CONTROL OF SOIL MOISTURE 



191 



of the soil is increased or, (b) the average water content 

 of the soil is decreased. If the crop is likely to suffer 

 from a deficiency of water, or from conditions associated 

 with a deficiency of water as food, we aim to in- 

 crease the moisture supply by conserving the rainfall, 

 or by direct additions of water. On the other hand, 

 in soils saturated with water, or which are too cold, 

 or too poorly aerated because of an excess of water, 

 it is desired to remove this excess either by drainage 

 or appropriate tillage 

 methods. 



91. Means of in- 

 creasing the water 

 content of the soil. 

 The average water 

 content of the soil 

 may be increased in 

 three ways: (1) By 

 decreasing the losses 

 from (a) percolation and (6) evaporation. (2) By 

 increasing the capacity of the soil for water (a) by 

 modifications of texture and structure, and (b) by in- 

 creasing the humus content. (3) By the direct addition 

 of water to the soil, which is irrigation. 



92. Decreasing loss. The water which comes on the 

 soil is subject to two forms of loss, (a) It may percolate 

 through the soil and beyond the reach of plant roots. 

 (b) It may evaporate. 



93. Percolation. The amount of loss in this way 

 is very great. (See page 192.) Water percolates most 

 rapidly in large spaces, and whether these large spaces 



Fig. 60. One - row toothed cultivator. 

 Adapted to shallow tillage and the mainte- 

 nance of a mulch. 



