DIVISION S-l-NOTES

SOIL WATER PRESSURE MEASUREMENTSIN SUBZERO AIR TEMPERATURES

D. S. GORDEN AND P. L. M. VENEMAN*

AbstractAir temperatures below freezing often preclude soil water pressure

measurements with tensiometers. The use of antifreeze solutions facili-tates tensiometer use in weather when air temperatures are below0°C yet the soil remains unfrozen. We studied the applicability ofmethanol concentrations ranging from 100 to 500 cm3 L*1. Soil waterpressures were measured with ceramic cup tensiometers installed at15-crn depth within 38-L-capacity buckets. Experimental temperatureswere - 2, 0, 5, and 15°C. The pressures ranged from 0 to - 60 kPa.Readings of the tensiometer with the methanol-water mixture werewithin 0.6 kPa of that of the tensiometer with pure water. No statisti-cally significant difference (a = 0.05) in soil water pressure was foundbetween the methanol-water mixtures and the pure water systemusing 1:1 lines and the Wilcoxon signed rank test for paired data.The results indicate that field tensiometer data may be collected inunfrozen soils by the use of the appropriate methanol concentrationbased on the temperature range expected.

TENSIOMETERS are used to measure soil water pressurein a wide range of environments. Tensiometer use

in the winter is limited because they become inoperablein freezing weather (Wendt et al., 1978). In freezingweather, tensiometers have been buried below groundlevel or installed in heated enclosures for protectionagainst freezing. Wendt et al. (1978) conducted a green-house study using an Olton loam soil (fine, mixed, ther-mic Aridic Paleustoll) to determine the effect of a 300cm3 L~' methanol-water mixture on tensiometer readingsand plant growth. Wendt et al. (1978) reported no majoreffects on either tensiometer readings or plant growth.The objectives of this particular study were to: (i) broadenthe temperature base of the Wendt et al. (1978) studyby assessing methanol-water and pure water variationsunder a range of temperatures more representative ofcold weather conditions, and (ii) extend the range inmethanol concentrations compared with the Wendt et al.(1978) study.

Materials and MethodsA thin layer of gravel was placed in five 38-L insulated

buckets that had several 0.1- and 0.3-cm2 holes drilled intothe bottom to provide drainage. The buckets were then filledwith a sandy loam soil obtained from the Bw horizon of amoderately well-drained Rainbow soil (coarse-loamy, mixed,mesic Aquic Dystrochrept) and brought to saturation. Tensio-meters were constructed by mounting 100 kPa porous cups to

D.S. Gorden, R.F. Weston, Inc., 99 South Bedford St., Suite 5, Burlington,MA 01803; and P.L.M. Veneman, Dep. of Plant and Soil Sciences, 19Stockbridge Hall, Univ. of Massachusetts, Amherst, MA01003. Received10 Jan. 1994. "Corresponding author (veneman@pssci.umass.edu).

Published in Soil Sci. Soc. Am. J. 59:1242-1243 (1995).

one end of a polyvinyl chloride (PVC) tube (22 mm o.d. by13 mm i.d.) and securing a clear plastic tube (15 mm o.d. by12 mm i.d.) of 10-cm length to the other end using epoxycement. The ends of the PVC tubes were widened to providea snug fit for both the neck of the porous cup and the clearplastic tube. Two 25-cm-long tensiometers were installed ata distance of = 10 cm in each one of five buckets, with themiddle of the cup situated at a depth of =15 cm below thesoil surface. One tensiometer in each bucket contained deairedwater, whereas the other tensiometer contained either a 100,200,300,400, or 500 cm3 L~' methanol-deaired water mixture.The buckets with tensiometers were placed in a regulatedtemperature chamber at 15°C and elevated slightly off theground to facilitate drainage. Soil water pressures were mea-sured daily at noon with a Tensimeter (Soil MeasurementSystems, Tucson, AZ) for at least 10 consecutive days or untilthe water pressure reached —50 kPa. Soil temperatures weremeasured at the same frequency in those buckets containingthe 100 and 500 cm3 L~' methanol mixtures with YSI thermis-tors (no. 15-176-22, Yellow Springs Instruments, YellowSprings, OH) installed at 15-cm depths. Each bucket of soilwas allowed to drain for a minimum of 10 consecutive daysor until the water pressure reached —60 kPa. After 10 d,the tensiometers were removed from the soil, the soil wasresaturated ( = 0 kPa), the temperature chamber was reset to5°C, and the experiment was repeated. Similar experimentswere carried out at temperatures of 0 and —2°C. The Wilcoxonsigned rank test for paired data was employed to test thestatistical significance of the data at the 95 % confidence level(a = 0.05).

Results and DiscussionWater pressure data for temperatures of 15, 5, and

0°C are shown in Fig. la, Ib, and Ic, respectively. TheWilcoxon signed rank test at the 95% confidence level(a = 0.05) indicated little variation (P values rangingfrom 0.0011 to 0.0360) between the readings of the purewater and the methanol-water solution at temperaturesof 15, 5, and 0°C. Higher methanol concentrations inthe tensiometer did not alter this relationship. At freezingtemperatures (-2°C), deviation in the readings of thepure water and the methanol-water solution was observedwhen the pure water started freezing after =2 d.

Wendt et al. (1978) observed that tensiometers con-taining the methanol-water solutions had slightly lessnegative water pressures than those containing pure wa-ter. We observed similar trends in this study. For atensiometer to operate, it is necessary for small quantitiesof the methanol-water mixture to move from the tensio-meter cup into the soil (Wendt et al., 1978). In general,pure water has a higher specific gravity and a lowersurface tension than methanol. Since the mixture hasthis difference in properties from pure water, some devia-tion in readings between tensiometers should be expected.A maximum difference of 0.6 kPa was observed betweenthe pure water and the methanol-water mixture at temper-atures of 15, 5, and 0°C. Wendt et al. (1978) concluded

Abbreviation: PVC, polyvinyl chloride.

1242

NOTES 1243

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Soil water pressure (kPa)

Fig. 1. Soil water pressures measured with mixtures of methanol-deaired water (x axis) vs. soil water pressures of deaired water(y axis) at (a) + 1S°C, (b) + 5°C, and (c) 0°C. Legend indicatesthe percentage of methanol by volume, curve indicates a 1:1 line.

that the observed differences between 1 and 2 kPa wereinsignificant when tensiometers were used for schedulingirrigations, and we draw similar conclusions with respectto most field morphological studies. When precise waterpressure values are required, the observed differencemay be of significance.

If differences between pure water tensiometer readingsand methanol-water tensiometer readings are statisticallyinsignificant for air temperatures ranging between 0 and15°C, it can be postulated that this same assessment istrue for air temperature readings below freezing eventhough this relationship cannot be properly assessed be-cause of the freezing pure water system. Extension ofthe monitoring season may be important for soil morpho-logical field studies. Wendt et al. (1978) presented atable indicating appropriate methanol concentrations toprevent freezing of tensiometers at a certain temperature.For example, the average daily winter temperature forAmherst, MA, is -3°C (Swenson, 1981). Measurementsin a local forested hemlock [Tsuga canadensis (L.) Car-riere] swamp indicated that the soil temperature neverfell below -0.1 °C at 25 cm or below 3.2°C at 50-cmdepth (Gorden, 1994). Use of a 300 cm3 L~' methanol-water mixture allowed us to obtain data essentiallythroughout the winter as long as the soil remained un-frozen. The use of methanol in a soil coupled withbiological degradation may lead to a methanol concentra-tion gradient. Microbial activity during the winter proba-bly is not extensive. However, when using methanol-water mixtures, the user should be aware of the possibilityof biologically induced artifacts. Monitoring possiblechanges in methanol concentration and the total liquidvolume may be useful under such circumstances.

Our data indicate that (i) tensiometers can be protectedfrom freezing by the use of methanol mixtures withoutsignificantly affecting gauge readings and without causingrapid deterioration of either tensiometers or the adhesiveglue used, and (ii) there is no statistically significantdifference between water pressures using pure water ormethanol-water mixtures at low temperatures. The use oftensiometers with the appropriate methanol concentrationunder freezing conditions offers researchers the abilityto determine soil water pressures during the winter seasonwhen air temperatures are below 0°C and the soil is notyet frozen.

AcknowledgmentsThis work was funded by the U.S. Army Corps of Engineers

Waterways Experiment Station under Contract DACW 39-91-k-0006.