IMEASUREMENTS OF SOIL WATER RELATIONS 257 



capacity of the soil. Even this latter procedure does not avoid a more funda- 

 mental error which is inherent in the technique of the method itself. When 

 water is allowed to drain from the soil in such determinations the lowest 

 layer of soil particles is in contact — not with the soil as is the situation under 

 field conditions — but with the atmosphere. At the soil-atmosphere interface 

 films of water develop between the soil particles which can only be displaced 

 by a very considerable force. These films interfere with free drainage and 

 cause the accumulation of considerably greater quantities of water in the soil 

 than would occur with free drainage under field conditions. In the field each 

 layer of soil particles is in turn in contact with a lower layer of soil particles 

 and if drainage can occur freely the water content of the soil is ordinarily 

 reduced to the point that capillary movement of the water ceases. 



3. Moisture Equivalent. — The moisture equivalent is defined as the per- 

 centage water content which a soil can retain in opposition to a pull one 

 thousand times that of gravity (Briggs and IMcLane, 1907). Such a pull 

 is equivalent to a pressure of about one atmosphere. It is determined by 

 placing samples of the soil in especially designed cups with a perforated bottom 

 and whirling them in a centrifuge for (usually) one-half hour (Veihmeyer, 

 et ah, 1924). This displaces all of the more loosely held water; that retained 

 by the soil is determined by the usual method of oven-drying, and expressed 

 as a percentage of the dry weight. 



The moisture equivalent of a soil is a purely empirical determination, but, 

 except in very sandy or in very heavy clay soils, it closely approximates the 

 field capacity (Veihmeyer and Hendrickson, I930- Moisture equivalents 

 therefore exhibit about the same range of numerical values as shown by field 

 capacities, ranging from about 5 per cent in very sandy soils to about 35 per 

 cent in clay loams. The principal utility of this determination is that it pro- 

 vides a laboratory measure which closely approximates the field capacity. 



4. Wilting Percentage. — The foregoing measurements of the water rela- 

 tions of soils are all purely physical determinations. The wilting percentage 

 (also called ivilting point or ivilting coefficient) is a physiological measure of 

 the water relations of soils. It is defined as the percentage water content of 

 a soil after the plant or plants growing in it have just reached the condition 

 of permanent wilting (Briggs and Shantz, \912a). A permanently wnlted 

 plant is usually considered to be one which will not recover its turgidity unless 

 water is supplied to the soil (Chap. XVIII). 



In order to determine the wilting percentage of a soil a sample is enclosed 

 in a waterproof vessel. The test plant is generally allowed to develop from 

 seed in the soil sample until it has attained a suitable size. The soil surface 

 is then sealed over so that all loss of water from the system occurs through 



