126 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



was 10,000 pounds dry matter per acre. The abundant supply 

 of water thus had a striking effect. 



Smith 1 has studied the relation of the rainfall to the yield of 

 corn. Figure 33 shows the relation between the average yield of 

 corn in bushels per acre, and the rainfall, in Ohio, Illinois, Indiana, 

 Iowa, Nebraska, Kansas, Missouri, and Kentucky for the years 

 1 883 to 1902 inclusive. 



Duty of Water. Duty of water is a term applied by irrigators as 

 the measure of the quantity of water used per acre, but the term is 

 also applied to the service which water may render in producing 

 crops. McGee 2 estimates that it requires approximately 1,000 

 pounds of water during the year to produce one pound of grains, 

 etc. This estimate allows for losses by evaporation, and its basis 

 is the aggregate yearly supply of water from all sources He 

 presents the following table, based on personal observation, show- 

 ing estimates of crop yields with varying amounts of water for 

 the entire year : 



These figures, are roughly approximate, as there are great 

 variations in conditions. According to this table, 36 inches yearly 

 rainfall will produce, on an average, 35 bushels of corn. 



Quantity of Water in Soil. The following figures of Hellriegel 3 

 are the result of an experiment to ascertain the effect of the 

 amount of water in the soil upon the crop production. The 

 experiment was carried out in pots. 



1 Yearbook, U. S. Dept. Agr., 1903, p. 216. 



2 Yearbook, U. S. Dept. Agr., 1910, p. 174. 



3 Jahresber, f. Agr. Chem., 1870-2, p. 161. 



