CHAPTER XVIII 



CONTROL OF MOISTURE 

 I. DRAINAGE 



VERY few places on the earth's surface have ample rainfall so 

 well distributed that no attention need he given to the control of 

 moisture. In many humid and superhumid areas the great prob- 

 lem is disposing of the excess of water, while in semi-arid regions 

 it is to conserve the rainfall for the crop, while in, the still drier 

 regions irrigation is the all-absorbing problem. Even in the humid 

 areas some seasons are so dry that the utmost care must be exercised 

 to hold the moisture for the crop. 



Removal of Excess of Water. Drainage. The average soil 

 has about 50 per cent of pore space. A waterlogged soil is one 

 having the pore space filled with water. It becomes necessary to 

 remove this excess of water so that the food-producing bacteria 

 and the roots of plants may be able to secure oxygen. The water 

 table in the soil must be from three to four feet below the sur- 

 face, sufficient to give room for the development .of large root 

 systems. If it is above this it must be lowered by drainage. Be- 

 sides the lowering of the water table many other benefits are de- 

 rived from drainage (Fig. 95). 



(a) Drainage gives stability to the soil. Ordinarily when a 

 heavy weight is applied to a very wet soil the particles are pushed 

 to one side, the excess of water weakens the cementing material 

 of the granules and acts somewhat as a lubricant to the particles. 

 This movement is very injurious to the tilth of the soil, since it 

 breaks down the granules, producing a puddled condition. This is 

 very likely to occur in any soil, but more particularly in a heavy 

 one. Freezing and thawing or wetting and drying may overcome 

 in time the condition produced if the soil is drained. Great dam- 

 age is sometimes done by pasturing wet soils during late winter 

 and early spring. 



(b) Soils containing an excess of water are rarely in good 

 physical condition. Granulation is produced by alternate wetting 

 and drying, and a soil that is saturated practically all of the time 

 cannot be subjected to these beneficial conditions. Freezing and 



222 



