26 AGEICULTUEE TN THE SEMIARID GREAT PLAINS. 



the soils are capable of holding between 10 and 17 inches of water 

 in the first 6 feet, but it is not always possible to get them filled to 

 their full capacity, nor can plant roots draw all the water out of the 

 soil. There will always be a considerable amount of water remain- 

 ing in the soil when plants cease to grow, and even when they die 

 on account of drought. The more rapid the evaporation, the greater 

 will be the quantity of water in the soil when plants begin to suffer, 

 because plants can not draw water as rapidly from a comparatively 

 dry soil as from a wetter one. The amount of water which soil still 

 ■ contains when plants have ceased to grow normally varies with the 

 character of the soil, being greatest in clay and least in sand. In most 

 of the Plains soils it is from 4 to 7 per cent of the dry weight of the 

 soil, or approximately that number of inches of water is distributed 

 through the first 6 feet of soil. On the other hand, plants will live, 

 though they will not grow much, till they have reduced the water 

 content of the soil nearly, though not quite, as low as the dry air 

 will be able to reduce it. Hence, it may be assumed that one-half to 

 three-fourths of the water which is stored in the soil is actually 

 available for normal plant growth. In a season, then, of 16 inches 

 of rainfall, if one-half of it is stored in the first 6 feet of soil there 

 will be 8 inches of water conserved. Probably 4 to 7 inches will be 

 actually available for the use of plants; that is, a reserve of 4 to 7 

 inches of water is carried over to supplement the rainfall of the suc- 

 ceeding season or to start winter grain and keep it growing till spring 

 rains come. This stored water, however, is much more valuable to 

 growing crops than an equal amount of rainfall, because it is down 

 so far in the soil that a much smaller percentage of it is lost by evapo- 

 ration from the surface than of the rain which falls upon the crop. 

 A small amount of water is often invaluable in enabling a crop to pass 

 successfully through a dry spell which it would not otherwise with- 

 stand. In this way even a very small reserve may determine the fate 

 of the crop. 



From this it will readily be seen that there are many places on the 

 Great Plains where it would not seem- probable that summer tillage 

 would conserve sufficient moisture, together with the rainfall of the 

 succeeding season, to produce a profitable crop. 



How much rainfall is absolutely necessary to ])roduce one crop in 

 two years is largely a matter of speculation. At present, however, 

 it does not seem that in the region under discussion profitable croj)s 

 can be expected without a precipitation of at least 15 or 16 inches in 

 one of the two years; that is, either while the ground is being sunnner- 

 tilled or while it is growing the crop. When a season with only 8 or 

 10 inches of rainfall is followed by one equally dry it does not seem 

 possible that even summer tillage will produce a paying crop. But 



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