SOIL WATER 131 



fall at the same rate would cost $113 per acre. From this it can be readily 

 understood that artificial means of supplying plants with water must be 

 done at a very low cost, otherwise it will not prove profitable. 



The amount of rain in any region is important in connection with 

 crop production. In all regions where the annual rainfall averages less 

 than twenty inches, failures from insufficient moisture in the soil are 

 frequent. The distribution of the rain is quite as important as the total 

 annual rainfall. That which falls during the crop-growing season is more 

 important than that which comes in the non-growing season. Conse- 

 quently, there are regions of comparatively low rainfall where the dis- 

 tribution is so favorable that crop failures are infrequent. In other 

 localities a large part of a good annual rainfall may come in the non- 

 crop-growing season, and as a result, crops frequently suffer from drought. 

 In moving from one region to another it is well to study the average rain- 

 fall and its distribution. 



Amount of Water Necessary to Produce Crops. — In the processes of 

 plant growth the amount of water transpired or given off by plants is 

 many times greater than that used in the plant tissues. Investigations 

 in different parts of the world and at several of the American experiment 

 stations show that in plant growth the amount of water required to pro- 

 duce a pound of dry matter ranges from 200 to 700 pounds. This amount 

 must actually pass through plants. Each ton of dry matter in alfalfa 

 takes 700 tons of water. Each ton of dry matter in wheat required about 

 400 tons of water; in oats, about 500 tons; and in corn, about 300 tons. 

 To produce three tons of alfalfa in one season requires from 16 to 17 

 inches of rainfall, all of which must pass through the plants. A 20-bushel 

 crop of wheat would require about 6 inches, and 40 bushels of oats 6|; 

 while 50 bushels of corn would require about 8| inches of rainfall. For 

 crops of the yields mentioned there should be more rainfall during the 

 growing season than above indicated, because of the loss of water by direct 

 evaporation from the soil, plus additional amounts that may flow from 

 the surface if the rain falls rapidly, together with some that may pass 

 through the soil into the underdrainage. 



Transpiration by Plants. — Transpiration, or the amount of water 

 that passes through the plant and is evaporated from the surface of the 

 leaves, varies greatly in different localities, and is influenced by a num- 

 ber of factors. Transpiration takes place most rapidly during the day- 

 time and in the presence of plenty of sunshine and warmth. During the 

 night-time it is reduced to a very small amount. Transpiration is increased 

 with a reduction of the humidity of the air, with rise in temperature and 

 with intensity of sunshine. It is also increased with an increase in the 

 movement of the air. An increase in plant food tends to decrease it, as 

 does also a rapid growth of the plant. Transpiration is more rapid in the 

 presence of an abundance of soil moisture than it is when the soil is dry. 



Experiments at the University of Illinois by Dr. Hunt showed an 



