76 SOILS 



to receive and hold a limited rainfall, he has taken 

 the most important step in solving the coordinate 

 problem of how to maintain its fertility. 



THE AMOUNT OF WATER NEEDED BY PLANTS 



It takes a very large quantity of water to mature 

 even an ordinary crop. Irrigation farmers appre- 

 ciate this much more than farmers in humid re- 

 gions, because they can see it in bulk. Hellriegel 

 has determined the amount of water necessary for 

 the growth of average crops of the following plants : 

 clover, 400 tons per acre; potatoes, 400 tons; 

 wheat, 350 tons; oats, 375 tons; corn, 300 tons; 

 grapes, 375 tons. This does not take into account 

 water that is constantly being evaporated from the 

 soil in which the crop is growing; it considers only 

 the water used by the plants themselves. At the 

 Iowa Agricultural Experiment Station it was found 

 that the loss of water in growing a ton of clover hay, 

 including what was used by the plants and what 

 evaporated from the soil, was about 1560 tons, or 

 enough to cover an acre 13.7 inches deep. The 

 loss of water in growing one ton of air-dried corn 

 fodder was 570 tons, or five inches per acre; of one 

 ton of oats, 1200 tons of water, or 11 inches per 

 acre; of 200 bushels of potatoes, 582 tons of water, 

 or 5.6 inches per acre. The loss of water in grow- 

 ing one acre of pasturage was 3223 tons, which is 

 equivalent to a rainfall of 28 inches per acre. These 

 interesting figures emphasise what every good 

 farmer already knows: that an abundant supply 

 of water is even more essential to a large crop than 

 an abundance of plant food, and that some crops 

 make larger demands upon the soil reservoir than 

 others. 



