I.S. M c Q u e e n a n d R . F . Miller
U . S . Geological Survey, W a t e r Resources Division, Soil a n d Moisture Conservation P r o g r a m , Federal Center, D e n v e r , Colorado, U . S . A .
A B S T R A C T : A moisture stress measurement for each gravimetric soil moisture sample, m a d e possible by development of a wide-range method using filter paper as stress sensors, is supplying answers to some difficult hydrologie problems. The moisture stress existing in a gravimetric sample can be measured within 2 percent and the range covered is from 0 to 1500 bars. Calibration of the method was accomplished in the laboratory and in the field using saturated salt solutions (15 to 1500 bars), a pressure m e m b r a n e extractor (1 to 15 bars), a pressure plate assembly (0.1 to 1 bars), and equilibrium above a water table (0 to 0.2 bars).
Disturbance of the sample has little effect on results at stress levels above 0.2 bars. A t stress levels below 0.2 bars, special handling and sampling techniques should be used to obtain samples with m i n i m u m disturbance.
R É S U M É : Des réponses à quelques difficiles problèmes hydrologiques peuvent être données par la mesure des tensions dues à l'humidité pour chaque échantillon d'humidité d u sol gravimétrique, mesure rendue possible par le développement d'une large méthode utilisant du papier filtre c o m m e élément sensitif aux tensions. Les tensions dues à l'humidité existant dans un échantillon gravimétrique peuvent être mesurées à 2 % pour un éventail de tensions comprises entre 0 et
1500 bars. Le tarage de la méthode se fait au laboratoire et en campagne en utilisant des solutions salines saturées (15 à 1500 bars), une m e m b r a n e à pression (1 à 15 bars), et un assemblage de plaques pour détermination des pressions (0 à 1 bar), et l'obtention de l'équilibre au-dessus de la nappe phréatique (0 à 0,2 bars).
D e légères modifications de l'échantillon n'ont qu'une faible action sur le niveau des tensions supérieures à 0,2 bars. Pour des tensions inférieures à 0,2 bars, une manipulation et un échantil-lonnage spéciaux doivent être utilisés pour obtenir des échantillons avec différences minimales.
1. Published with permission of the Director of the U . S . Geological Survey as partial results of projects titled: 'Interrelationships between ion distribution and water m o v e m e n t in soils and associated vegetation' and 'Soil-moisture energy relationships under and adjacent to riparian vegetation in Arizona'.
I. INTRODUCTION
The soil-moisture research staff of the Soil and Moisture Conservation Program, Water Resources Division, U . S. Geological Survey recognized a need for a wide-range method for measuring moisture stress in soils while conducting studies on the moisture require-ments of arid land plants in the western United States. The standard or popular methods for measuring moisture stress either did not cover the range of stress values expected on arid lands or they were not adaptable to use on field samples.
Various methods, described in the literature, were examined to determine if they could be used for arid lands research. A method proposed by Robert Gardner about 1936 using filter papers as indirect moisture stress sensors was investigated, modified, and eventually adopted.
II. H I S T O R Y
The use of paper as a moisture stress sensor has gradually evolved in Europe and the United States. Hansen (1926) working at the University of Copenhagen used blotting paper as a carrier for sugar solutions. Blotting paper strips, saturated with four different concentrations of sugar solutions, were exposed to soil samples in closed chambers.
The sugar solution that did not lose or gain weight was assumed to represent the stress level in the sample. Stocker (1930) used a similar procedure with a larger number of sugar solution concentrations for better accuracy. G r a d m a n n (1934) improved the method by using a single strip of blotting paper soaked in a salt solution and then calibrated for weight versus stress. These sensors were enclosed with soil samples until complete equilibrium was reached. This method with some minor refinements has been used in France by Eckardt (1960).
The first use of paper as a moisture stress sensor without a hygroscopic salt and probably the only previous use in the United States w a s reported by Gardner (1937).
III. M E T H O D
The basic concepts of using filter paper as a passive gravimetric moisture stress sensor as proposed by Gardner have been followed by the authors but details of the method have been changed to eliminate s o m e hazards and difficulties and adapt it to use with routine gravimetric soil moisture sampling programs.
A . A P P A R A T U S A N D SUPPLIES
In addition to the equipment required for routine gravimetric soil moisture tests, the following are needed:
1. an analytical balance accurate to 0.0002 g;
2. small lightweight weighing boxes such as Soiltest Catalog N o . L T - 1 5 ; 3. constant temperature chamber (20 ° C ) ;
4. filter paper — Schleicher and Schuell N o . 589 White Ribbon, 5 ^ c m dia. circles were used in this study (other grades of paper m a y require calibration);
5. pentachlorophenol ' D o w c i d e - 7 ' reagent grade or equivalent if obtainable;
6. ethanol or methanol reagent grade solvent ; 7. plastic electrical tape to seal soil moisture cans.
B. PROCEDURE
1. Filter paper discs are pretreated by dipping them into a 2 percent solution of pentachlo-rophenol in ethanol and allowing to air dry.
2. O n e treated filter paper disc is placed in the top of each gravimetric soil moisture sample w h e n it is obtained in the field and the can is sealed with plastic tape.
3. The samples are transported to a laboratory and allowed to equilibrate in a constant temperature chamber at 20 °C for one week.
4. T h e filter paper is removed from the soil sample can and its moisture content is accurately determined. T h e average dry weight of the filter papers is 0.2 g. W h e n exposed to air there m a y be rapid changes in their moisture contents. Special weighing techniques are used to limit exposure to air. T h e soil sample m a y be treated as a routine soil moisture content sample after the filter paper is removed.
5. T h e calibration curve for the filter papers (fig. 1) m a y be used to determine the total soil moisture stress from the moisture content of the filter paper or the stress m a y be computed.
F I G U R E 1. Moisture content of filter paper (per cent of dry weight)
IV. C A L I B R A T I O N
The filter papers were calibrated under conditions and procedures that were as much as possible like those that exist during normal use of the method. However, additional conditions and procedures were investigated to help define limits of accuracy and time requirements for equilibrium.
A . F O R H I G H S T R E S S
For stress levels above 15 bars, filter papers were exposed to saturated salt solutions in closed containers in a constant temperature chamber. Periodic weihings were m a d e of
both initially wet a n d initially dry papers to define the time required for equilibrium and to determine equilibrium moisture contents. Technical data for this phase of the calibra-tion are given in table 1.
T A B L E 1. Calibration data for stress levels controlled by saturated salt solutions in a constant temperature chamber
1. From a compilation of data by O'Brien (1948)
2. Computed from vapor pressure data in International Critical Tables, Washburn (1926)
3. Computed from Kelvin equation SQ =(RTlg)\n p\p0, where Rig — 4.60376 and plpQ relative vapor pressure 4. Exposed over distilled water for 2 days prior to exposure to saturated salt solution
5. Started at air dry condition
Results agree with the corresponding portion of Gardner's calibration curve for which he exposed the papers to sulfuric acid solutions in evacuated chambers. Available data for relative humidity above saturated salt solutions d o not agree but the range of disagree-ment s h o w n in table 1 is small.
B. F O R MEDIUM STRESS
For stress levels from o n e bar to 15 bars, samples of several soils were brought to given stress levels o n a pressure m e m b r a n e extractor and then sealed in cans with wet and/or dry filter papers. F o r points below one bar stress, soil samples were brought to given stress o n a pressure plate assembly and then sealed in cans with wet and/or dry filter papers.
T A B L E 2. S u m m a r y of calibration data from samples prepared on pressure membrane extractor (M) and pressure plate assembly (P)
Stress
1. See Richards (1954), page 109 2. M = ( 3 . 2 3 8 - l o g1 0SB) -f- 0.0723 3. M = [(9.8966- 1 0 ) - log,0SB] -f- 0.01025
4. Several samples rejected because of leakage in pressure plate