Michigan State University Agricultural Experiment Station. - 15 - 14. without becoming polarized, and must be sturdy enough to withstand considerable shaking and jarring. Manufacture of Platinum.Microelectrodes. A 10 inch length of 20 gauge copper wire is fused with 6 to 8 millimeters of 22 gauge pure platinum wire. The wire is then placed in a piece of glass tubing 9 inches long and 5 millimeters in diameter. One end of this glass tubing has been previously drawn to a taper and almost sealed. The platinum wire is placed at the tapered end.of the tubing with.2-3 millimeters of platinum exposed. A liquid plastic material (Castolite) is drawn up into the glass tubing through the tapered end by placing auction at the opposite end. When the desired height of plastic is drawn up into the tube, the suction is removed and the end containing the plati- num wire is pushed gently, but firmly, into a small piece of cork to seal it and prevent the plastic from draining out of the glass tube. After allowing the plastic to set in a 60°C oven for at least 48 hours, the glass tubing is chipped away from the plastic. Enough plastic is removed from the platinum so that exactly 4 millimeters of the metal is exposed. A sketch of'the microelectrode is shown in Figure l. The microelectrode in various stages of construction is illustrated in Figure 2. 15. copper wire lastic (Castolite) shaft 6 inches minimum fused Cu-Pt junction 4 wiry-22 gauge platinum wire FIGURE 1: Diagram of Completed Microelectrode Fused wires inside glass tubing. Copper-platinum wires fused. Completed microelectrode. n O .1. t m t S n .O C 0.. O 8 O 8 a t S 3 . u 0 .1 r a v . n .1 ,8 8 ”O.” t O 6 1 ..e .0 r O .1 us 1. 2. 5. FIGURE 2: 17. The use of plastic for the shaft of the electrode works quite well. The tip can be tapered with a metal file to enable the electrode to be inserted into the soil with a minimum of disturbance of the structure. The plastic is less brittle than glass and will with- stand some stress without breaking. If breakage does occur, the shaft can be mended with masking tape. Even though the electrode is fairly sturdy and resiliant, appreciable breakage does occur. However, the electrodes can be produced on a mass production basis fer approxi- mately 35 cents apiece. Manufacture of the Electrode Control Box. The box is designed so that ten microelectrodes may be operated simultaneously. ‘Figure 3 presents the top view of the control box. Table I is an explanation of symbols used in this figure and in Figures 4 and 6. All of the switches, plugs, leads and microammeters are set in a lightweight aluminum instrument chassis 17 inches long, 9 1/2 inches wide, and 2 1/4 inches deep. The wiring diagram in‘Figure 4 shows the con- nections which enable the control box to operate. lanufacture of the Standard Reference Electrode. The saturated silver-silver chloride reference electrode was decided upon as the type most likely We"? FIGURE 5: Microelectrode Control Box 19. TABLE I Legend for Microelectrode Control Box Figures P1 BAT. + BAT. '- POT. + and - S.C. L.M. SOM. 10 hole plug for connecting lO electrodes to the circuit simultaneously. positive terminal of the 2 volt wet cell battery. negative terminal of the 2 volt wet cell battery. connection for a voltmeter to measure voltage applied between the standard cell and the microelectrode. saturated silver-silver chloride standard cell terminal. rheostat for regulating the potential applied from the battery. double pole-single throw switch to connect battery to circuit. double pole-single throw switch to allow measurement of the applied.voltage with a. V0 ltmeter e 5 position rotary shorting switch for con- necting an electrode to the large micro— ammeter plus the electrical circuit (desig- nated by'L.M. next to switch), to only the electrical circuit (designated by ON next to switch), or to the small ammeter plus the electrical circuit (designated by S.M. next to switch). large microammeter, range of 0-100 microamps. small microammeter, range of 0-20 microamps. 20. sepauoueomgw 4d 01 .2... I» .2.m .r& “I 0 pl, ® 7 +3 |$ 3| H... O. .+ I“D Mom doapeoo 323.323: a8 anemia met“: 3 smears 2l. to withstand rigorous field conditions and still operate correctly. A 50 inch length of pure silver foil 4 inches wide is accordioned into a plastic food container 8 inches high and 4 inches square. A small strip of silver, still attached to the main body of the metal, is led through a slit in the plastic case and sealed with Epon VI adhesive (Shell Chemical Corporation). A short piece of glass tubing is sealed into another side of the plastic case so that a length is exposed both in- side and outside the case. The cell is charged by passing current through the cell containing approximately 0.1 N hydrochloric acid. A large platinum gauze is used as one electrode and the silver acts as the other. Silver chloride is plated out onto the surface of the silver foil as the silver is oxidized to silver, valence plus 1, by the current. The cell is then rinsed out and filled with saturated potassium chloride. A piece of Tygon plastic tubing is attached between the glass tubing sealed into the plastic case and a porous clay cup. This acts as the salt bridge to the soil. A pinch clamp is used to close off this contact when desired. Two views of the completed standard cell are presented in Figure 5. The electrical circuit is shown in Figure 6. The potential of -0.8 volt, as measured with a voltmeter, is applied between the platinum microelectrode and FIGURE 5: Standard Silver-Silver Chloride Reference Electrode, Side View FTGURE 5: a Standard Si Electrode, ff.3‘3;1.&':'. . w‘.‘ '4 ‘5 o: .o, 5‘ "‘T-‘j‘WVrI O, a“ -. _ .-'b a IIIIIIII ‘55 . - .ckavdiéligf it“: ,7," ".5' . lver-oilver Chloride Reference Top View 23, duo needed IIIIIIIIIII. :8 2856 III/VIII)! .004 .3 .8. .sm . oeoauooaeoaeax ea can u sou passage Hsoanuooam ea aim 88860222 3.25.; e» 24. the standard cell by the battery. Oxygen is reduced at the surface of the platinum, and current flows through the above ground circuit, through.the standard cell into the ground. The salts in the soil complete the electri- cal circuit between the porous cup and the platinum mic roelectrode . .Assembly of the Apparatus. The terminals of a 2 volt wet cell battery are connected with the appropriate terminals of the control box. An alligator clip is attached to the strip of silver protruding from the reference cell and a lead is attached to the 8.0. terminal on the control box. Ten electrodes are soldered to 5 foot lengths of insu- lated wire and these wires are soldered to a male plug which is inserted into the 10 hole plug (Pl) on the control box. The entire apparatus, as assembled, is shown in‘Figure 7. Operation. The two switches, X and Y, are turned on and the rheostat, R, is adjusted.so that the potential between the standard cell and the platinum microelectrodes is exactly -O.8 volt. The porous clay cup is inserted into the soil, the soil wetted with more potassium chloride if necessary, and the screw clamp Opened to permit electrical contact with the soil. The microelectrodes 30303033: Escape: on» «3.2. soanauda semhno no nsogasese: .30: you @3939? no hanaeaed up 95lo 26. are pushed approximately 4 inches into the soil and the measurements are ready to be taken. A stopwatch is started and the first switch, 81’ is turned to L.N. The farthest point of swing of the needle is recorded as the initial reading and the switch is turned to ON.‘ This process is repeated rapidly for the other 9 switches, allowing only enough time between readings to permit the meter needle to come to rest at zero after the meter is disconnected from the circuit. At the end of 5 minutes, switch 81 is turned to Sun. and.the reading is recorded as the 5 minutes reading, or it, and the switch turned back to ON. This process is repeated rapidly for the remaining switches. All switches, Sn, x and Y, are then turned off, the electrodes removed from the soil, replaced at a different location and the entire process repeated with the exception of setting the rheostat. If the battery potential is con- stant with time, the rheostat need only be checked periodically to be sure that is has not been changed accidentally. Using this apparatus, ten diffusion readings of 5 minute duration and the initial measure of oxygen concentration for each can be taken in about 6 minutes, saving a great deal of time and allowing more measure- ments to be taken during a day's work. LABORA'IO RY Before taking the microelectrodes into the field several problems had to be resolved. Polaregram with the Stationary Platinum Nicroelectrode. Doubts had arisen that a polarogram could be made properly with a stationary platinum microelectrode. It was argued that eddy currents would be set up at the elec- trode due to the formation of water at the surface of the platinum, and that these currents would cause varying diffusion rates of oxygen to the electrode. A laboratory experiment was set up to discover if this were actually the case. A container was built which could be completely closed off to the atmosphere. A dilute solution of potassium chloride was introduced and gas of known oxygen content bubbled through the solution until the solution and gas in the chamber were of the same oxygen content as the bubbled gas. A platinum microelectrode and potassium chloride contact to a saturated calomel electrode had previously been placed in the solution. The solution was stirred with a magnetic stirrer for 1 minute, allowed to stand for 5 minutes, and the prOper potential applied for 1 minute. The diffusion current was read at the end of that time. The potential was then removed and the process repeated at a different -27- 28. potential. Potentials of -O.l to -l.4 volt were applied and solutions of 0.4, 8.4 and 38 parts per million oxygen were measured. The polarograms are plotted in Figure 8. At the end of 1 minute the microammeter showed the current to be steady, and it is assumed that eddy currents, if produced, had no effect on the mea- surement of diffusion. Lemon and Erickson (8) published a graph similar to that in'Pigure 8 with the exception that the solution was a 1:1 soil to water suspension of Brookston clay loam. The "plateau" regions of the curves of the 1:1 suspension occurred between -0.7 and -0.8 volt. These same potentials lie on the plateaus of the curves in Figure 8, confirming the use of -0.8 volt as that poten- tial which produces the limiting current. It is logical to assume that if these two types of polarograms can be determined without disturbing effects of eddy currents, the same type of polarogram could be determined in the soil where resistance to such currents would be greater than in.a soil-water suspension or in a water solution. In no case during all the field measurements was there -any evidence of eddy currents. More or less violent fluctuajdon in diffusion current was observed when the control box was wetted by rain and.shorted to the ground. In this case the measurements were of no value anyway. #.H N. 29. *1 a Huaeno> ca ”saucepan o.a m.o 0.0 0 ad to .. Tm seasonao addenduom z v.0 moo |'\‘ 0.0 I <0 1 3 sdmmoaotw u: aueaano 30. Determinat ion of Moisture Film on the Platinum Micro- aloof-freak One of the limitations of this determination is that the platinum microelectrode must be completely covered with a moisture film in order to Operate as a reducing surface for oxygen. If it were not so com— pletely covered, the effective size of the electrode would be reduced, causing differences in size between electrodes. Equation (1.3) would then no longer be valid. Another laboratory experiment was set up to evaluate the moisture stress necessary to rupture the complete moisture film surrounding the microelectrode. Natural cores of the top 3 inches of several types of soil were taken during 1954. These cores were satu- rated with water and placed on p? tables to obtain the desired moisture tensions. After the cores had come to equilibrium they were removed from the tables and placed on a moist porous plate. Three electrodes were placed in each core and the potassium chloride bridge connected to the porous plate to complete the circuit. The initial reading was taken and the diffusion current was recorded after 5 minutes. For moisture tensions greater than 60 centimeters of water, a pressure cooker was used. The weight of the core was recorded after each reading and after all the oxygen diffusion measurements were taken the core was oven dried at 105°C. The moisture 31. content in percent by weight was calculated for each core at the various moisture tensions. According to equation (15): Swiss. Relating this equation to equation (10) and 6., page 11, equation (14) is obtained. (14) smc'xc = $5.9. where K =:nFA and K' and KO have the same mean- ing as before. or (14s) SK'KonF = 25:22:; It is assumed that n, F and A were all constants when the proportionality constant K was introduced. After the moisture film on the platinum microelectrode has burst, as soil moisture comes under successively greater stress, this film will shrink in area, effectively reducing the operating size of the electrode, A. In order to maintain the equality of equation (14a), assuming constant 8 in each core, 1t9 must decrease as A diminishes. Or, if A remains constant, Eggimust remain constant. To find out if this is true, 3&2 has been calculated at each moisture tension for the different cores and is reported in Table II. In many cases the values were quite uniform until the large change in moisture tension from 60 to 342 centimeters. Then a marked increase in the 2&2 value was observed. This was quite unexpected and indicated that some variable not previously evaluated had caused this value to increase instead of remain constant or to decrease. 52. m.e a.o m.m s.e 0.0 n.« a.» n.m . Assoc sesame omuwsamansa a o.e e.n H.¢ >.e m.» s.m m.a s.m . Asses sesame omuooassm H.ea H.na H.oa n.» n.¢ a.» n.e o.e Ahsaov ms-eaousoaaa m.m «.0 n.n a.» e.m e.a m.a e.a “assoc mousaasaom.. mg. to to to o4 «3 m4 1» _ Assam eased 5.883%: a. e.e m.¢ m.n m.¢ n.n a.» n.e a.» .1 “sesame «buoomesaass e.¢ o.n o.n n.m m.m o.m m.a m.m Assoc epsom. se-soenaooam o.a n.a >.a . s.m m.H m.a «.H >.H A xasoq_h>somv oe-so>oaoo 31%. MW Mm". mm. .04. mm mum. mm 3.80 hope: we nheuefluseo ca seduces 9253.6: esoausoa cheese: 3.6.3.; 94 3.30 Aeneas: assepom sou mousagaso .9qu «o more: HH Mada 55. A comparison of the initial readings in Table III with the 3'53 values given in Table II shows that the large increase in the latter value is accompanied by a large increase in initial reading. It seems reasonable that these two large increases may be related to one another. It may be postulated that as the increased oxygen content at the surface of the microelectrode causes a larger and larger initial reading, the inertia of the mic roammeter has a greater effect upon the reading. In other words, as the needle deflection increases, or tends to increase, the inertia inherent to the micro- ammeter reduces the reading to a greater extent. The following procedure was used to discover if this were actually the case. Let equation (14) be changed so that the term C has an emonent, a. This exponent is due entirely to the characteristics of the microammeter and could be termed the microammeter correction factor. . -—1t9’ 14b SKK‘ — ‘ ’ K" 1: Then (15) log ite = a log 0 + log SKK'Ko Equation (15) is that of a straight line with a the slaps and log SKK'Kcme intercept. Log ice versus log 0 was plotted for all. the cores and a typical example is shown in Figure 9. The slopes for all the curves are recorded in Table IV. 54. or nr 0* cm am om no no GNOH as «e we so on no no can mam. am no as as mm as on no mum mm om no an an em pa mm mm. on an as be on ea a an mm as on on mm mm on m mm .mm as as as am mm m a pa mm HA bd on ma Ha Na 0H aope3.uo necessauneo mw usoaunes ensundoa HHH manda enoaenea cascade: usounsb as em . meanness om . coasam we - esouaoam so . sausaom E. .. 335.32 we I concedes: soa_aopmsoosm we I ne>osoo uOhOO nonoo demons: Hsne>em sou seasoned: :4 50V uwcaeoom asaaasu 2. 2.4 1.8P FIGURE 9: 54a. Log 1t9 Versus Log 0 for Natural Cores 55. TABLE IV: Slepes for Curves of Log ifie Versus LogAC Core Number Slope 66 1.17 74 1.08 77 1.20 69 1.46 72 1.45 80 1.21 86 1.44 Average 1.3 The average slope of all lines was 1.3. This value '33 selected for a and £¥2: was calculated for all cores at each moisture tension did recorded in Table V. It is evident that if this value is constant for each core, the effective surface, A, of the microelectrode is constant. In other words, as the moisture tension increases, if.%§$% remains constant, then the moisture film.is intact on the surface of the microelectrode and is no longer a limitation to field application. As can be seen in Table V, the values are relatively constant for each core, except for the Marinesco, number 74, which gave a downward trend indicative of a shrinking moisture film. This soil is the sandiest of the eight studied and this fact may explain why the moisture fihm started to shrink prior to 1 atmosphere tension. There are several reasons why the values in Table V are only relatively constant. Up to 60 centimeters tension the cores were not covered so that evaporation may have caused some discrepancies with the observed 56. e.m m.m m.m a.» m.m m.H m.a m.a om - madness: o.H o.H H.a m.a e.H m.s m.o n.H om . oocsam m.» m.¢ H.m H.m m.a m.a o.m s.a ms - eaoosoam o.a n.a m.a e.H H.” v.0 m.o s.o me . asaaaom H.m m.m o.m m.H m.a e.H o.m H.m es - concedes: m.H m.H m.H o.a m.m n.m n.m 0.” as - concedes: n.H o.a a.o o.a m.H H.a o.H >.H so . sopuaoosm 0.0 «.0 n.o m.H s.o o.o o.o m.o no - sosoeoo mm». mm mm”. mm. mm“ .onm mm and. «.88 hope: no openness—co a... unoaneea end—pane: one: son. can» 30: 3622, as m . IF”. o 0.30 Hagen Acheson non cocci—ease ¢na no needed, > Eda. 37. moisture data. Also, the microelectrodes were left in place and some poisoning of the platinum may have occurred. For tensions above 60 centimeters of water, the cores were placed in pressure cookers. To do this the micro- electrodes had to be removed and could be replaced only after the cores were taken from the cookers to be mea- sured. Many holes were thereby dug in the core where the electrodes had been. This could cause variation in S and therefore in.%¥%s, but the accuracy is suffi- cient to validate the reasoning throughout this paper. In conclusion it may be stated that over a wide range of soils a moisture tension as high.as 1 atmosphere and probably higher will maintain a complete moisture film on the surface of the platinum microelectrode. As a sidelight to this study the data were analyzed to see if the exponent on-e should be a value other than unity. Correlations between it and-e-were calculated. In no case was the coefficient of correlation less than -0.83 indicating a linear relationship between it and 8. Therefore, if the exponent on it is unity, the exponent on-e must also be unity. Equation (13) may now be written in final form to be used to analyze data from field work. 1t9 (16) 8 ((5.173 FIELD EXPERIMENTS Now that the theory of oxygen diffusion measurements has been developed, a parameter for measuring and com- paning soil structures introduced, and equipment both sensitive and sturdy constructed, the apparatus is ready to be taken into the field. Ferden Field Experiments, Chesaning, lichigan. The first field study was completed in 1954 on a series of plots designed to study the effects of soil conditioners on structure and plant growth. The plots are 21 feet square and are laid out at random on an acre of flat Brookston clay loam. The treatments studied, with four replicates of each, are as follows: 10 ChOCk 2. IBMA (2000 pounds acre) 5. IEMA (250 pounds acre) 10. Crude Abeidic Acid (3000 pounds/acre) 14. VAMA (1000 pounds/acre) 17. Lignin Sulfate (15,000 pounds/acre) Pea beans were grown and harvested, with the weight of the beans per plot recorded at harvest time. Only one set of the oxygen diffusion measurements can be of’any use because only on this particular day were moisture samples taken. At the same time oxygen diffusion as measured with the nitrogen filled tube developed by Raney (9) was performed to supplement the diffusion measured with the platinum microelectrode. - 38 - 39. The comparative data are given in Table VI. The data obtained with the Raney tubes cannot be used to characterize structure because they do not take into account the oxygen level of the soil atmosphere and the soil moisture content. They are used as a soil aeration factor and can be compared with the diffusion reading, 1t' a comparable factor obtained with the microelectrodes. Such a comparison yields absolutely no significant results or correlation. This can be explained on the basis of what the two methods for measuring oxygen diffusion actually determine. The Haney tubes are macro in size and measure the diffusion through large pores, cracks and worm holes. These plots were very dry and large cracks existed in the surface. Measurements of this nature under such conditions may yield data not indicative of the entire aeration con- dition of the soil. 0n the other hand, the microelec- trodes will not function if they happen to be placed in such a crack or void. They Operate when surrounded by soil particles and a complete moisture film. In such surroundings, they measure diffusion through the smaller pores, as affected by the supply of oxygen which can be circulated through the larger pores and cracks. There- fbre, these measurements are believed to be a more accurate measure of the whole soil aeration than that obtained with the Raney tubes. 40. nom0.03 eemn.ou nu» apnoesum wab.o www.o mnm.o me.o nmmoo bmw.o mon.o incapaaoev onxo apnoaenznuoa onus modem «sea .0“ sonaopaom .npoam eonoapaeaoo Haom no upcoaoaenmoa dodaaumun commas no upasmom announce "caoah none the on\n noospon coapwaonpoo "daeah done one o.mm m.aa m.¢H ¢.mH m.mm c.0H ¢.¢H m.mm A093 0:53 E and bb.o a®.o mm.o no.0 mo.H bo.H OH.H OH.H «masonoaa an . o mmww noowuon :oaumaopeoo o.m n.mH o.mH N.HH o.HH @.HH $.ma m.na H> Wanda upsoaonsaeo: oooauooaeoaeas poam\monsom macaw seem «H a m ha OH ea N 4mm<<<<.0 wmb.0 0b0.0 sense hensm do eumoaonsusos noandmuaa nomfiNo no mpazuom .caamp non hanmnonm cadence 0mm oven mafia :0 mama oaumhhmv b91010 “300* as 0 000 CO 0000 .sags H0.0 H0.0 0«.0 nb.0 00.0 00.0 00.0 00.0 H0.0 00.0 $0.0 00.0 0b.0 adadoaoai .94 v.0 uouonuooaoonoaa Escapeam neon .usoam soups: onaawso umaom HH> mqmo be no>oao poosm we on nu ma moose mm madam N ha mno>o oases: ee on name» m hao>o mm ne>oao woos» am one cannea :uom mm name» m heo>0 ma ao>oao acorn .oz comm :ofimuom 44. that the structure had not changed appreciably. Since no cultural practices occurred in the interum between sampling dates, and only one very light shower fell during that time, there was little if any tendency for structure to be changed. Therefore, the agreement of the data indicates the validity of the factor, #3 Studies on Cropping Rotations - 1955. The other study made in 1955 was on rotations at the'Ferden farm designed to 810w the differences in crop yield strictly on the basis of the result of the rota- tions. These are five year rotations of: (1) corn, sugar beets, barley and 2 years alfalfa-brome mixture, (2) sugar beets, carn, barley and 2 years alfalfa-brome and (6) corn, sugar beets, barley, beans and wheat. Each year each rotation is replicated 4 times so there are 20 plots for each rotation. Only the sugar beet and corn plots of these three rotations were measured. A great many measurements were made during the first three months of the growing season. All of the plots studied were not measured on the same day so that direct comparison of all the data would not be satisfactory. Therefbre, the data from 6 days in which all the corn plots were measured are averaged. Similarly, the data are averaged for 6 days in which all the sugar beets were measured. These data are reported in Table VIII. Table VIII Average of Structure Factor g§$3 for Six.Days on Cash.Crop Rotations, 1955 Sugar Bee ts____ A: Replicates Rotation B I '—B 2 BiS' B 4 l 1.10 1.06 0.99 0.97 2 1.10 1.00 0.74 0.76 6 1.01 1.06 0.98 0.85 Corn _4_ Replicates Rotation 1:1 B 2 l 0.87 1.06 2 1.07 1.05 6 1.14 1.00 46. It should be noted that the plots run north to south in order from replicates l to replicates 4. The plots become sandier moving from north to south, and the struc- ture is visibly poorer on the southern side of the plots. This structural gradation is evident from the data in Table VIII and is due to soil differences and not neces- sarily to differences due to rotations. All in all there is very little difference in struc- ture between the soils in the three different rotations. The reason is fairly obvious. At no time this year has more than 0.76 inch of rain fallen in one day, and the rains that have fallen have been very gentle. The plots were worked at optimum moisture conditions for tillage, and there was no reason fbr a poorer structure to develop on some plots than on others Just on the basis of crop- ping practices. Oxygen diffusion was detennined with Raney tubes, but few data were accumulated and the data obtained showed no significant differences in aeration between rotations. It might be of interest to follow the structure of some of these plots as measured with the platinum micro- electrode through the first 5 months of the growing season. Admittedly these were selected plots on the basis of the data obtained. -All of the plots did not follow the same pattern with rainfall and cultural practices, and this behavior is difficult to explain. 47. The data notreported were not all erratic, because there were indications of seasonal trends, but there were many deviations from the season pattern. The explanation which seems most logical is that a field will not have uniform structure throughout, but will vary within cer- tain limits. Whether or not this variation in structure caused the variation in.the structure factor can only be speculated at the present time. In Table II are shown the 1t9' values and dates 2:175 When the measurements were taken for several plots, plus the dates of cultivation and the accumulation of rainfall between.measurement dates. From this table the seasonal variations of soil structure can be followed. All the plots had.a given structure at the starting point, May 6. The structure factor in rotation 1 plots was low and did not change appreciably until the plots were plowed and planted. The structure factor for rota- .tion 6 soil was a little better to start with, but de- creased as the rains came and became about equal to that of rotation l bef0re plowing. Three weeks after planting and just after several light rains the soils were mea- sured again and showed an increase in the structure factor due to the once over tillage used in planting the corn. Following this measurement the corn was spiked. This was followed two days later by a half inch of rain. The structure measurements after this cultivation and 48. oo.a am.” sH.H oa.a nm\o aaoo eooasataeo oa\o mo.o 0H.H mo.a os.o na\o moumasasoou Haauaaaa .aa oa.a Ha-s\o aaoo eoaa>aaaao e\o no.0 on.a oa.a eo.o ”\o assesses .an oo.o aa\o aaoo eoaaom sm\m om.a ma.a ma.a oo.H mmxm dopdasesood assesses .aa Hm.o mmaem\m mm.a . om.o eopaaao one eoaoaa a\m so.o mm.o eo.o oo.o am\s as.o oo.o am.o no.0 smxe coueasadoom assesses .sa oo.o mm-em\e os.o mm.o mm.o No.0. . mm\e Haauaaaa .aa os.o aa\a ma.a oH.H «o.a me.o ow\e\e um-num m.mnm. um- -: aoaoapuoaso open oapnfipu noon: on o soaoapom oEonn adHHdH Hd ado can an a soapaaom some . oaoo . oooam ooaaaoom dose ease and Hamhcddm anaconda one aooaaoaaa Homepage an dopoouud as .mndd cannonnpm aaom Mo coupeaaa> Hmcomsom RH mqmda 49. rain showed a decrease in structure in all but one plot. It may be noted that this cultivation had no beneficial effects on soil aeration or structure and the only bene- fit, if any, was the control of weeds. The corn was cultivated two days after the last measurement, and the cultivation was again followed by rain. Measurements of structure two days after the rain showed a continued general decrease in the structure factor. However, between the next cultivation and the last measurement on June 25, there was no rainfall and the structure in all cases showed some improvement, presumably due to the cultivation. No general conclusions can be drawn from this analy- sis of data because the data were selected on the basis of the results, but there is introduced a possible appli- cation of the platinum microelectrode measurements of oxygen diffusion for determining the effects of various fbrms of cultivation on soil structure. One more study was made on the sugar beet soil in the cash crop rotations which was designed to compare' structure in the plow layer with that below the plow layer. Measurements were made at the surface and 10 to 12 inches below the surface. The results are presented in Table X. In every case the sub-plow layer structure was considerably poorer than the surface structure, which may be expected on the basis of the cultural practices TABLE X 50. Comparison of Structure of Plow Layer and Sub-plow Layer RotationLReplication in Cash Crop Rotations, 1955 1 1 Date 5/15/55 5/15 5/15 5/15 5/15 5/15 5/15 5/15 5/15 5/15 5/15 5/15 its 1:9 4 inches 10-12 inches CI°3 deep 01'3 deep 1.12 0.41 1.51 0.45 1.11 0.50 1.25 0.72 1.55 0.45 0.95 0.57 1.62 0.55 0.98 0.46 0.90 0.55 1.02 0.49 1.06 0.44 0.85 0.45 51. followed in this experiment. 0f the 4 plots in which 2 'measurements were made the results of the 2 sub-plow layer measurements are in close agreement with but one exception. This again tends to confirm the validity and accuracy of the 139' structure parameter. 01.5 SUMMARY AND CONCLUSIONS The method of measuring oxygen diffusion in the soil with the platinum microelectrode as developed by Lemon is refined, both as to procedure and equipment, and is adapted to field.work. The theory of Lemon and Lemon and Erickson is expanded, yielding a factor called the structure parameter. Field measurements with the platinum microelectrode are analyzed and discussed. The laboratory and field data presented in this paper indicate the validity and usefulness of the method and its application. Much work has been done to improve equipment and measurement techniques, and much more work remains to be done. For the sake of quantitative evalu- ation of oxygen diffusion as related to soil physical conditions, the mobility of oxygen through the soil, water and across soil-water interfaces should be evaluated and the initial reading calibrated in terms of actual units of oxygen concentration in the soil. Also, the exact effect of ionic strength of the soil solution and of soil temperature on diffusion should be determined. With the development of quantitative oxygen diffusion measurements and the absolute evaluation of soil factors influencing the diffusion, an unlimited field for appli- cation of thkatechnique unfolds. Factors affecting soil - 52 - 55. structure, such as tillage, cropping systems, incorpora- tion of organic matter and meteorlogical phenomenon can be evaluated.ig_gitg as they occur. Critical aeration values for plants can be determined, both as to duration and severity. With further development of this measure- ment of oxygen diffusion, valuable contributions to the fields of soil physics and crop production appear imminent. 5. 6. 7. 8. 9. 10. 11. 12. 13. BIBLIOGRAPHY Archibald, A.J., Effect of Soil Aeration on Germi- nation and Development of Sugar Beets and Oats, M.S. Thesis, Michigan State College, 1952. Baver L.D. Soil Physics John Wiley and Sons Inc. New 13m, 1545'”. ' ' ' Blake, G.R. and Page, J.B., Direct Measurement of Gaseous Diffusion in Soils, Soil Sci. Soc. Amer. Evans, D.D. and Scott, A.D., A Polarographic Method of Measuring Dissolved Oxygen in Saturated Soil, Soil Sci. Soc. Amer. Proc. 19: 12-16, 1955. Kolthoff, 1.x. and.Lingane, J.J., Polarograghy, Interscience Publishers, Inc., New or , . Lemon, E.R., Soil Aeration and Its Characterization, Ph.D. Thesis, Hichigan State College, 1952. Lenon, E.R. and Erickson, A.E., The Measurement of Oxygen Diffusion in the Soil with a Platinum Micro- electrode, Soil Sci. ScC.Amer. Proc. 16: 160-165, 1952. -------- , Principle of the Platinum Microelectrode as a.uethod of Characterizing Soil Aeration, Soil Science 79: 585-592, 1955. Raney, W.A.,'Field Measurement of Oxygen Diffusion Through Soil, Soil Sci. Soc. Amer. Proc. 14: 55-61, 1949. Russell, M.B., Soil Physical Conditions and Plant Growth, Academic Press, Inc., New York, 1952. Scott, A.D. and.Evans, D.D., Dissolved Oxygen in Saturated Soil, Soil Sci. Soc. Amer. Proc. 19: 7-12 , 1955. Taylor, S.A., Oxygen Diffusion in Porous Media as a Measure of Soil Aeration, Soil Sci. Soc. Amer. Proc. 14: 55-61, 1949. 'Wiersma, D. and Mortland, M.M., Response of Sugar Beets to Peroxide Fertilization and its Relationship to Oxygen Diffusion, Soil Science 75: 555-560, 1955. -54- *~ F”"J"n gr».-_ ,._ 2'". it‘d! t.a.___.— . ,. d it ~ seam-2-..): s Laid-1“ FE 2’], '56 3581736 MICHIGAN STATE UNIVERSITY LIBRARIES ‘I II IIILIIIIIIIIII 3 1293 03 77 3439