What causes soybean root nodules to be few and small?

Soybean roots have root nodules that can perform symbiotic nitrogen fixation, which is an important source of nitrogen for legume crops. But in actual production, sometimes soybean root nodules are few and small. What causes this?

Expert: When encountering particularly severe symptoms, the first thing to consider is that this may be caused by a deficiency of the trace element molybdenum. Since molybdenum is an essential trace element for nitrogen fixation by legume root nodules, the absolute content in general crops is at the level of one part per million, while soybean's requirement for molybdenum is dozens of times higher than that of other crops. The main role of molybdenum is to promote the formation of soybean root nodules, increase the number of root nodules, and enhance the nitrogen fixation capacity of root nodules, which can enhance drought resistance.

The control measures for molybdenum deficiency in soybean include the following methods:

First, treat seeds with molybdenum fertilizer. The commonly used molybdenum fertilizer is ammonium molybdate. When treating seeds, use only 2 grams of ammonium molybdate per kilogram of soybean seed. First dissolve the ammonium molybdate in 5 times its volume of hot water, stir thoroughly, and after cooling, treat the seeds. Second, spray molybdenum fertilizer on the leaves before and after the flowering period. The spray concentration is a 0.005%-0.01% ammonium molybdate solution. Foliar spraying generally requires spraying more than twice. Third, since molybdenum deficiency symptoms mostly occur on acidic soil, the soil acidity can be reduced by applying an appropriate amount of lime to improve the availability of molybdenum. It should be noted that molybdenum application mainly uses the foliar fertilization method, and the soil application method is generally not used, in order to control the dosage. Therefore, if the soil application method is used, 200 grams of molybdenum fertilizer per mu should be thoroughly mixed with organic fertilizer before application, and it should not be applied continuously in the same field year after year. Fourth, before applying molybdenum fertilizer to soybean, first determine whether the soil is deficient in phosphorus. If it is deficient in phosphorus, supplement phosphate fertilizer; otherwise, applying molybdenum fertilizer alone will instead reduce root nodules.

The condition of soybean root nodules is also related to the use of nitrogen fertilizer. If too much chemical nitrogen fertilizer is applied, it will also inhibit nitrogen fixation by root nodules, and will likewise show a reduced number of soybean root nodules, small nodules, and a lower nitrogen fixation rate.

In addition, phosphorus plays a very important role in the growth and development of soybean root nodules and nitrogen fixation. Phosphorus not only promotes root growth, but also benefits the development of root nodules and improves the nitrogen fixation capacity of root nodules. Therefore, on legume crops, attention should be paid to applying phosphate fertilizer. By applying phosphate fertilizer to promote nitrogen fixation, the effect of "using phosphorus to increase nitrogen" can be achieved.