How to determine the timing of topdressing for soybeans and the fertilizer amount per mu

Soybean is a relatively common cash crop in China, so it is grown in many regions. But to grow soybeans well, you need to do a good job with basic management such as water and fertilizer, light, and temperature. So when is the best time to fertilize soybeans? How much fertilizer is suitable per mu? How should you fertilize?

I. The importance of topdressing soybeans and their fertilizer requirements

Topdressing is fertilizer applied during the crop's growth period, based on the crop's nutrient needs, to make up for a shortage of base fertilizer. Topdressing is extremely important for high soybean yields. To promptly meet the nutrient needs of growth and development, quick-acting fertilizers should be used for topdressing, such as chemical fertilizers and well-decomposed organic fertilizers. Topdressing methods for soybeans include broadcasting, furrow application, hole application, and spraying. The type, amount, method, and number of topdressings should be determined according to the weather, soil, and the growth condition of the soybeans.

For every 100 kilograms of soybean grain produced, 7.0-9.5 kilograms of nitrogen, 1.3-1.9 kilograms of phosphorus, and 2.5-4.0 kilograms of potassium need to be absorbed; the ratio of nitrogen, phosphorus, and potassium is 1:0.2:0.4. Of the nitrogen absorbed by soybeans, 1/3-1/2 comes from nitrogen fixation by the soybean's own rhizobia, and the rest comes from the soil and fertilizer. Phosphorus and potassium are mainly supplied by the soil. Soybeans also especially need boron and molybdenum, among which molybdenum is related to the nitrogen-fixing ability of rhizobia.

The fertilizer requirement characteristics of soybeans at each growth stage are closely related to plant growth. The amount of nitrogen absorbed by soybeans from emergence to flowering accounts for 20% of the total absorbed over the whole life cycle; from flowering to the pod-filling stage, nitrogen absorption accounts for 55%; from the pod-filling stage to maturity, nitrogen absorption accounts for 25%.

From emergence to the initial flowering stage, soybeans absorb only 15% of total phosphorus; from flowering to the pod-setting stage, they absorb 60%, reaching a peak; from pod setting to pod filling, they absorb 20%.

Potassium absorption from emergence to flowering accounts for 32.2% of the amount absorbed over the whole life cycle of soybeans, higher than the absorption ratios of nitrogen and phosphorus; from flowering to pod filling, absorption accounts for 61.9%; while from the pod-filling stage to maturity, absorption accounts for only 5.9%. The pod-setting stage is the period when soybeans absorb the most nitrogen, phosphorus, and potassium nutrients, and the absorption intensity is high, which can easily lead to fertilizer deficiency.

II. How should soybeans be topdressed?

Generally, soybeans enter their most vigorous growth stage starting from the flowering period. Plant height, leaf area index, and dry matter accumulation all increase relatively quickly, and the nutrients required are also greater. However, at this time the available nutrients in the soil usually drop to a low level, and the nutrient supply is insufficient. Therefore, topdressing quick-acting nitrogen fertilizer at the initial flowering stage of soybeans can significantly increase dry matter accumulation and the photosynthetic rate, and is an effective measure for high soybean yields.

Topdressing nitrogen fertilizer should also be handled flexibly according to the plant growth condition and soil fertility. For example, if the soybean seedlings are thin and weak and the soil is poor, the topdressing time should be appropriately moved earlier to promote vegetative growth and build a good high-yield framework. For plots that have already been topdressed at the seedling stage, if fertilizer is still lacking at the flowering stage, one more topdressing can be applied. If the soybean seedlings grow robustly, the leaf area index is too large, and the soil alkaline hydrolyzable nitrogen content is above 80 milligrams/kilogram, then nitrogen fertilizer does not need to be topdressed at the initial flowering stage, so as to avoid excessive vegetative growth and lodging. Apply 10-15 kilograms of urea per mu; poor land and weak seedlings can be topdressed with a bit more. Furrow application is preferable for topdressing, though urea can also be broadcast between the rows. It is best to combine this with intertillage and irrigation to ensure fertilizer efficiency. When broadcasting, avoid spreading fertilizer on the leaves to prevent burning them.

Phosphorus fertilizer is generally used as a base fertilizer. If phosphorus is deficient, phosphorus fertilizer can be applied in furrows at the soybean seedling stage or initial flowering stage. If phosphorus is severely deficient, phosphorus fertilizer still needs to be topdressed at the pod-setting stage. The topdressing amount is generally about 25 kilograms of superphosphate or an equivalent amount of other quick-acting phosphorus fertilizer per mu, and a bit more can be applied when phosphorus is severely deficient. After the pod-setting stage, soybeans should be topdressed with less phosphorus. Because phosphorus moves very little in the soil, it must be applied in the plow layer, preferably by furrow application; when topdressing phosphorus fertilizer at the soybean pod-filling stage, foliar spraying is best.

Potassium fertilizer should be topdressed early, best from the soybean seedling stage to the initial flowering stage, applying 7.5-10 kilograms of potassium sulfate or an equivalent amount of other quick-acting potassium fertilizer per mu, applied in furrows in the plow layer.

Topdressing soybeans can be combined with watering, intertillage, and earthing up. Gengzhongbang suggests intertilling and earthing up immediately after topdressing, which can prevent fertilizer loss.

III. How should soybeans be fertilized?

Foliar spraying is a topdressing method used outside the plant roots. Generally, a sprayer is used to spray fertilizer solution or dry powder onto the crop's leaves or the underside of the leaves. Foliar spraying allows fertilizer to be absorbed directly through the leaves and quickly transported to various organs, participating in biochemical processes within the plant, and can significantly increase yield and improve quality. Because this method does not pass through the soil, the nutrients will not be fixed by the soil, and fertilizer use efficiency is high. But this is only one method of foliar topdressing and cannot replace root topdressing.

Fertilizers commonly used for foliar spraying on soybeans include urea and monopotassium phosphate, with a spraying concentration of 1%-2%; the spraying concentration of trace element boron, manganese, copper, and zinc salts is generally 0.1%, and the spraying concentration of molybdate is 0.01%-0.05%.

The mixing ratio for foliar spraying topdressing on soybeans is generally 0.5 kilograms of urea, 0.5 kilograms of monopotassium phosphate or 1.5 kilograms of superphosphate, and a water extract of 0.75 kilograms of potassium sulfate, diluted with 50 kilograms of water, and sprayed onto the plant leaves. Foliar spraying on soybeans should be carried out on cloudy days or after 4 p.m. on sunny days. Spraying on the underside of the plant leaves gives a higher absorption and use efficiency than spraying on the upper side. Generally, it can be absorbed and used by the soybean plants 2-8 hours after spraying.