THE INFLUENCE OF IRRIGATION 223 



Plant-food Carried to a Soil by Irrigation Water. — The 



Valley of the Nile has became famous in irrigation history not 

 because it was among the iirst irrigated districts of the world but 

 because of its extremely fertile fields, the fertility of which has 

 been maintained throughout the ages. Other soils just as fertile 

 to begin with have become barren. Why the difference? The 

 Valley of the Nile owes its lasting fertility to the flood waters 

 which carry to it rich deposits of silt each year. Hence, we find 

 that a soil's fertility may increase and not decrease due to the 

 water applied to it. 



Moderate quantities of irrigation water can be applied to a soil 

 without leaching from it soluble plant food* This water, as it 

 evaporates deposits within the soil the soluble and insoluble plant 

 food which it had carried to the soil. 



Now, let us examine some of the results which have been ob- 

 tained during the last few years at the Utah Experiment Station 

 in a study of the irrigation waters of the intermountain region, 

 bearing in mind that the constituents which are most usually lack- 

 ing in soils are potassium, phosphorus, and nitrogen. 



Hundreds of samples of water representing fifty-eight streams, 

 the majority of which are extensively used for irrigation purposes, 

 have been analyzed. These waters vary in potassium content 

 from 49 parts per million to only .70 part per million. Slightly 

 over one-half of the waters contained 5 parts per million. The 

 importance of these results becomes more obvious when we ex- 

 amine the pounds of potassium carried to an acre of soil by two- 

 acre-feet of water. This varies from 266,6 pounds in the case 

 of the highest to 4.4 pounds in the case of the lowest with an 

 average potassium content of 33.4 pounds per acre. 



These results are not without economic significance, for the 

 potassium in the highest would be sufiicient to produce 370 bushels 

 of corn, 230 bushels of wheat, or 34 tons of sugar-beets. The 

 average for the streams is sufiicient to produce 47 bushels of corn, 

 29 bushels of wheat, or 4 tons of sug|r-beets. 



Many of the soils of the intermountain region are rich in potas- 

 sium; hence, this element is not as important as is phosphorus. 



