Making More of Irrigation

by B. T. SHAW and O. J. KELLEY

WE HAVE a long way to go before we reach a ceiling on crop yields. Just lately we have realized how great are the returns obtained from the use of several good practices in combination how a farmer can put two and two together and get more than four.

An example is given by L. M. Ware of the Alabama Agricultural Experiment Station. Sweet corn failed completely in a field at Auburn, when no commercial fertilizer was used, he reported in 1945; the plant food in the soil was inadequate to support corn production. But when a commercial fertilizer was added, a yield of corn valued at $153 an acre was obtained. When a commercial fertilizer was added and the corn irrigated, a crop was produced that had a value of $253 an acre. When a commercial fertilizer and manure were added, the crop was valued at $439.50 an acre. When a commercial fertilizer and manure were added and the corn was irrigated, a crop was produced that had a value of $553 an acre. When a commercial fertilizer and manure were added, a crop of vetch was turned, and the corn was irrigated, a yield was produced that had a value of $699.50. Each fertility factor artificially supplied helped to provide more nearly the optimum conditions for corn production. The effect on the yield of corn was cumulative. When all factors were supplied, a yield of 13,845 pounds of marketable green corn was produced.

Another illustration of our thesis comes from B. A. Krantz of the North Carolina Agricultural Experiment Station. In 1944, yields of corn of more than 100 bushels an acre were reported in a section of the country where corn normally yields from 10 to 20 bushels an acre. Under ordinary conditions of spacing (about 4,000 to 5,000 plants on an acre), fertilization (0 to 30 pounds of nitrogen to the acre), and cultivation (too deep and too close to the plant so that roots are pruned), and with ordinary open-pollinated varieties, 20 bushels would have been normal. With shallow cultivation and ordinary spacing and fertilization, adapted hybrids yielded up to 54 bushels an acre. North Carolina hybrid 1028 gave a yield of 84 bushels when the nitrogen fertilization was boosted to 120 pounds. Then, by increasing the number of plants per acre to 8,000, a yield of 100 bushels was obtained.

During the war, research was initiated to see what could be done to speed up rubber production with guayule under irrigation. The standard practice for growing guayule then was to transplant nursery-grown plants 20 inches apart in rows that were 28 inches apart. The plants were irrigated until they were established and then were left to depend on normal rainfall. The yield of shrub in 19 months was 0.86 ton an acre. At this moisture level, increasing the number of plants on an acre raised the yield to 1.20 tons because of the more efficient utilization of water. Nitrogen fertilization was without effect. Increasing the supply of water but keeping plant spacing the same, increased the yield to 1.38 tons without nitrogen and to 1.61 tons with it. Increasing the number of plants at this higher moisture level by cutting the distance between plants in the row to 6% inches boosted the yield to 2.21 tons without nitrogen and 2.59 tons with it. This yield of 2.59 tons is three times that obtained by standard practice.

Great as the responses in the examples were, one wonders if they might not have been much higher. How high would the return have gone in the Alabama experiment if spacing and variable moisture had entered the experiment as in the guayule study, and different hybrids as in the North Carolina test? What would have been the effect of supplemental water on the North Carolina results? How high would the yield of guayule have been if a strain of guayule bred for high levels of nutrition and water had been used? How would controlling insect damage have affected all results? These are unanswered questions, of course, but, to repeat, it seems that we have a long way to go before we reach a ceiling on crop yields.

So much for research results. How can the farmer use them?

It is not necessary to tell anyone who has herded a stream of water over a farm that irrigation farming is different. The irrigator is beset with most of the hazards that plague ordinary farming: Weeds, insect Pests, and plant diseases; the problem of selecting the best crop, variety, or hybrid; growing the selected crops in the best sequence; the uncertainty of continued soil fertility and the problems of maintaining it. Besides, farmers under irrigation face problems of salt accumulation in soil, the best use of available irrigation water, and erosion due to irrigation. Also, under irrigation, a greater proportion of the costs of Production are in the form of fixed charges that must be met regardless of the yield of the crop, or even if there is a total crop failure. Thus, while the possibilities of getting larger returns are greater where the hazards of drought have been eliminated, the financial risks are correspondingly increased unless superior yields are assured.

Most irrigation farmers know and practice many features of good soil management. They realize that all irrigation waters contain salts, and that it is necessary, therefore, to leach the soil occasionally or salts will accumulate. They know the value of maintaining a high organic matter content in soil, which usually can be accomplished through good crop rotations and liberal use of manure. Irrigation farmers who have sloping lands realize that often it is necessary to plant on the contour to reduce erosion to a minimum. While these and other good practices for irrigation are well known and, if used, usually will result in relatively .high yields, the yields without a doubt are not the highest economic yields obtainable.