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Effective nitrogen management in soybeans is crucial

October 2023

In South Africa the soybean production area has grown considerably, with new areas being planted throughout the country. The growing conditions are very different from the east to the west of the country. Farmers have to consider each situation that could influence the treatments of the nitrogen-fixing bacteria.

Every farmer should decide which product to use on his soybean crop and how to manage the treatment. In this article, the basic guidelines of how to make this decision are explained.

Nitrogen-fixing bacteria fall under the pesticide registration process at the Department of Agriculture in South Africa and all products must be registered under this act to be sold. 

All soybean varieties rely on Bradyrhizobium japonicum for biological nitrogen fixation. The Agricultural Research Council (ARC) isolated B. japonicum WB74 many years ago, but there are various strains around the world which can and will fix high volumes of nitrogen. All the strains registered in South Africa are effective, but they will have different application guidelines to be truly effective. Speak to your representative to find the best way.

Multi-strain products may sound great to the ear, as you could get a dual or triple mode of action, but in biological products the active ingredient concentration is extremely critical to the performance of the product. Once there is a saturated solution, you need to reduce the numbers of the one B. japonicum strain to add another strain to the formulation.

Now there are two or more strains of B. japonicum in the product, of which one will be more dominant to perform the targeted action – in this case, nitrogen fixation on soybean. So, in essence now there is a reduced volume of the dominant strain of bacteria, which is performing the task.

The quality of the product is a very important aspect of B. japonicum and will influence its ability to be effective in the field. Due to the slow growth rate of these microorganisms, the final product must be free of contaminants and must contain a high amount of the microorganism. 

Looking back, ten years ago most of the products in the market were manufactured with a one-to-six-month shelf life, with 24 hours to treat and plant the product. However, with new technologies, products can be produced with an 18-month shelf life and 60 days between treatment and planting. This shows that the products are now more stable and higher quality, with added technology to protect the bacteria on the seed. 

The placement of nodules is also very important, as the quantity of plant-available nitrogen it can produce is ten times more on the primary root than on the secondary root structure. 

Be careful not to look at the number of nodules only, but rather at the positioning on the root zone. This is very important, as the same amount of nitrogen can be fixed by one nodule of the same size in dry weight on the primary root as ten nodules on the secondary root. 

Planting conditions are extremely important, as bacteria and seed require moisture to germinate and grow. This is often easier said than done with the growing season in South Africa. Farmers are constantly pushing this boundary to plant or not due to dry soil and very hot top 5 cm of soil at 40°C to 50°C. When soybeans are planted, the best conditions are essential for good nodulation and germination of the plants. 

When treating biological products to your seed via seed treatment or through infurrow application, you need to take the temperature and sunlight into account. Store B. japonicum at a maximum of 25°C ambient temperature and out of direct sunlight. 

The operation needs to be managed more strictly. For instance, if you did a seed treatment, take only the seed that you will use for the morning planting. It is advisable to rather collect fresh treated seed more frequently from your shed, where it should be stored carefully and responsibly. Management of the treatment and treated seed on the farm is extremely important in order to get good results.

The physical treatment of seed is vital to a good start. Ensure that every seed is treated with the same amount of B. japonicum. The treatment machine should also be clean and free of any chemicals that can influence the bacterial treatment.

Daily calibration of the seed treatment machines is very important, especially when changing from one variety to the next because of the different seed sizes. If this is not done, you could see varied results in the product. Make sure you follow the rules with the treatment method. 

The role of molybdenum (Mo) and cobalt (Co) is vital to the functioning of the nitrogenase enzyme, which acts as a catalyst to nitrogen fixation in soybean and legume plants. Without Mo and Co, the nitrogen fixation process will be limited. 

Certified seed farmers would apply three to four applications of Mo in the production cycle of the seed to increase the molybdenum in the seed. Molybdenum and cobalt can be applied as a seed treatment or foliar application in the growing season, as it will help the nitrogenase activity.

There are some warnings, as both Mo and Co products are salts which will dry out bacteria on the seed when treating them together with a rhizobium. So make sure that if you apply Mo and Co to the seed treatment, planting is done within four to six hours after the seed treatment. Make sure you have a very high-quality rhizobium inoculant with osmoprotection technology to increase the survival of the bacteria, as this will be essential to your nodulation. 

Fertilisers are important for the soybean yield, but management of the type and placement is very important. 

  • Too much nitrogen is very problematic, as about 10 kg/ha is more than sufficient for planting. Any more than this will hamper nodulation and delay the process. 
  • Secondly the placement needs to be a minimum of 50 mm below and 50 mm on both sides of the seed to ensure that moisture and salt burn will not happen. 

Use phosphorus (P), potassium (K), sulphur (S), calcium (Ca), magnesium (Mg) and micro-elements to increase the soybean yield. However, it needs to be especially created for your soybean crop. 

As growers and agronomists in the field, R1 would be a stage at which you need to evaluate the nodulation process and assess if this has happened successfully. If not, you need to act and apply high volumes of nitrogen fertiliser of 100 kg to 200 kg. Talk to your local agronomist for final details of this application.

By analysing many trials statistically and commercially in side-by-side trials, both methods of rhizobium application are effective and it really depends on the grower’s management of the farming operation.

If you feel comfortable with both methods, choose the best one for your situation. Infurrow treatment results in slightly but not significantly more secondary nodulation and can look more effective in some cases. But once harvested, you generally do not see any significant yield advantage due to the placement and efficacy of the nodules on the secondary root structure. Many farmers treat the seed and apply product infurrow to cover any mistakes in the treatment, which works well to ensure effective results. 

As more and more planting is done in sandy soils in the western region, nematode pressure and control are vital for good soybean production. This also influences the rhizobium nodulation, nitrogen fixation and ability to have space on the root structure. Please contact your chemical agent for registered options available.

Soybean inoculants are very important for your farming operation, providing high amounts of nitrogen for your soybean crop. The impact can range from 200 kg to 1 500 kg of yield per hectare. Depending on the soil, the region and yield of your planted crop, it could equate to as much as 40% of your crop’s yield. It is therefore critical to get the treatment correct via seed treatment or in-furrow applications.

Too much nitrogen will hamper nodulation.

Publication: October 2023

Section: Pula/Imvula

Author: Jonathan Etherington, director of MBFi (Pty) Ltd