70 



SOILS AND FERTILIZERS 



The most economical utilization of water was secured by 

 a medium water supply. 



78. Effect of humidity, wind and temperature of the air. 

 — A dry atmosphere and a high temperature increase the 

 transpiration ratio. For this reason crops require a large 

 amount of water in arid regions and in regions of high summer 

 temperatures. A high and constant wind movement 

 also tends to raise the transpiration ratio. In parts of the 

 country requiring irrigation the economical use of water must 

 be considered. Such a region is likely to have much sun- 

 shine associated with high temperatures and dry atmosphere. 



79. Effect of soil fertility on transpiration. — A soil high 

 in available plant-food material has, in general, the property 

 of producing crops with a small unit expenditure of water. 

 Experiments in Nebraska gave the following results : 



Table 15. — Relative Water Requirements of Corn on 

 Different Types of Nebraska Soils 



80. Quantity of water required to mature a crop. — A 

 rough estimate of the quantity of water required to bring 

 to maturity a crop of wheat may be calculated as follows : 

 Assuming the yield to be forty bushels or about two tons of 

 dry matter in straw and grain and the transpiration ratio 

 to be 400, the quantity of water actually used by the plants 

 would be 800 tons to the acre, or equivalent to about 7 

 inches rainfall. In addition to this there would be an equal 

 or larger quantity of water evaporated directly from the 



