THE CONTROL OF SOIL MOISTURE 265 



It is immediately obvious that as the losses by run-off, 

 Iraching, and evaporation increase, the amount of water 

 left for crop utilization decreases. In arid and semiarid 

 regions this is fatal to plant growth, while in humid regions 

 it may be such a factor in periods of drought as to se- 

 riously reduce the harvest. Control of moisture is there- 

 fore necessary in all regions, and this really consists in 

 so adjusting run-off, leaching, and evaporation as to 

 maintain optimum moisture conditions in the soil at all 

 times. This control should result in a proper and eco- 

 nomical utilization of the soil water by the plant. 



185. Run-off losses. In regions of heavy rainfall 

 or in areas where the land is sloping or rather impervious 

 to water, a considerable amount of the moisture received 

 as rain is likely to be lost by running away over the sur- 

 face. Under such conditions two considerations are im- 

 portant : (1) by not entering the soil the water is lost for 

 plant use ; and (2) washing of the soil may occur, which 

 if allowed to proceed may entirely ruin the land. The 

 amount of run-off varies with the rainfall, the slope, and 

 the character of the soil. In some regions it may rise 

 as high as 50 per cent of the rainfall, while in arid regions 

 it is of course very nearly zero. As a general thing, this 

 loss is estimated with the losses by leaching, the two being 

 expressed as one figure. 



186. Percolation losses. When at any time the 

 amount of rainfall entering a soil becomes greater than 

 the water-holding capacity of the soil, losses by percola- 

 tion will result. The losses will depend largely on the 

 amount and distribution of the rainfall and the capa- 

 bility of the soil to hold moisture. The bad effects of 

 excessive percolation are twofold : (1) the actual loss of 

 water, and (2) the leaching-out of salts that may function 



