Addition of elements or other materials to the soil to increase or maintain plant yields. Fertilizers may be organic or inorganic. Organic fertilizers are usually manures and waste materials which in addition to providing small amounts of growth elements also serve as conditioners for the soil. Commercial fertilizers are most often inorganic. Fertilizer

Methods of applying fertilizers vary widely and depend on such factors as kind of crop and stage of growth, application rates, physical and chemical properties of the fertilizer, and soil type. Two basic application methods are used, bulk spreading and precision placement. Time and labor are saved by the practice of bulk spreading, in which the fertilizer is broadcast over the entire area by using large machines which cover many acres in a short time. Precision placement, in which the fertilizer is applied in one or more bands in a definite relationship to the seed or plants, requires more equipment and time, but usually smaller amounts of fertilizer are needed to produce a given yield increase.

For some deep-rooted plants, subsoil fertilization to depths of 12–20 in. (30–50 cm) is advantageous. This is usually a separate operation from planting, and uses a modified subsoil plow followed by equipment to bed soil over the plow furrow, thereby eliminating rough soil conditions unfavorable for good seed germination. Top-dressings are usually applied by broadcasting over the soil surface for closely spaced crops such as small grains.

Since solid fertilizers range from dense heavy materials to light powders and liquid fertilizers range from high pressure to zero pressure, a variety of equipment is required for accurate metering and placement. In addition, application rates may be as low as 50 lb/acre (56 kg/hectare) or as high as 6 tons/acre (13.3 metric tons/hectare). Large bulk spreaders usually use drag chains or augers to force the material through a gate or opening whose size is varied to regulate the amount passing through and falling on the spreader.

Liquid fertilizer of the high-pressure type (for example, anhydrous ammonia) is usually regulated by valves or positive displacement pumps. The size of the orifice may be controlled manually or automatically by pressure-regulating valves. Low-pressure solutions may be metered by gravity flow through orifices, but greater accuracy is obtained by using compressed air or other gases to maintain a constant pressure in the tank. This method eliminates the effect of temperature and volume changes. Nonpressure solutions may be metered by gravity or by gear, roller, piston, centrifugal, or hose pumps. The accuracy of the gravity type can be improved by the use of a constant head device by which all air is introduced into the tank at the bottom.

Nonvolatile fertilizer solutions are often pumped into the supply lines of irrigation systems to allow simultaneous fertilization and irrigation. With the exception of bulk spreaders and other broadcasters, most fertilizer application devices are built as attachments which can be mounted in conjunction with planters, cultivators, and herbicide applicators. Often the tanks, pumps, and controls used for liquid fertilizers are also used for applying other chemicals such as insecticides.