What is the NPK ratio for fertilizing soybeans?

During the seedling stage of soybeans, applying an appropriate amount of nitrogen fertilizer makes the plants grow stronger; generally, applying 7.5 to 10 kg of urea per mu is suitable. During the flowering stage, spraying a 0.2% to 0.3% aqueous solution of monopotassium phosphate or superphosphate as a foliar spray can increase the nitrogen content of the seeds and has a clear yield-increasing effect; spraying a 0.1% aqueous solution of borax, copper sulfate, or manganese sulfate at the flowering stage can promote full seeds and increase the oil content of soybeans.

I. Nutrient Requirements of Soybeans

Among grain and oil crops, soybeans have relatively high economic value. Although soybeans need a lot of nitrogen, they can fix nitrogen through root nodules, generally obtaining 5-7.5 kg per mu from the atmosphere, which is about 40%-60% of the soybean's nitrogen requirement. For every 100 kg of soybeans produced, 1.8-10.1 kg of nitrogen, 1.8-3 kg of phosphorus, and 2.9-3 kg of potassium need to be absorbed from the soil. The nutrients absorbed by soybeans are far higher than those of rice, wheat, and corn.

II. Nutrient Uptake Patterns of Soybeans

The growth and development of soybeans is divided into the seedling stage, branching stage, flowering stage, pod-setting stage, seed-filling stage, and maturity stage. The entire growth period is 90-130 days, and its nutrient uptake patterns are as follows: Nitrogen uptake rate. The emergence and branching stages account for 15% of the total nitrogen uptake during the entire growth period, the branching to full flowering stages account for 16.4%, the full flowering to pod-setting stages account for 28.3%, and the seed-filling stage accounts for 24%. The flowering to seed-filling period is the peak period of nitrogen uptake in soybeans. Phosphorus uptake rate. The seedling stage to early flowering stage accounts for 17%, the early flowering to seed-filling stage accounts for 70%, and the seed-filling to maturity stage accounts for 13%. Soybeans have the greatest demand for phosphorus in the mid-growth period. Potassium uptake rate. The cumulative potassium uptake before flowering accounts for 43%, the flowering to seed-filling stage accounts for 39.5%, and the seed-filling to maturity stage still requires the absorption of 17.2% of potassium. It can be seen that the flowering to seed-filling period is both the peak period of dry matter accumulation in soybeans and the peak period of nitrogen, phosphorus, and potassium nutrient uptake.

III. Key Points of Soybean Fertilization Techniques

The fertilization system for soybeans generally consists of base fertilizer, seed fertilizer, and topdressing. The principle of fertilization is to ensure that soybeans have sufficient nutrition while also giving play to the nitrogen-fixing role of rhizobia. Therefore, whether in the early or late growth stage, nitrogen application should not be excessive, so as not to affect the growth of rhizobia or cause lodging.

But on the other hand, the mistaken concept that “soybeans have rhizobia, so they do not need nitrogen fertilizer” must also be corrected. Fertilization should achieve a reasonable combination of major elements such as nitrogen, phosphorus, and potassium fertilizers with trace elements such as boron and molybdenum, and should use both slow-release and quick-acting fertilizers.

Soybean fertilization requires mastering the patterns, so that soybeans can grow better, have higher oil content, and yield higher profits.