200 SOILS: PROPERTIES AND MANAGEMENT 



obvious, not only in comparing the water content of the 

 same soil, but in comparing different soils as well. 



In using a percentage of moisture based on the dry 

 soil instead of on the wet, the first of the above objections 

 is eliminated. Consequently this method of expression 

 is perfectly legitimate as long as soils having about the 

 same apparent specific gravity are compared. As soon 

 as soils of different weights are considered, however, a 

 more nearly accurate method must be employed. Ob- 

 viously, then, the only really rational mode of moisture 

 statement is by the volume method. In ordinary calcu- 

 lations of water, however, the percentage by dry weight 

 is generally used because of its simplicity and the facility 

 of expression that it affords. It is also much easier to 

 establish than a percentage based on volume. 



The first and second methods of volume expression are 

 of about equal value as far as direct comparison goes. For 

 the actual water present the number of cubic inches to a 

 cubic foot of soil is perhaps preferable, as it shows the exact 

 amount of water contained and may easily be converted 

 to pounds to a cubic foot or tons to an acre as the case 

 may be. The third volume statement is generally used in 

 field practice, especially in irrigated regions, where water 

 is measured in inches in depth to an acre of area. Such 

 a statement of the available water in a soil not only is 

 convenient, but also gives a direct comparison with the 

 probable rainfall of the growing season. 



132. Kinds of water in the soil. As has already been 

 demonstrated, a soil of a definite volume weight has a 

 definite pore space which may be occupied by air or by 

 water, or shared by both, as the case may be. Of course, 

 an ideal soil for plant growth is one in which there is 

 both air and water, the proportions depending on the 



