Fertilization

In spite of the increasing use of synthetic fertilizers in agricultural practice and the subsequent raise of crop yields, it is found generally that, to maintain a high level of inherent, their use must be accompanied by periodic applications of bulky organic manure, either in the form of farm compost, or other organic by-product, either of the farm of certain industries.

The maintenance of high productivity depends on a number of factors; the soil must be in the optimum condition for crop growth and must be in a position to furnish both food and water; it must have a suitable reaction in the chemical sense, and it must contain no substances toxic to growth.

It must be physically amenable to cultivation, resistant to forces of erosion, and the micro-flora and micro-fauna must be of a character to ameliorate the general chemical and physical properties of the soil and the soil-plant relationship. Any substance which when added to the soil brings about an improvement in any one of those directions could be considered as a fertilizer or manure using the terms in their widest sense, and it is in connection with these indirect benefits that organic manures are of such great importance in fertilizer practice. As a source of plant food, farmyard manure contains all the important nutrients although their availability is variab­le. The potassium present is readily soluble and immediately available; the nitrogen is present in both available forms, and in compounds which only slowly break down; the phosphate compounds also decompose slowly. This is one reason why, for particular crops, applications of manure need to be supple­mented with dressings of inorganic fertilizers and in parti­cular with available phosphorus compounds. The less avail­able portions of the farmyard manure are slowly released and become of value to succeeding crops.

Whatever the type of soil, applications of manure can have a beneficial action upon the physical properties. Heavy soils can be made easier to work, and the aeration and drainage improved by means of the increased organic-matter content, whilst on sandy soils the moisture-holding capacity of the manure increases drought resistance, the binding effect of the organic matter controls erosion and the increased base-exchange capacity improves the power of the soil to retain plant nutrients. Soil colour will be darkened by the incorpo­ration of humus, with an increase in the heat-absorbing power.

If a crop is growing under conditions of a single nutrient deficiency, the addition of appropriate fertilizer will give an increase in yield, but increases so observed are not direct­ly proportional to the quantity of fertilizer used. As the rate of application increases, a given extra increment of fertilizer gives a smaller increase in crop until a maximum yield is reached. Beyond this figure further additions may either have no effect or may even do damage and cause crop reduction.

The growth responses following the provision of adequate supplies of two or more limiting nutrients are not necessarily the arithmetical sum of the separately obtained responses. Quite frequently it is found that the increase in yield of a crop when supplied with two different fertilizers is much greater than the sum of the individual yield increases; the two fertilizers are then said to have a positive interaction.

The use of compound or mixed fertilizers is increasing and possesses many advantages from a management point of view. Labour costs are considerably lowered by the reduction of the number of operations necessary, and frequently compounds can be obtained commercially containing the nutrients in concentrated form, thus reducing the quantity to be applied in any particular instance.

base-exchange capacity — емкость катионного обмена