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Why don't soybeans have nodules on their roots?

Dlaczego soja nie ma brodawek na korzeniach?

The most common causes of poor soybean nodulation in Poland

Soybean, as a large-seeded legume species, has been "equipped" by nature with the ability to symbiosis with the nodule bacteria Bradyrhizobium japonicum , thanks to which it is able to use atmospheric nitrogen. Bradyrhizobium japonicum bacteria do not occur naturally in Polish soil, so soybean seeds are inoculated with special bacterial vaccines. Unfortunately, bacteria, as living organisms, are quite sensitive to a number of external factors, which means that the process of establishing symbiosis itself is vulnerable to failure and it happens that no root nodules are visible on soybean plants despite vaccination. The reasons may be different, but Emilia Fink-Podyma, president of the Polish Soja Association, points to the two most common ones.

– The fact whether soybeans are in a given field for the first time or are returning to it again is the main factor that determines whether warts are likely to appear and to what extent. The problem of lack of nodules most often occurs in places where soybean has never been grown before. When soybeans are returned to the same site, the chances of nodulation are very high. The second factor that strongly limits warts is the lack of water. Establishing symbiosis with bacteria is a process that needs water to occur. In drought conditions, there are many more plantations without warts. Of course, we have no influence on the amount of rainfall, but we can take care of the soil humus content, thanks to which more water will be retained in the soil. There are also other factors, such as soil sealing or low temperatures, but they have a smaller impact on nodulation and occur relatively less frequently – explains Emilia Fink-Podyma in an interview with farmer.pl.

As the expert adds, this year the problem of lack of root nodules in places where soybean first appeared is quite common. However, new growers are comforted by the fact that after sowing soybean inoculated with nodule bacteria, the microorganisms will be able to survive in the soil for several years and in such "superinfected" fields it will be easier for nodulation to occur in the future.

It is estimated that the optimal conditions for the nodulation process are soil moisture of 60-70%. water capacity and temperature in the range of 14-28°C. Unfortunately, during the soybean sowing and initial development period (May and June), drought is increasingly common in Poland. In the study by Michałek (2004), in conditions of deep water deficit, the mass of soybean nodules was over three times lower than in the case of optimal soil moisture.

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Problem with soybean nodulation – soil reaction may be to blame

Another frequently cited possible cause of poor soybean nodulation is its cultivation in acidic and very acidic soils. Soil culture, especially its reaction, significantly affects the number and survival of nodule bacteria. The optimal soil reaction for Bradyrhizobium japonicum and the nodulation process is considered to be pH 6-7. In acidified sites, excess hydrogen and aluminum ions negatively affect the multiplication and survival of bacteria, the development of soybean roots, the penetration of bacteria into the roots, the formation of nodules and the fixation of atmospheric nitrogen. In soils with a pH below 5.5-5, the problem of no or few nodules on soybean roots is quite common.

– Soil pH affects nodulation and below pH 5 it negatively affects the survival of bacteria. However, there are signals from farmers that with very favorable conditions and high soil culture, we are able to compensate for the impact of low pH and warts will appear. Of course, this is not a rule, but such cases also happen, says Emilia Fink-Podyma.

There are also reports that soybeans and symbiotic bacteria "do not like fresh lime" and sowing vaccinated seeds shortly after liming the soil may also reduce nodulation. It is recommended to regulate the pH at least two years before soybean sowing in a given site, but according to our interlocutor, liming for soybeans is in a sense a matter of "weighing the risks", and increasing the pH in the season before soybean sowing will have more benefits than negative effects. and is less risky to warts than low pH.

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Soybean fertilization and nodulation problems

Fertilizing soybeans with nitrogen is a controversial issue among growers. There is an opinion that soybean is self-sufficient in this respect and does not need additional nitrogen fertilization. On the other hand, there are recommendations to provide plants with small amounts of nitrogen for the initial period of growth and development. Regardless of the approach, remember that too much nitrogen from mineral fertilizers or/or high nitrogen content in the soil hinders symbiosis and, as a result, may limit nodulation. A macronutrient that has a very positive effect on the symbiosis between soybeans and bacteria is phosphorus.

Soybeans can be foliar fed with micronutrients, especially zinc, boron, iron and molybdenum. This last element is especially important in establishing symbiosis and its deficiency may negatively affect the number of root nodules. Note – providing nutrients in seed fertilizers may in some cases limit the activity of symbiotic bacteria and the formation of nodules. A safer option is foliar feeding. As Emilia Fink-Podyma points out, the Polish Soja Association plans to conduct research on the impact of the use of microelements in Polish conditions on the development of soy, including: the wart process.

Vaccination errors may result in the absence of warts

Soybean seed producers graft the seeds before selling them. However, it is recommended to vaccinate soybeans twice: first, "factory", and then by the farmer immediately before sowing. When sowing seeds from your own propagation, this is a mandatory activity. Unfortunately, bacteria are sensitive organisms and must be handled gently. At the seed grafting stage, a lot can go wrong. Seed material should not be inoculated in high sunlight and high temperatures. If a given vaccine requires dissolution in water, it cannot be chlorinated water. The sowing of vaccinated seeds should not be delayed – it is best to sow within 24 hours of inoculation. The effectiveness of vaccination may be limited by the combined use of the vaccine with some seed fertilizers and fungicide treatments – it is best to apply such products before grafting the seeds. It is also important to ensure that the equipment used to apply the vaccine is clean, without any residues of other chemical preparations.

– Farmers' vaccination with factory-vaccinated seeds is becoming the norm. This is a very good phenomenon. Just a few years ago, any appearance of warts on the roots was a success. Today, there are fewer and fewer plantations without warts, which is due to agrotechnical progress, including double vaccination. The process of grafting seeds on farms generally goes well. We must be aware that farmers are not able to create laboratory conditions for the treatment, but the methods used in economic conditions are sufficient and give good results – says Emilia Fink-Podyma.

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