128 THE IRRIGATION AGE. 



acre inches should permit of a yield of 70 bushels of shelled corn per 

 acre; while 8.27 acre-inches are needed for 400 bushels of potatoes. 



One, two, three and four tons of clover hay per acre, according to 

 our observed rate of consumption, call for 2.08, 4.16, 6.24 and 8.32 

 acre-inches of water respectively when the hay is cured to contain 15 

 per cent of moisture. Corn silage containing 70% of water willire- 

 quire 16.9 acre-inches of water for 12 tons per acre, and 26.2 acre- 

 inches for 20 tons. 



These figures must be regarded as showing the minimum amounts 

 of water which will bring the crops named to full maturity, so as to 

 produce the yields specified, only under conditions of no loss by surface 

 or under drainage, and. where the evaporation from the soil itself is as 

 small as it can well be. It must be further understood that the soil 

 at seeding time already possesses the needful amount of water for the 

 best conditions and that, at the end of the growing season, it is yet so 

 moist that no check to normal vigorous growth has occurred. Further 

 than this the soil must have possessed all "of the essential plant foods 

 in abundance. 



Let us next consider what the available rainfall is in various parts 

 of the United States, what yields per acre are realized from it, and 

 what may reasonably be expected, under the best treatment. 



To make the discussion as concrete, definite and pointed as possi- 

 ble let us draw our date from the states of Illinois, Indiana, Iowa, 

 eastern Kansas, Maine, southern Michigan, Missouri, Minnesota, New 

 York, Ohio, Pennsylvania, Vermont and Wisconsin. In these states 

 what is the amount of rainfall available for crop production? 



It is evident that from the mean annual rainfall of these states 

 must be deducted the annual run-off as not available for crop pro- 

 duction. But if the water borne away in the drainage is deducted 

 from the rainfall the difference will be too large, for very many show- 

 ers are too slight to be of any service whatever; and not only this, 

 but very light rains often do positive injury by destroying the effect- 

 iveness of earth mulches, which have been developed by tillage, caus- 

 ing a loss of a portion of the deeper soil moisture with that which fell 

 as rain. It is further necessary to consider the rainfall of the grow- 

 ing season of the specific crop under consideration in order to know 

 whether tillage alone will make possible large crops unaided by irri- 

 gation. 



The first crop of clover, for example, must be largely made by 

 the rains of May and June, in the states named, while the crop of 

 potatoes will be more largely influenced by the rains of June to 

 October. The period for barley would extend from May nearly 

 through July;*oats*from May to the middle of August; and corn from 

 the middle of May to the middle of September. For the growing 

 season of the barley and^oats I have estimated that two inches of rain- 

 fall are lost by percolation, and for that of corn 1.5 inches. The in- 



