SOIL WATER 73 



to have the moisture pass into the soil and this is encouraged 

 by any of the operations and conditions that favor the main- 

 tenance of good tilth. Fall plowing and early spring plow- 

 ing also serve this end. In arid and semi-arid regions run- 

 off is usually not of any moment. Terracing a hillside is 

 often resorted to as a preventive of run-off, especially in 

 the south Atlantic states where the rainfall is often tor- 

 rential. 



85. Percolation. — Water that enters a soil is either 

 retained by the capillary spaces or eventually percolates 

 into the subsoil. The percolate is lost to crops, except that 

 part which remains in the subsoil and is later raised by 

 capillarity to within reach of roots. The chief consideration 

 is to maintain the soil in good tilth, which gives a large 

 capillary capacity, thus storing within easy reach of the 

 roots a maximum quantity of the descending water. The 

 more rapidly the gravitational water is disposed of the better, 

 because its presence prevents aeration of the soil together 

 with those beneficial processes that good ventilation encour- 

 ages. One of the most frequent causes of saturation of soil 

 is lack of facility for the water to escape from the lower 

 depths. This difficulty is best relieved by tile drainage. 



86. Evaporation. — It has been concluded from experi- 

 ments conducted at Rothamsted, England, that with an 

 annual rainfall of twenty-eight inches, one-half is lost by 

 percolation. The quantity of water required to produce an 

 average crop in a humid region is about seven inches, which 

 is one-half of the water retained by the soil. The other half 

 is presumably lost by evaporation. A rough estimate of 

 the disposition of rain water in a humid region would there- 

 fore be one-half lost by percolation, one-fourth by evapora- 

 tion and one-fourth used by the growing crop. The ratio 

 of quantity lost by evaporation to quantity used by crop 

 may vary by reason of a number of factors, among which is 



