The irrigation farmer should not be content with using water to eliminate the hazard of drought, even though this results in an annual yield of alfalfa of 8 tons an acre, as compared to annual yields of 3 to 4 tons in the humid regions. He should ask himself, why not 12 tons? He cannot afford to use row widths and spacings of plants in the row that were found to be satisfactory in the humid region where water limited yields. He cannot assume that since his yields are higher than those of humid-region farmers he cannot make profitable use of fertilizers. He must not be content with superior strains of plants developed in the humid region, but must select plants adapted to the favorable environment he can create. In short, the irrigation farmer should ask himself : "Now that I have eliminated the hazard of drought, how should I modify my other farming practices to take full advantage of my opportunities?"

One modification on many irrigation farms that would result in higher returns is an increased use of commercial fertilizers. For a long time western farmers boasted of their fertile soils and too many still believe that it is a reflection on their farming if they have to use fertilizers to maintain yields. This prejudice is gradually breaking down, as evidenced by the rapid increases in fertilizer consumption during recent years. The country as a whole used 7.4 percent more plant food in 1944 than in 1943, but in the West the increase was 30.6 percent. When compared with the 5-year (1935-39) average, usage in 1944 was 243 percent of prewar in the western region. This is marked progress, but even so, western farmers have only begun to exploit the possibilities of using fertilizers in crop production under irrigation.

Some irrigators have tried using fertilizers and have concluded that the yield responses obtained were not sufficient to be economic. Others have reaped large increases through fertilization. One important fact in this connection is that intelligent use of fertilizers will require that other management practices be adjusted in keeping with the increase in plant nutrients. With higher levels of fertility we will want more plants per acre and we will use more water than was used in growing half of the potential crop.

Consider again the guayule experiment. It is readily seen that plants that received the amount of water under the standard practice did not respond to nitrogen. This was true regardless of the spacing at which the plants were grown. Had this been the only moisture treatment used in the experiment, it would have been concluded, certainly, that there was ample nitrogen in the soil for guayule and that added nitrogen would not increase yield. But when we consider all the data, it is quite evident that under the conditions of that moisture treatment, water was the factor limiting plant growth, and that regardless of the fertilizer added, there was not enough moisture to produce yields potentially possible with the natural soil fertility. We begin to see evidence that when a sufficient number of plants was supplied with sufficient water they produced enough growth to cause nitrogen to be a limiting factor in growth. All Plants that received even more moisture benefited from added nitrogen. The thicker the plants, the more benefit obtained.

This experiment points up the need for studying in combination the various important factors affecting plant growth. It also shows how extremely important it is for farmers to have the right combination of practices if they are to obtain potential maximum production.

On any farm, at whatever level of soil fertility, maximum plant growth cannot be obtained if moisture is a limiting factor. Where moisture and plant nutrients are supplied in adequate amounts, the highest production will be obtained only when there are enough plants to utilize fully the space available for growth. It is only when the best combination of the various practices is obtained that the farmer may expect a maximum yield.

THE AUTHORS

B. T. Shaw became assistant administrator of the Agricultural Research Administration in 1947, after 4 years as agronomist in the Bureau of Plant Industry, Soils, and Agricultural Engineering. He grew up on irrigated farms in Utah and Idaho. His undergraduate training was completed at Utah State Agricultural College and his postgraduate studies at the Ohio State University.

O. J. Kelley is a soil scientist, Division of Soils, Fertilizers, and Irrigation, Bureau of Plant Industry, Soils, and Agricultural Engineering. He also is a westerner. He did his undergraduate work at Colorado State College and received his postgraduate training at Ohio State University.