What is the principle of rational topdressing of seedlings?

Fertilization during seedling growth period is called topdressing. The purpose is to supplement the needs of seedlings for different nutrients in each growth period in time.

(a) the principle of reasonable topdressing:

In order to carry out rational topdressing, it is necessary to comprehensively consider the weather, soil and seedling growth, correctly determine the proportion of nitrogen, phosphorus, potassium and trace elements, and determine the most suitable fertilization period and dosage.

1. Fertilize according to the weather:

When the weather is hot and rainy, we should master the principle of applying a small amount of fertilizer frequently to avoid nutrient loss. When the weather is cold, in order to exert fertilizer efficiency quickly, topdressing organic fertilizer must be fully decomposed. In years with high temperature, the first topdressing time can be advanced.

2. See soil fertilization:

Different soils contain different kinds and quantities of nutrients, and topdressing should be based on the principle of "supplementing what is lacking". As far as general soil is concerned, topdressing should be the main nitrogen fertilizer. If the soil is rich in nitrogen, the ratio of phosphorus to potassium should be increased. Calcareous soil or strongly acidic soil is prone to phosphorus deficiency, so the proportion of phosphorus fertilizer should be increased when topdressing. In addition, the soil texture is related to the ability of maintaining fertilizer. The more fine particles are contained, the stronger the ability of maintaining fertilizer, the more fertilizer can be applied and the longer the interval can be.

3. Seed seedling fertilization:

Different growth and development stages of seedlings require different nutrients. Nitrogen fertilizer and phosphorus fertilizer are needed in the early growth stage. The fast-growing period needs a lot of nitrogen, phosphorus, potassium fertilizer and other necessary trace elements. In the late growth stage, potassium fertilizer is the main fertilizer, supplemented by phosphorus fertilizer, and nitrogen fertilizer is avoided.

In addition to reasonable topdressing according to different stages of seedlings, plant nutrition diagnosis can also be carried out according to the morphological performance of seedlings and the determination of nutrient content in seedling tissues, which can be used as the basis for topdressing at that time.

(1) Under nitrogen deficiency, the seedlings are short and thin, with small leaves, light green or yellow-green. Old leaves of conifer seedlings turn yellow or fall off. The lateral buds of broadleaf seedlings died and the growth of shoot tips stagnated.

(2) Under the condition of phosphorus deficiency, the root system is underdeveloped, small and thick. In severe cases, the lateral buds degenerate and the branches become shorter. The leaves are small and thin, with dark green and brown spots, and the old leaves are red. The lower leaves are easy to wither and fall off, and their growth is inhibited. The needle tip is gray, blue-green and brown, which is smaller than normal leaves.

(3) When potassium is deficient, the stems of seedlings are weak and the growth of roots is inhibited. The leaves are small, partially contracted, with brown edges, yellow needle tips and gradual color changes.

The above are just the general manifestations of seedlings lacking certain nutrients, and the symptoms of seedlings of different tree species are often different. When environmental conditions are not suitable for seedling growth, symptoms similar to fertilizer deficiency often appear, so it is sometimes easy to be confused. Therefore, it is necessary to have some experience in plant nutrition diagnosis. It is necessary to combine the specific analysis of soil properties in order to make an accurate judgment.