Irrigation and Soil, I

Irrigation and Soil By J. E. Greaves, Chemist and Bacteriologist Utah Experiment Station I. — Effect of Irrigation Water on the Fertility of Soil Water is supplied to the soil to meet the needs of the grow- ing crop. But what influence has it upon the fertility? Does it increase the soluble plant food within the soil, leaving it there for the use of the growing plant, or does it drain from the soil the nitrogen, potassium, or phosphorus, thus leaving the soil poorer in potential fertility? Is it increasing or decreasing the activity of the farmer's nursable friends the bacteria? Does it carry with it and deposit within the soil injurious alkali salts which, in time, will render the soil sterile, or does it in- crease the total quantity of plant food in the soil, thus making it more fertile? The answer which we give to these questions will be gov- erned by the nature of the soil, the composition of the irri- gation water, and the knowledge of the individual applying it. The application of irrigation water to a soil may change the chemical, physical, or biological nature of a soil so that the food available to the growing plant is materially increased or decreased. Although pure water is a universal solvent, yet its action is materially increased when it becomes charged with carbon dioxide. Work has amply demonstrated that the main factor which produces carbon dioxide — the bacteria of the soil — is directly influenced by the quantity of water supplied to a soil. The quantity of carbon dioxide produced in twenty-four hours in a good arable soil supplied with the optimum amount of moisture is enormous. Some results indicate that this at times may be as much as 67 pounds per acre to a depth of 16 inches produced daily. This comes in contact with the in- soluble potassium and phosphorus of the soil, and renders it available to the growing plant, as is seen from increased yield which is obtained. One investigator found that soil constantly supplied with carbon dioxide through a pipe buried in the ground gave an increased yield of 12.2 per cent over crops grown on adjoining untreated soil. All plants except the legumes — beans, peas, vetches, etc., — get their nitrogen directly from the soil. This comes from the plant residues in the soil, or from the applied fertilizer. All soil and fertilizer nitrogen, except nitrates, must be rendered available for the growing plant by bacteria. They transform the protein nitrogen into ammonia, but there are few plants

528 IMPROVEMENT ERA which can use this. Other groups of bacteria transform the am- monia into nitrates in which form it is available to the higher plant. The speed with which these transformations occur is governed by the quantity of water in the soil. If we consider the quantity of water which a soil will hold when it is just saturated as 100, then the quantity of ammonia produced at the various moisture contents becomes as follows: Per cent water Ammonia Per cent water Ammonia in soil Produced in soil Produced 10 2 60 100 20 8 70 78 30 32 80 57 40 68 90 \ 49 50 85 100 45 Hence, when we have soil with 60 per cent of its air space filled with water, and the other 40 per cent with air, we have a maximum transformation of the nitrogen of plant residues over into ammonia. When the water applied is below or above this quantity, the transformation is not as rapid. It is interest- ing to note that excessive quantities of water are more detri- mental than a small quantity. But how about the speed with which the ammonia is trans- ferred into available nitrates? Is this dependent upon the water supply? This is answered by the result given below: Per cent water Per cent Ni- Per-cent water Percent Ni- in soil trates formed in soil trates formed 10 11 50 85 20 17 60 100 30 31 70 40 40 62 80 9 90 none Here again the greatest activity occcurs when the soil has 60 per cent of its spare space filled with water but when it becomes much greater nitrification rapidly decreases and eventually ceases, and it is interesting to note that numerous experiments have demonstrated that this is the water content which produces maximum crops. - This is due to the fact that at this water content the nitrogen of the soil is being rapidly rendered available for the growing plant. Moreover, the resulting by-products of bacteria, granish, acids, are rapidly formed and these come into contact with the soil phosphorus and potassium, and render them available to the plant. Hence, we must ask ourselves the question: Are we applying too little or too much water to our soil to maintain sixty per cent of the air space filled with water? Logan, Utah