Fertilise your soybean crop with plant nutrients

September 2025

SOYBEANS ARE PART OF THE LEGUME FAMILY, WHICH IS UNIQUE IN THE SENSE THAT IT HAS THE ABILITY TO EFFECTIVELY UTILISE RESIDUAL (BUILT-UP) SOIL FERTILITY. NITROGEN (N) CAN BE FIXED BY LEGUMES THROUGH A SYMBIOTIC RELATIONSHIP WITH BRADYRHIZOBIUM JAPONICUM, COMMONLY KNOWN AS NODULATING BACTERIA.

Due to this unique ability, it is often assumed that soybeans do not respond well to fertilisation. However, the starting point remains to understand which nutrients are available for uptake by the soybean plants. A soil analysis will provide a very good indication of the potentially available plant nutrients, such as phosphate (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg).

AVAILABILITY OF NUTRIENTS
The effective uptake of these nutrients is not guaranteed, as environmental factors such as plant-available moisture, the soil pH and the presence of nematodes can play a major role in the availability of nutrients. It is therefore essential to ensure that nutrient uptake can occur as easily as possible.

A good planting mixture that is band-placed (at least 5 cm away and 5 cm below the seed), can help the plants to establish better and develop stronger root systems. This is especially true in cold, wet soils and sandy soils.

As with any other crop, the yield of the previous harvest and the conditions during that season must be considered. Nutrient carry-over can occur in cases of a lower-than-expected yield. The opposite of this is soil depletion, which happens when the previous crop yields more than what the soil was fertilised for.

If the residual nitrogen from a previous maize crop is high due to over-fertilisation or drought, there is a good chance that the subsequent soybean crop will not form many nodulating bacteria. The reason for this is that the plant receives enough nitrogen from the soil and therefore does not need the symbiotic relationship.

The plants then usually grow vigorously with long internodes but have relatively low yields because the residual nitrogen is depleted later, leading to a shortage during pod filling. This happens because the reduced number of bacteria cannot produce enough nitrogen for the plant.

Nitrogen
On sandy soils, a small amount of nitrogen can be beneficial because sandy soils retain relatively low levels of residual nitrogen. This gives the plant a good start, while the symbiotic relationship between the soybean plant and the bacteria only becomes active later.

Therefore it is advisable to discuss the findings and planning with a knowledgeable crop advisor to find an appropriate solution with the lowest possible risk.

NUTRIENT removal
According to the Fertilizer Association of Southern Africa (Fertasa), soybeans, like any other plant, require nutrients to produce optimally. However, soybeans extract more nutrients per ton of grain harvested than other crops, such as maize and wheat. Table 1 shows the nutrient removal of 1 ton of soybeans compared to that of maize.


The belief that soybeans transfer nitrogen to the next crop has been well researched in South Africa. Dr André Nel (formerly of ARC-Grain Crops) states that soybeans should rather be considered as nitrogen-neutral. This means that nitrogen is not necessarily transferred to the next crop. The yield increase of maize planted after soybeans could rather be attributed to an improved root system (16% more roots than maize after maize), as well as overall root health.

IMPORTANCE OF FERTILISATION
According to Prof. Fred Below and Dr Ross Bender from the University of Illinois, fertilisation – besides weather conditions – is extremely important for ensuring success with soybeans. Their research shows that the peak of nutrient uptake occurs between the R5 and R6 growth stages.

If the soil can meet the plant’s nutrient demands during these stages, there will be little response to fertilisation. The opposite is also true if the soil cannot meet the plant’s needs at that time.

UPTAKE CURVES
Graphs 1 to 3 show the phosphorus (P), potassium (K) and sulphur (S) uptake curves for short-season soybeans (fast-growing soybeans) planted in the United States of America. The information in the graphs is based on a yield of 4 t/ha.

Nutrient reserves such as phosphorus (Graph 1) are depleted very quickly by soybeans. About 80% of the phosphorus taken up from the soil ends up in the grain. Approximately 60% of the potassium (Graph 2) and sulphur (Graph 3) taken up by soybeans also ends up in the grain.

HOW TO APPROACH FERTILISATION
Fertilising soybeans can be approached in various ways. For grain removal, it can be done if the soil analysis is normal, while lower soil analysis levels may require fertilisation beyond just nutrient replacement. Soil correction can also be done, for example, to raise the phosphate status with 20 mg/kg to 25 mg/kg (Bray 1). Fertilisation can also be applied to replace nutrients removed after the soybeans have been harvested, thereby maintaining soil nutrient levels.

Lime application is crucial to ensure that plant nutrients are optimally available for uptake by the roots, and also to create a favourable pH for the nodulating bacteria. Micro-elements (zinc, iron, manganese, copper, boron and molybdenum) also require attention, as any form of deficiency or poor uptake can significantly impact the yield.

High yields are being harvested more frequently these days, particularly when the climate during the growing season was favourable and when the soil could meet the nutritional demands of the higher yield (Table 2). To supply large amounts of nutrients at specific growth stages, the soil’s status of the specific nutrient must be high or sufficient fresh nutrients must be applied.


IMPROVE SOIL THROUGH PRECISION CORRECTIONS
Soils, where soybeans are regularly cultivated but not fertilised or improved through precision corrections will experience a decline in nutrient status, and over time, yields will decrease. It has been found that the phosphorus and potassium levels significantly dropped over a five-year period where soybeans were grown in a 50/50 maize rotation system.

Phosphorus decreased from 18 mg/kg to 9 mg/kg (Bray 1), and potassium dropped from 95 mg/kg to 52 mg/kg (ammonium acetate). It is evident that the soil has become severely depleted and that its potential has declined. Restoring the soil to optimal levels for crop production will be a costly endeavour.

When environmental factors such as the rainfall (which the farmer has no control over) are no longer the limiting factor for yields and it is instead the soil’s poor nutrient status that determines the yield, the risk of economically cultivating soybeans becomes very high.

Fertilising soybeans should, in principle, be approached like any other crop – what is removed must be replenished. Balanced fertilisation that includes most plant nutrients will deliver the best yield results over the long term.

Publication: September 2025

Section: Pula/Imvula

Author: KOBUS VAN ZYL, SENIOR AGRICULTURIST, AND PETRUS VAN DER MERWE, AGRICULTURIST, OMNIA NUTRIOLOGY^®