164 THE PRINCIPLES OF SOIL MANAGEMENT 



is directly beneficial, and the process constitutes a form 

 of natural sub-irrigation. 



The figures presented above illustrate the effect of 

 texture on the total water capacity of a soil, and upon 

 the proportion of gravitational water. Anything which 

 increases the pore space increases the total water capac- 

 ity. When there is not a corresponding increase in 

 the capillary capacity, as happens in a sandy soil, the 

 total amount of gravitational water is thereby increased. 

 That is, in such a soil, there is a larger amount of water 

 which may be lost by percolation. In so far as organic 

 matter alters the structure of the soil, it modifies the 

 gravitational water content of a soil in the manner 

 just outlined. 



79. Amount and rate of loss. Near the outset of 

 the discussion of soil moisture, it was stated that the 

 amount of water in a soil depends upon the extent and 

 rate of loss of water, as well as upon the factors which 

 have just been explained. For example, fifteen inches 

 of water is far more efficient in crop production when 

 applied to a loam soil in a humid region, like the New 

 England states, than when applied to the sand of the 

 Imperial Desert, California. In the latter case, the loss 

 by percolation and evaporation is so great and so rapid 

 that the amount of moisture available to crops is very 

 small. The two forms of loss which affect the moisture 

 in the soil are: (1) Percolation. (2) Evaporation. 



Percolation is the gravitational flow of water through 

 the pores of a soil. Percolation concerns the gravitational 

 water. The total loss in any given soil will depend upon 

 the distribution of the rainfall or the irrigation supply. 



