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Soybean fertilization

Competition for carbohydrates

KALI Academy
Soybean is an attractive forage and protein crop. This article deals with the competition between rhizobia and pods. It furthermore gets into detail regarding the demands of soybeans on plant nutrition.

The ideal conditions for growing soybeans are a pH value in the neutral range (pH value 6-7) and a humus-rich and airy soil capable of warming up as easily as possible. Nodule bacteria are known to take over the supply of nitrogen from the air. It should be observed that a high nitrogen supply from the soil at the beginning of the crop's growth can have a negative effect on the development of the nodule bacteria. Once the rhizobia have established, they reliably fix enough nitrogen from the air to meet 70 to 80 percent of the plant's needs. Further requirements for optimum growth are covered by N mineralization from the soil or, if necessary, by fertilization.

Nodule bacteria provide nitrogen to the plant in exchange for carbohydrates

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Due to magnesium deficiency, soyabean plants cannot transport enough photosynthesis carbohydrates to the roots on the left side (- Mg). Therefore, root growth is inhibited compared to a plant well supplied with magnesium (+ Mg) and also the nodule bacteria do not develop optimally.

In return for nitrogen fixation, the nodule bacteria need carbohydrates (e.g. sugar) for their own nutrition, which are provided by the plant. This is where the nutrients potassium and magnesium come into play. Both are essential for the smooth functioning of photosynthesis, the process by which carbohydrates are produced. In addition, both magnesium and potassium play an important role in the transport of carbohydrates within the plant - for example to the roots or to the harvesting organs.

Carbohydrates can only be transported to the nodule bacteria in the roots if there is a good supply of potassium and magnesium. They feed the rhizobia and also facilitate the formation of a good root network - so water and nutrients from the soil can be better accessed by the plant. If potassium or magnesium are not present in sufficient quantities, a large proportion of the carbohydrates remain in the leaves and are not available to the roots and rhizobia.

Competition for sugar & Co.

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With the beginning of the soybean grain filling phase, the plant’s need for sugar and other carbohydrates produced during photosynthesis increases. The nodular bacteria, however, also continue to claim large quantities of these carbohydrates for themselves. At this stage at the latest, competition begins between rhizobia and pods for carbohydrates. In the event of potassium or magnesium deficiency, this competition not only leads to plant malnutrition but also to “undernourished” rhizobia and, as a result, to reduced N fixation – with negative consequences for yield and thousand grain weight.

In practice, the application of potassium and magnesium fertilizers must ensure that the soil is of class C content and replace deprivation resulting from harvested products. A balanced K : Mg ratio is important. At a yield level of 30 dt/ha (content class C), the use of 2 dt/ha ESTA Kieserit is recommended.

Pay attention to the supply of sulfur

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Sulfur is needed at the beginning of the vegetation period for bacterial protein build-up. Experimental results in which the weight of the nodule bacteria was measured have revealed a doubling of the rhizobia weight through the use of sulfate-sulfur (see graph). Magnesium further increased the development of the nodule bacteria by providing them with a better supply of carbohydrates. 

Once the rhizobia have formed sufficient protein of their own, the available sulfur is used to form protein in the plant, enabling the production of high-quality and protein-rich feed.

Part of the sulfur demand is covered by mineralization in the soil, which depends on temperature, moisture, and the amount of bound sulfur. In practice, however, it has been observed that especially at the beginning of vegetation, the sulfur supply is not always provided by mineralization alone. Therefore, fertilization with rapidly available sulfate-sulfur is recommended for ensuring optimal development of nodule bacteria at the beginning of vegetation and for yield optimization. Fertilization with ESTA Kieserit is the ideal way to cover this need.

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Increase and secure yields

In a field trial in Austria, the application of ESTA Kieserit significantly increased soybean yields (see graph: Effect of ESTA Kieserit and EPSO Top on soybean yields). The combination with an additional EPSO Top leaf fertilizer has revealed that the yield improved once again, as the temporary peak in need was optimally covered. 

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Depending on the content, targeted potassium fertilizer use produces significant yield effects, as is illustrated by the following test result. Potassium regulates the water balance and enables the soyabean plant to make optimum use of the available water for yield formation, especially in dry years. In this trial too (see graph: soybean yield increase with potassium fertilizer use and leaf fertilization using EPSO Microtop), leaf fertilization also resulted in a further increase in grain yields.

Conclusion

Potassium and magnesium for a high-grade, protein-rich quality

Potassium and magnesium ensure a good supply of vital carbohydrates for the soybean plants and the important rhizobia. Potassium and magnesium are essential for photosynthesis and the transportation of assimilates in the plant. Sufficient sulfur is also essential for the protein production of nodule bacteria and soybeans. Fertilization with sulfate sulfur is the best choice for meeting this sulfur requirement, as it is immediately available to the plant. Feeding plants with these nutrients in accordance with their needs enables good yields and a high-grade, protein-rich quality of the harvested products. 

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