256 SOILS AND SOIL WATER RELATIONS 



Laboratory Measurements of Soil Water Relations. — The problem of 

 the water relations of soils has been approached both from the standpoint of 

 laboratory and field studies. Some of the broader generalizations resulting 

 from field studies have been just described. Of the numerous laboratory meas- 

 urements of the water relations of soils which have been devised, only a few 

 of the better known will be discussed. 



1. Water Content. — The water content of a soil is commonly expressed as 

 the percentage of water present in terms of the dry weight of the soil. It is 

 measured by drying a sample of the soil in an oven to constant weight at 

 (usually) 105° C. and assuming that the loss in weight represents water. 

 There is however nothing critical about the temperature 105° C. The 

 relation between the water content as determined by this method and the 

 temperature at which the soil is dried is a linear one over a wide range of 

 temperatures extending on both sides of this point. This shows that there 

 is no especial significance to be attached to the amount of water which is lost 

 from a soil when dried at this temperature. There is also a small error in 

 such determinations due to some decomposition of organic matter at this 

 temperature. 



Determinations of the total water content of the soil contribute very little 

 to an understanding of the absorption of water from the soil by plants. As 

 will be shown later the amount of water available to plants may be very differ- 

 ent in two soils of identical water contents. 



2. Water Retaining Capacity. — One of the most familiar laboratory de- 

 terminations of soil water is that of the so-called water retaining capacity 

 (also called moisture holding capacity, ivater holding capacity, water capacity. 

 etc.). This determination is usually made as follows: A shallow cylindrical 

 pan with a perforated bottom is filled with dry soil, immersed in water to the 

 depth of a few millimeters, and allowed to stand until it becomes saturated. 

 The soil is then allowed to drain free of all the water which will drip out 

 under the influence of the pull of gravity. The water content of the soil 

 after it has been subjected to this treatment, determined by the usual method 

 of oven-drying, is regarded as its water retaining capacity. 



It has often been assumed that a determination such as that just described 

 is an indication of the amount of water which would be held by that sod m 

 the field in opposition to the pull of gravity but such an assumption is entirely 

 unjustified. In the first place in making such determinations the vessel is 

 filled with the soil in such a way that the distribution of particles in the soil 

 mass is very different from that which obtains in the field. This error is 

 sometimes minimized by cutting out a core of soil in the field by means of a 

 steel cylinder and using this core for a determination of the water retaining 



