Topdressing

the addition of fertilizers to crops during the growing season in order to improve plant nutrition and boost yields. Fertilizer is often applied to the soil, so that the nutrients are taken up by the roots. The plants may also be sprayed with solutions of fertilizers, and the nutrients penetrate the plants through the leaves and stems.

Topdressing was introduced by the German scientist P. Wagner in the late 19th century. In Russia, the first experiments with topdressing, using potassium nitrate, were carried out on oat shoots at the Derebchin experimental field (Ukraine) in the late 19th century. Despite the beneficial results—a yield increase of about 6 centners per hectare (ha)— the method was not widely adopted. It was not until 1935 that topdressing with mineral and organic fertilizers was used over large areas in the USSR.

The following techniques of topdressing are common. (1) Dry fertilizer is scattered by fertilizer distributors, by fertilizer spreaders, or from an airplane (winter crops, rice). Sometimes the fertilizer is applied manually. (2) Dry fertilizer is scattered and worked into the soil with harrows, cultivators, or other implements. (3) Water and fertilizer solutions are applied with waterers or from overhead sprinklers.

The effectiveness of topdressing depends on the weather and on the properties of the fertilizers— their solubility and the extent to which they migrate in the soil. Mineral fertilizers that are quickly soluble in water are most often used for topdressing. These include nitrogen fertilizers, including ammonium nitrate, urea, an aqueous solution of ammonia, and sodium nitrate (for sugar beets); all potassium fertilizers; and superphosphate. Local fertilizers (ash, liquid manure, poultry manure, rotted manure) and microfertilizers are also used.

Early spring topdressing of winter crops is very important. It is often carried out on frozen soil. Winter crops consume the most nutrients from initial spring growth to ear formation. At this time there is usually an inadequate supply of nutrients in the soil. The fertilizers added before sowing are absorbed by the soil, washed out by rain, and used by plants and microorganisms, and the nitrification process is still surpressed by the low temperature and high moisture content. The maximum increases in yield are obtained by using a complete fertilizer containing N, P₂ O₅, and K₂O as topdressing. In regions with adequate moisture, nitrogen fertilizers should be applied twice— in early spring and before the stem extension stage. The average amount of fertilizers used as topdressing for winter crops is about 60 kg/ha of NPK, which produces an average increase in yield of 3-5 centners per ha. Row crops that should be topdressed during irrigation include cotton, sugar beets, and rice. In the Soviet Union, about 40 million ha have been topdressed every year since 1965. Top dressing is widely used in France, Italy, other European countries, the United States, and Canada.