Irrigation and Soil, II

Irrigation and Soil By J. E. Greaves, Chemist, Utah Agricultural Experiment Station II — How much fertility is Irrigation Water Carrying From Your Soil? Water is a universal solvent. It dissolves to varying extents everything with which it comes in contact. This is exemplified by the fact that pure water is a chemical curiosity found only in the chemical laboratory, and kept only by carefully planned and executed laboratory work. Hence, we can expect that the water applied to a soil will dissolve more or less of its con- stituents, depending upon the nature of the soil and the water applied. If only sufficient water is added to a soil to moisten or even saturate it, the subsequent evaporation of the water redeposits the constituents in the soil and nothing is lost. But when more water is added than the soil can hold, it passes through the soil and carries with it the plant food. The mag- nitude of this factor is exemplified by the enormous quantities of the various salts found in the waters of some lakes and oceans. These owe their origin to the water seeping through the soil, becoming laden with salts which are carried with the water to the lakes and ocean ; when the water evaporates the salt remains, thus ever becoming more concentrated. It is estimated that the rivers of North America carry to the ocean each year 474,000,000 tons of soluble constituents, which would be a quantity sufficient to cover one hundred acres to the depth of nearly three thousand feet. Although the greater part of this is common salt and other non-valuable agricultural compounds, yet there is an appreciable quantity of soil fertil- ity carried in these waters as is witnessed by the 33.4 pounds of potassium, the 3.46 pounds of phosphorus, and the 22,8 pounds of nitrogen which, as an average, was found in two acre-feet of water from the stream of the intermountain region. Carefully controlled erperiments have been conducted at the Rothamsted Experimental Station, in England, where the yearly rainfall is about 30 inches, and the annual loss of nitro- gen was found to be 35.5 pounds per acre. The work of the Utah Experiment Station, covering a period of ten years, in- dicates that large quantities of nitrogen are lost in the drain- age water when excessive quantities, 25 inches and over, of irrigation water are applied to a soil during the season.

626 IMPROVEMENT ERA This loss of nitrogen results in a decreased yield, for the nitrogen is removed by the water as fast as rendered available in place of being taken up by the growing plant and built into valuable plant constituents. Moreover, for this same reason, the grain which is grown on such soil is lower in protein. Un- doubtedly the great merits of the dry-farm wheat, as a bread- maker, is due, in a large measure, to the fact that the scanty rainfall is not sufficient to remove the soluble nitrogen from roots of the growing plants. There is still another factor, ex- cessive quantities of water rapidly deplete a soil of its fertility, which viewed from the economic standpoint is no trifle for the thirty-five pounds of nitrogen removed by the drain waters from the Rothamsted Station would cost, at thirty cents a pound, $10.50 per acre annually. The remedy then is, first, in those districts where irrigation is practised, the quantity of water applied should be such that the drain water is at a minimum on the majority of soils. Most soils produce better crops with 16 inches of water than with more. Furthermore, small frequent applications are not nearly as likely to remove soil fertility as is the same quantity of water applied in large quantities. Where the rainfall is such that there may be a loss of plant- food, the only practical method of reducing this loss is by the use of growing plants, the roots of which may absorb the plant- food about as rapidly as it is made available. If required, these may be returned to the soil when the plant-food will be ren- dered available as needed by the next growing crop. The use of rye, rape, or other green manures, seeded in the fall and plowed under the next spring on land that would otherwise be bare during fall, winter, and early spring, will not only conserve plant-food that would otherwise be lost but it will greatly in- crease the organic matter of the soil, and this usually improves its physical, chemical, and biological properties. Logan, Utah A Comparison Dark the day when grim death beckons To our precious ones below; It seems past our understanding That they are compelled to go. Death compare then to the twilight Ushering in the durkest night; Dawn brings in a glorious morning, Immortal heaven in sight. Delta, Utah H. L. Johnstone