CHAPTER X 

 THE CONTROL OF SOIL-MOISTURE 



In the discussion of the water requirements of plants it 

 was apparent that for a normal yield of any crop, the amount 

 used by the plant alone was very great, varying from five to 

 ten acre-inches according to conditions. Were this the only 

 loss of water, the question of raising crops with given amounts 

 of rainfall would be a simple one. Three further sources of 

 water loss, however, are usually operating in the soil and tend- 

 ing to lower the water that would go toward transpiration, a 

 loss absolutely necessary for proper growth. The various 

 ways by which water finds an exit from a soil are: (1) tran- 

 spiration, (2) run-off over the surface, (3) percolation, and 

 (4) evaporation. The diagram (Fig. 36) makes clear their 

 relationships. 



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

 leaching, and evaporation increase, the amount of water left 

 for crop utilization decreases. Some control of soil-water 

 is, therefore, necessary both in an arid and a humid region. 

 Under arid and semi-arid conditions, where run-off and per- 

 colation are not of such great importance except where irriga- 

 tion is practiced, loss by evaporation is of especial consequence, 

 as it competes directly with the plant. Under humid condi- 

 tions, losses by percolation and run-off seem to merit the 

 greater attention, because of the loss of nutrients with the 

 former and the erosion damage from the latter. The influence 

 of evaporation, however, is not to be under-estimated or ne- 

 glected. Control of moisture is, therefore, necessary in all 

 regions. This control consists in so adjusting run-off, leach- 



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