66 COBN CROPS 



An average corn soil in good tilth will store about 5 to 

 6 inches of available water in the upper 4 feet. A 50- 

 bushel crop would then require at least 6 inches addi- 

 tional rainfall during the growing season, and prob- 

 ably more than this, as corn seldom grows well when 

 required to exhaust the soil moisture to low limits. A 

 75-bushel crop would require an additional rainfall of 

 10 inches and a 100-bushel crop at least 15 inches during 

 the growing season, in addition to that stored in the soil. 

 When the run-off is large, as on hills or with torrential 

 rains, or when there is seepage, the above estimate should 

 be increased. This estimate is on the assumption that 

 the soil is fertile. No amount of rain would make a poor 

 soil productive. For example, the average rainfall for 

 June, July, and August in the eight surplus corn States 

 is about 12 inches, but the average yield is 28.5 bushels. 

 Other factors than total rainfall here limit the yield, 

 one important factor being that the rainfall is not always 

 properly distributed. 



51. Any system of culture that will serve to prevent 

 run-off on the one hand and to decrease evaporation on 

 the other, will proportionally increase the available water 

 supply for the crop. 



Not only the total amount, but the distribution, of the 

 season's rainfall is of great importance. Figure 25 shows 

 the precipitation for June, July, and August for a period 

 of fifteen years and the yield for eight surplus corn States, 

 namely, Ohio, Indiana, Illinois, Iowa, Nebraska, Kansas, 

 Missouri, and Kentucky.' 



Here is shown a very close relationship between rainfall 

 and yield, when large areas are considered. 



> Smith, J. Waeren. Relation of Precipitation to Yield of Corn. 

 U. S. Dept. Agr. Year Book, 1903 : 215.-224. 



