72 SOILS AND FERTILIZERS 



The range is doubtless greater for a soil in good tilth than 

 for one in poor condition, and the wider the range of 

 optimum moisture content the less likely is a crop to suffer 

 from either extreme. 



83. The control of soil moisture. — Since there may be 

 too much or too little water in a soil for its most effective 

 crop production, the problem of moisture control is to remove 

 the excess and to conserve the remainder, attempting to 

 maintain the supply within the range of the optimum mois- 

 ture content. In heavy soils there is likely to be a surplus 

 of water in the spring and in sandy soils a deficit in midsum- 

 mer. The excessive water content in the spring is also 

 objectionable because it delays plowing, planting, and germi- 

 nation of seed as well as the early growth of crops. The 

 ways in which water leaves soil are by (1) run-off over the 

 surface ; (2) percolation ; (3) evaporation ; (4) absorption by 

 plants. The last of these is to be encouraged, at least when 

 it is economically accomplished. Run-off should generally 

 be prevented. Percolation and evaporation should be con- 

 trolled within certain limits. 



84. Run-off. — Removal of water in this way is objection- 

 able because the rivulets carry with them the fine particles, 

 which are frequently the most valuable part of a soil, and 

 gullies are formed that may interfere with the working of 

 the land. In regions in which the rainfall is large, and 

 particularly where it falls in torrential showers, more than 

 half of the precipitation may escape in this way. The 

 water so removed is, of course, entirely lost so far as its 

 utilization by plants is concerned. The proportion lost by 

 run-off is greater on slopes than on level land, and on com- 

 pact soil than on sandy soil or on soil in good tilth. 



The removal of excess water by means of open ditches is, 

 to some extent, a utilization of run-off to drain land, but 

 it is not so desirable a method as tile drainage. It is better 



