and four - plant physiologyfor plaster of paris absorption units an arbitrary resistance value of...

A COMPARISON OF ELECTRIC RESISTANCE UNITS FOR MAKINGA CONTINUOUS MEASUREMENT OF SOIL MOISTURE

UNDER FIELD CONDITIONS1 \

G. J. BouYoucos AND A. H. MICK

(WITH FOUR FIGURES)

Received January 14, 1948

Since its introduction in 1940, the plaster of Paris block method ofmeasuring soil moisture by means of electrical resistance (4) has beenwidely employed in agricultural fields. Hydrologic research2 (16), warproduction problems involving guayule culture (11, 12), and many otherinvestigations ranging from field irrigation to greenhouse studies havemade use of this technique. Briefly, the method consists of imbeddingwithin the soil a plaster of Paris block containing two electrodes. At aconstant temperature, the electrical resistance of this absorbent materialvaries with its moisture content, which, in turn, varies according to themagnitude of the forces exerted by the surrounding soil. A determina-tion of block resistance thus gives a measure of these soil moisture forces,a measurement which can be made with a high degree of accuracy and re-producibility within the moisture range that is of significant importanceto growing plants (12).

Fundamental principlesBecause an absorption block indicates the force with which moisture is

held by the soil environment, it may be considered to portray more accu-rately than conventional volume or weight percentages soil moisture condi-tions as they exist with respect to actively transpiring plants. The con-ventional percentages have, of course, long been recognized as inherentlyfallacious in this regard because they cannot express free energy factorsoperating in moisture phenomena. Figure 1 is a typical curve in the soilmoisture-block resistance relationship. It illustrates that the absorptionblock may offer at least a partial solution to soil water measurement prob-lems. Other investigators (1, 8, 10, 11, 13, 16) have confirmed the shapeand general characteristics of this curve, which, it is interesting to note,exhibits a marked resemblance to the moisture stress-soil moisture curvesrecently developed by WADLEIGH (15). This resemblance confirms thefundamental nature of the absorption block technique.

Unique features of the absorption block are that (a) the relationshipbetween moisture in the soil and block resistance is basically one of freeenergy and (b) the measurable range of sensitivity coincides with the

1 Authorized by the director for publication as journal article No. 955 of the Michii-gan Agricultural Experiment Station.

2 Garstka, W. U. Unpublished data. Michigan Hydrologie Research Project, SoilConservation Service, U. S. Department of Agriculture.

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BOUYOUCOS AND MICK: SOIL MOISTURE

critical variation of soil water. In other words, the range of the tech-nique corresponds with the quantity of moisture between the permanentwilting point and approximately the moisture equivalent. In terms ofthe absorption block technique, this available water is the water held in

u 1 5 10 1i 20 1 253.4 22.4

Permanent wilting Moisture equivalentPercent of water in the soil

FIG. 1. Curve illustrating the relationship between absorption block resistance andsoil moisture.

the soil by forces of such low magnitude that the block resistance is lessthan the asymptotic values which the curve approaches with respect to themoisture index. An arbitrary average resistance of 75,000 ohms for plasterof Paris absorption blocks has proved practical as an indicator of themaximum forces against which plants can obtain moisture from the soil

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(5). The steep gradient of that portion of the curve in question meansthat exceedingly small changes in the volume of total soil water give riseto relatively great resistance changes. These are of little practical sig-nificance, however, because of the small moisture volume changes. More-over, it is a generally accepted concept that the wilting point (by virtueof its definition) is a narrow range rather than a specific value (9, 14), andtherefore an arbitrary resistance value becomes a practical necessity.

At the other extreme, when the curve becomes asymptotic with respectto iesis-,ance, that is, whenl block resistance falls to a minimum constantlevel, the soil contains abundant moisture and is perhaps losing some waterin response to gravitational forces. Here again, experience has indicatedthat a convenient, although arbitrary, critical minimum value is about 600ohms. Rising resistance values indicate that the available soil moisture isdecreasing and that the portion remaining in the soil is being held withcorresponidingly greater forces.

For plaster of Paris absorption units an arbitrary resistance value ofat1out 75,000 ohms characterizes the permanent wilting point of a fairlylarge number of widely different soil samples, whereas resistance values of450 to 650 ohms characterize their moisture equivalents (2, 3, 5). Thisevidence suggests that, for practical purposes, these resistance values rep-resent their respective extremes of moisture status for all soils (exceptwhere high salt concentration, and thus high osmotic potentials, are en-countered). The validity of these relationships is confirmed by presentinformation which indicates that the soil moisture potential is about thesame for all soils when they are at their permanent wilting percentage or

at their moisture equivalent percentage (14). This feature of the absorp-tion block technique is thought to possess great practical implications, sinceit eliminates the necessity for calibrating the instrument except for exact-ing applications.

A more accurate and more fundamental concept is that for any givenresistance, different soils hold water with approximately the same force.This statement is comparatively exact when limited to the drying portionof the soil water fluctuation cycle, which is the phase of primary interestin forecasting crop water requirements. It is because of hysteresis exhib-ited between drying and wetting tension curves, because at any givenmoisture content different soils do not exhibit the same pressure deficien-cies, and because of osmotic differences that an apparent variation is en-

countered in any attempt to correlate thermodynamic measurements withmore or less empirical constants. Statistical significance cannot be ex-pected to obtain, at least not to a high degree, between such attributes.Nor is a high degree of significance necessary, because the moisture con-stants are of little value in actual field management practices. Despitetheir inherent inadequacies, however, the constants are here employed tofacilitate the presentation of this stechnique, because they are familiarterms, and because thev have been widely, although often erroneously,used.

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Detailed studies may be accomplished by calibrating absorption blockresistances against soil moisture, correcting all measurements to a standardtemperature. Other investigators (10) have perfected a fairly rapid andsatisfactory calibration technique which enlarges the scope of this method,adapting it to many agricultural, industrial, and even fundamental re-search studies involving soil moisture forces, stresses, flows, and also vol-ume and mass relationships. It is emphasized, however, that calibrationis not necessary to the use of the standard block resistance as an indicatorof the force with which moisture is held by the soil, or as an indicator ofapproximate volume and mass relationships, that is, the amount of avail-able water present in the soil.

Enhancing the value of the absorption unit resistance as a soil moistureindicator for use in large-scale installations are the following advantages:

1. After the initial installation, which is relatively simple, the soil neednot be disturbed.

2. Readings may be made by unskilled labor.3. Single readings require a minimum of time, generally not in excess

of 1 minute; several hundred readings can be made by a single operatorin the course of a working day.

4. Absorption units may be completely buried so that their presencedoes not interfere with surface tillage or plant growth or in any other waywith crop production.

5. Absorption units are not costly; this, together with the ease of ob-taining individual measurements, makes feasible a large number of repli-cations.

Another advantage of this method is that it gives an indication of thetime at which soil freezes, a factor that is sometimes of interest in hydro-logic investigations (7, 16). Freezing within the soil mass is indicated byan abrupt rise in the resistance of the absorption unit, caused bv a sharpdecrease in its conductivity as moisture within it changes from the liquidto the solid state. This transition point cannot be obtained by the use ofthermometers, since temperatures considerably below the critical point maynot result in freezing within the soil.

Plaster of Paris units

Under Michigan climatic conditions, plaster of Paris blocks have func-tioned satisfactorily in well-drained soil profiles for more than five years.On the other extreme, waterlogged conditions encountered in low topo-graphic positions reduce their useful life to a single growing season; thisis particularly true in organic soils where the CaSO4 dissolves rather rap-idly. In intermediate drainage positions, the useful life of the block ap-pears to be determined bv the relative length of time it is exposed to satu-rated conditions. In general, the higher and drier its position, the longerit will function. Freezing conditions do not appear to deteriorate theblocks as rapidly as does solution; considerable physical disintegration,

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PLANT PHYSIOLOGY

especially at the corners, is observed after exposure to repeated freezingand thawing, but this superficial disintegration has little effect on the re-sistance characteristics of the block, unless the volume between the elec-trodes has been substantially reduced.

Investigations concerning the design of the absorption block have led tothe conclusion that the original specifications are indeed practical, althoughperhaps not so theoretically sound as, for example, a coaxial design. Studiesof cylindrical blocks containing concentrically arranged electrodes showedthat the effects of external electrical fields were minimized. Difficultieswere encountered, however, in casting such blocks with a high degree ofuniformity. The relative simplicity of the present rectangular pattern,and the ease and cheapness with which such absorption units can be manu-factured appear to offset any theoretical advantages of a coaxial absorptionunit. Large quantities of commercially manufactured blocks made accord-ing to the original specifications (4) include an average of two defectiveunits in every hundred.

Many varieties of material are marketed as plaster of Paris. Differencesin both the physical and chemical characteristics of these materials arecaused by the varying quantities of hasteners or retarders which are addedto control setting speeds and otherwise standardize a given product for com-mercial and technical uses. In general, these regulating agents have anunfavorable influence on the soil water-resistance relationships of the blocks;chemically pure materials exhibit a wider range of resistance change for agiven change in soil moisture and therefore contribute to the sensitivity ofthe method. Pure gypsum (sold by the United States Gypsum Companyas No. 1 White Molding Plaster of Paris) has proved extremely durable andthe best material so far investigated. It sets in about 30 minutes andpossesses a very porous, although relatively soft, structure.

Two materials of a somewhat similar nature, marketed as Hydrostoneand Hydrocal, have been used, because of their extreme hardness, by severalinvestigators. It was thought that this hardness might indicate greaterdurability and that blocks made of hard materials would last longer whenburied in a soil. Experience with these materials has shown, however, thattheir high density is complemented by a low porosity which reduces notonly the relative proportion of water that can be absorbed but also thespeed with which it moves within the block and between the block and thesoil. The sensitive range of these blocks is narrow, possibly because of thenarrow pore-size distribution characteristics. A third objection is the lowsolubility of these hard materials, which reduces their buffer capacity withrespect to salt concentration changes within the soil solution.

Other kinds of absorption unitsFor the agronomist and other investigators interested in plant-soil re-

lationships, plaster of Paris block appears to be an ideal unit because itcovers the range of available water, indicating both the field capacity and

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the wilting point of a soil. An absorption unit with a wider range would,however, prove a useful tool especially to the engineer interested in hydrol-ogy or construction. An extensive search has therefore been conducted tofind absorbent materials with energy characteristics and physical propertiessuperior to those of plaster of Paris, and with the capability of indicatingmoisture changes up to the saturation point. A large number of clay andcement substances were examined. Also tested were many fibrous materialssuch as glass fabrics and nylon. Specially designed contact electrodes werere-examined, with the view toward minimizing their former inadequacies.

In Figure 2 are shown six types of resistance units which have givengood laboratory performances. With the exception of Unit 5 all of thesedevices consist essentially of three parts: (1) a porous absorbent (2) elec-

FIG. 2. Six types of experimental absorption units. Their construction is de-seribed in the text.

trodes of some resistant metal aiid (3) well-inisulated, weather-proof leadswhich serve to join the electrodes to a resistance measuring apparatus.These units are dissimilar in that their electrode patterns differ and thattheir absorbent materials are not alike, either physically or chemically. Theconstruction of these units is briefly described below:

UTNIT ELECTRODES1. Single-strand monel wires, widely-spaced, and

woven into the fabric.2. Triplicate stainless steel wires arranged alter-

nately in series, and woven into the fabric3. Medium spaced monel wires stitched to fabric4. Heavy i-inch monel hardware cloth5. Heavy i-inch galvanized iron hardware cloth6. Multiple-strand tinned lamp cable

ABSORBENT

Fiberglas or nylon

NylonNylon or fiberglasNylon or fiberglasNonePlaster of Paris

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PLANT PHYSIOLOGY

In all units but No. 5 the resistance that develops between the electrodeswhen the unit is imbedded in a drying soil is caused to a large extent bythe decrease in conductivity of the porous absorbent. In Unit 5, however,

Soil moisture in per cent0 10 20 30 40 50 60

0 10 20 30 40 50 0 10 20 30 40

0 10 20 30 40 50 60

Soil moisture in per cent

FIG. 3. Typical moisture-resistance curves for the several types of soil moisturemeasuring units. Moisture indices, in terms of dry weight percentages, are indicatedabove and below for each of the upper and lower curves, respectively. Unit 6 wasimbedded in a light sandy loam, all others were in different kinds of silt loams.

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the resistance increases as the soil dries because of decreased conductivitywithin the soil mass proper. Figure 3 shows a selection of typical labora-tory curves obtained with these various units imbedded in different soils.These curves are all similar. They approach minimum values when thesoil is at or near saturation. On the opposite extreme they approach maxi-mum values at minimum moistures. Depending on the pore-size distribu-tion of the absorbent materials all of these units possess the capability ofyielding a moisture index over a wide or narrow range. With the excep-tion of Unit 6 all units are sensitive nearly to the point of saturation.

It has already been pointed out that Unit 6 is unique in that its curveyields a close approximation of the permanent wilting per cent. and fieldcapacity. Unit 4 is of special interest because it appears to operate overan extremely wide moisture range. Preliminary tests indicate that ityields fairly accurate and reproducible results from air-dry conditions tosaturation and for this reason it is being developed and refined. Becauseexperimental errors cannot be ignored, the range of these curves is only oneaspect of their utility. Unit 6, the standard plaster of Paris block, althoughoperating over the smallest range, exhibits the smallest scatter of itemswithin its range. Not only does this unit show the smallest experimentalerror within a single drying cycle, but it also shows the least hysteriesisbetween successive drying cycles, thus illustrating the unique buffering,function of its slightly soluble gypsum absorbent.

Despite the advantage of being responsive over a wide range of mois-ture conditions, units other than 6 possess certain inherent disadvantagesin addition to their relatively large experimental errors. Pressure changesresulting from swelling and shrinkage not only contribute to these inherenterrors but they may, under extreme conditions of dryness, result in a separ-ation of the absorbent from the soil matrix which interrupts the operationof the unit. Plaster of Paris units not only withstand these pressurechanges but they will continue to function as long as a small area of con-tact remains betweeni the absorbent wall and the soil. The plaster of Parisunits are therefore favored, especially where plant-soil relationships are in-volved, because they permit more quantitative measurements than do theother types, even though lthe latter are more sensitive to moisture changes.Where it is desirable to measure the entire range of moisture conditions,from the air-dry state to saturation, then Unit 4 is recommended. A care-ful application of this technique will yield satisfactorv results.

The resistance bridgeA major obstacle in the development of a practical procedure for field

use was the lack of a satisfactory resistance measuring device. Commer-cial meters already on the market possessed many disadvantages chief ofwhich wAere a limited racge, delicate conistruction, bulkiness, and high cost.In order to overcome these obstacles and to benefit from recent advances ininstrumentation it was necessary to construct a special resistance bridge.

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PLANT PHYSIOLOGY

As now manufactured, this improved bridge combines rugged, compactconstruction with a high degree of sensitivity. Both the resistance bridgeand the plaster of Paris blocks are manufactured and sold by The Wood andMetal Products Company, Bloomfield Hills, Michigan. The improvedX~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

C A:

FIG. 4. The resistance bridge used with the absorption units described. The uniti.s self-conitained and easily portable, weighing about 20 pounds.

bridge is a completely self-contained unit (Fig. 4) which is designed to mea-sure resistances in circuits containing considerable capacitance, such as isencountered in installationis where the blocks may be connected by up to200 feet of commercial multiple-strand rubber-coated copper wires.

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BOUYOUCOS AND MICK: SOIL MIOISTURE

Experience with many designs has indicated that a wooden case helpsto shield the instrument from ground currents, a factor in metal-cased in-struments that invariably causes difficulties under certain combinations ofcircumstances frequently encountered in the field. Headphones are greatlypreferred to an "electric eye" indicator. The electric eye, although asatisfactory null indicator in the laboratory, is difficult to read under fieldlight unless a shadowbox or curtain is used. This increases the bulkinessof the instrument and adds to the difficulties of manipulation. Galvan-ometers, likewise, proved unsatisfactory for general field use, inexpensivecommercial models being too delicate or else, on the other extreme, lackingin sensitivity. A relatively inexpensive headphone set, however, has proveddurable and efficient for general outdoor use. Making use of the soundcharacteristics of the necessity oscillating current, headphones have provedto be the most useful type of null indicator. In conjunction with the pres-ent circuit design, a great contrast in tone volume, which rapidly fadesto a minimum level within an extremely narrow range, contributes to the easeof adjusting the instrument. For prolonged operation, sponge rubber cush-ions over the phones have been found to be helpful by reducing the inter-ference of extraneous sounds such as are produced by wind currents. Theyalso distribute the mechanical pressure over the entire ear cartilage, whichadds to the comfort of the operator.

The circuits of the improved bridge are based on the Wheatstone prin-ciple. To avoid the influence of various capacitance factors present infield installations, a large condenser has been included which contributesgreatly in obtaining a good null balance within the bridge. The instrumentis powered by dry-cell batteries feeding through a 2000-cycle electronicoscillator. An extremely wide range of sensitivity is obtained by insertingtwo series of standardized resistances in opposite arms of the bridge; theproper conmbination is selected bv means of multiplier switches, and the finalnull-point is obtained by adjustinig a logarithmic potentiometric rheostatfitted with a 6-inch graduated dial. The bridge is thus balanced, or tunedto the null point, by manipulating five dials in a matter of seconds. Anohm-miieter modification of this circuit has been successfully used for severalseasons to control irrigation on experimental fields. When the absorp-tion block leads are plugged into this moisture meter, movement of a needleacross the dial indicates the extent to which soil moisture has been depleted.

Summary1. The electrical resistance technique of obtaining a continuous measure

of soil moisture in situ under field conditions by means of absorption unitsis discussed in the light of additional knowledge and experience gainedsince the inception of the method in 1940.

2. Fundamental considerations of the characteristics of this techniquereveal that for most practical purposes standard plaster of Paris absorp-tion blocks need not be calibrated. Resistance readings may be directly

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PLANT PHYSIOLOGY

interpreted in terms of available soil water; in all soils the percentage ofavailable water is approximately the same for any given resistance value.

3. Several types of adsorption units are described and compared bymeans of laboratory calibration curves. Some of these units offer greatpromise of measuring soil moisture from saturation to dryness.

4. The advantages of the method are summarized.DEPARTMENT OF SOIL SCIENCE

MICHIGAN STATE COLLEGEEAST LANSING, MICHIGAN

LITERATURE CITED

1. ANDERSON, A. B. C., and EDLEFSON, N. E. Laboratory study of the re-sponse of 2- and 4-electrode plaster of paris blocks as soil moisturecontent indicators. Soil Sci. 53: 413-428. 1942.

2. Bouyoucos, G. J. A comparison between the suction method and thecentrifuge method for determining the moisture equivalent of soils.Soil Sci. 40: 165-171. 1935.

3. Bouyoucos, G. J. The dilatometer method as an indirect means ofdetermining the permanent wilting point of soils. Soil Sci. 42:217-223. 1936.

4. Bouyoucos, G. J., and MICK, A. H. An electrical resistance methodfor the continuous measurement of soil moisture under field condi-tions. Michigan Agr. Exp. Sta. Tech. Bul. 172. 1940.

5. BouYoucos,. G. J., and MICK, A. H. Improvements in the plaster ofparis absorption block electrical resistance method for measuringsoil moisture under field conditions. Soil Sci. 63: 255-265. 1947.

6. COLEMAN, E. A. The place of electrical soil-moisture meters in hydro-logic research. Trans., Amer. Geophys. Union 27: 847-853. 1946.

7. COLEMAN, E. A. Manual of instructions for use of the fiberglas soil-moisture instrument. California Forestry and Range Exp. Sta.,Berkeley. 1947.

8. CUMMINGS, R. W., and CHANDLER, R. W., JR. A field comparison ofelectrothermal and gypsum block electrical resistance meth,odswith the tensiometer method for estimating soil moisture in situ.Soil Sci. Soc. Amer. Proc. 5: 80-85. 1941.

9. EDLEFSON, N. E., and ANDERSON, A. B. C. Thermodynamics of soilmoisture. Hilgardia. 15: 31-298. 1943.

10. KELLEY, 0. J. A rapid method of calibrating various instrunmentsfor measuring soil moisture in situ. Soil Sci. 58: 433-440. 1944.

11. ]KELLEY, 0. J., HUNTER, A. S., and HOBBS, C. H. The effect of mois-ture stress on nursery-grown guayule with respect to the amountand type of growth and growth response on transplanting. Jour.Amer. Soc. Agron. 37: 194-216. 1945.

12. KELLEY, 0. J. et al. A comparison of methods of measuring soil mois-ture under field conditions. Jour. Amer. Soc. Agron. 38: 759-784. 1946.

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BOUYOUCOS AND MICK: SOIL MOISTURE 543

13. SLATER, C. S., and BRYANT, J. C. Comparison of four methods of soilmoisture measurement. Soil Sci. 61: 131-156. 1946.

14. VEIHMEYER, F. J. Report of the committee on physics of soil mois-ture, 1936-37. Trans. Amer. Geophys, Union. 18: 302-318. 1937.

15. WADLEIGH, C. H. The integrated soil moisture stress upon a root sys-tem in a large container of saline soil. Soil Sci. 61: 225-238.1946.

16. WHITE, R. G. Installations f-or noting the water and thermiial rela-tionships in soils. Agr. Eng. 37: 21-25. 1946.



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