Irrigation and Soil, III

Irrigation and Soil By J E. Greaves, Cliemist, Utah Agricultural Experiment Staiion III — Is Irrigation Water Increasing the Fertility of Your Soil? The Valley of the Nile has become famous in irrigation history, not because it was among the first irrigated districts of the world, but due to its extremely fertile fields, the fertility of which has been maintained through the ages. Other soils just as fertile have become barren, but the Valley of the Nile continues fertile through the waters which are continually carry- ing rich deposits of silt to it each year. Hence we find that a soil's fertility may increase and not decrease due to the irri- gation water applied to it. Is your soil gaining or losing in plant food due to the water applied? The answer to this ques- tion will be determined by the nature of the water applied and the knowledge of the farmer applying it. If the quantity of irrigation water used is large there is a constant drainage through the soil, the probable effect of which would be the carrying out of certain soluble constituents. Whether this is greater or less than that brought to the soil will vary, depending upon the composition of the water, nature of the soil, and the quantity of drainage. On the other hand, when water is added in moderation nothing is carried from the soil; moreover, the water applied to the soil evaporates and deposits within the soil its soluble and insoluble plant food. Now let us examine some of the results which have been obtained 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 lacking in soils are potassium, phosphorus, and nitrogen. During 1916-17 the Chemistry Department of the Utah Ag- ricultural College collected hundreds of samples of water, repre- senting fifty-eight streams, the majority of which are extensively used for irrigation purposes. These waters vary in potassium content from 49 parts per million to only .79 parts per million. Slightly over one-half of the waters contained five parts per million. The importance of these results becomes more obvious when we examine 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

696 IMPROVEMENT ERA lowest, with an average potassium content of 33.4 pounds per acre. These results are not without economic significance, for the potassium in highest would be sufficient to produce 370 bushels of corn, 230 bushels of wheat, or 34 tons of sugar-beets. The average for the streams is sufficient to produce 47 bushels of corn, 29 bushels of wheat, or 4 tons of sugar-beets. Many of the soils of the intermountain region are rich in potassium; hence, this element is not as important as is phos- phorus, which, although used by the crop in smaller quantities, is nevertheless at times the limiting factor in crop production. The total phosphorus of the irrigation waters analyzed varied from traces to 5.47 parts per million. The great majority of them, however, contained less than one part per million. The average in two acre-feet of the streams was 3.46 pounds, for the wells 3.36 pounds, and for the drains 1.82 pounds. The phosphorus in two acre-feet of the water from the richest stream is sufficient for the production of 175 bushels of corn, 120 bushels of wheat, or 33 tons of sugar-beets. In the case of all the other streams, while not as high, it undoubtedly plays a part in maintaining the phosphorus content of the soil. Even more important than the phosphorus is the nitrogen of the waters, for nitrogen is the limiting factor of crop pro- duction in most of the soils of the intermountain region. This varies from traces up to 24.3 parts per million. The average quantity of nitrogen in two acre-feet of the irrigation is 22.8 pounds, while that in the highest is 132.2 pounds per acre. This would be sufficient to produce 186 bushels of corn, 114 bushels of wheat, or 17 tons of sugar-beets. These results probably help to explain the remarkable fertility of many of the irrigated soils of the arid regions. Some of them have been producing crops undiminished in quantity for upward of fifty years, and there is no reason why a limited few soils on which the richer irrigation waters are being used cannot continue for another fifty or one hundred years to produce maximum crops. Greed To be always getting and hoarding and planning, A way to grasp more with eyes never scanning The day to perceive another's dire need, Doth fill well the mouth of insatiable greed. To be always getting and never once giving. Ah me! such a life would not be worth living; But like unto some stagnant pool must appear With depths growing deadlier year after year. Grace Ingles Frost