More particularly, this application is directed to the foliar application of ammoniumpolyphosphate fertilizer solution to field crops to improve the growth rate, stress resistance, and maximum planting density of such crops.
The ground application of fertilizer and, in particular, of phosphate-containing fertilizers such as ammonium phosphate to increase crop yields is, of course, well known. In the case of seed-grown annual crops, such phosphatecontaining fertilizers can be applied in either liquid or solid form prior to, simultaneously with, or subsequent 'to, the planting of the seeds from which the crops will grow. Such fertilizers are generally of the type wherein the phosphate is rendered assimilable by the plant through the action of soil bacteria so as to provide for assimilation b the plant over a prolonged period.
The purpose of fertilizer application is, of course, to increase the crop output per unit of land. Since phosphorous, which is assimilated by the plant as phosphate ion, is one of the essential elements required in major amounts for plant growth, one of the limiting factors in determining the size and the number of plants that can grow to maturity on a given area of land is the amount of phosphorous available to the plant during its growth cycle from seed to maturity. Since virtually all soils naturally contain a comparatively limited amount of phosphorous, in general, the greater the amount of added phosphorous, the greater the number of plants which will grow on a given piece of land.
This is subject to three qualifications:
(1) Phosphorous is, of course, not the only essential element for plant growth. Other elements required by plants in comparatively large quantity are nitrogen, potassium, calcium, magnesium, and sulfur. Also, the so-called micronutrients, i.e., copper, zinc, manganese, molybdenum, chlorine, boron, and iron must also be present albeit in comparatively limited amount;
(2) The phosphorous must be present in a form that is either directly assimilable by the plant (phosphate anion) or, alternatively, is transformable by water and soil bacteria into such assimilable form;
(3) A saturation point is ultimately reached beyond which additional quantities of soil-applied phosphorous achieve a very limited increase in the crop yield per unit of land. The saturation point, of course, varies depending upon the particular crop, the soil, and climatic conditions, the availability of water and sunlight, temperature, and the like. However, even under conditions of abundant water, sunshine, etc., a point is reached after which further addition of phosphate fertilizer to the soil only marginally increases crop output. The point of diminishing economic returns is almost invariably reached before this saturation point is reached; that is, the additional phosphorous-containing fertilizer and its application costs more than the increased crop output is worth.