88 ^IRRIGATION PRACTICE 



drainage. It is then possible to calculate the probable 

 quantities of soluble salts deposited in the soil to a depth 

 of 10 to 15 feet during one season's irrigation. In the 

 arid regions, 250 parts of dissolved substances in 1,000,000 

 parts of water are accounted unusually low, unless obnox- 

 ious substances are admixed. Such a water, applied to the 

 soil to a depth of 2 feet throughout the season, allowing 

 for no drainage, would leave in an acre of soil throughout 

 the season, approximately 1,300 pounds of solid matter. 

 This repeated, year after year, would naturally run into 

 large amounts, although some would, undoubtedly, be 

 taken up by the plants in their growth and used for the 

 elaboration of plant tissues. 



At the Utah Station, a large number of analyses were 

 made of crops grown under irrigation, and it was found 

 that in wheat kernels the ash content was about 2.5 per 

 cent and, in wheat straw, about 10 per cent. A thirty- 

 bushel wheat crop would then abstract from the soil 

 about 345 pounds of mineral matter, or a little more than 

 one-fourth of the total quantity added by irrigation. 

 Lucern contained about 8.5 per cent of ash materials, in 

 which case a crop of 10,000 pounds would contain approx- 

 imately 850 pounds, or a little more than two-thirds of 

 the materials' left by the irrigation water. None of the 

 crops ordinarily grown under irrigation takes up the 

 quantity of soluble substances added to the soil by 2 

 acre-feet of water, providing drainage is prevented. It 

 must be remembered, in this connection, that irrigation 

 waters do not always contain all the essential plant-foods, 

 or in the right proportion. While a water may add to 

 the soil more solid matter than the crop needs, the indi- 

 vidual constituents may be wholly or in part absent, and 

 must be supplied by the soil. 



