soya seed

Inoculate soya seed for healthier plants AND low fertiliser bills

The small-scale farmer faces many challenges because he must do everything by hand. But some things are just worth doing and inoculation of soya seed is one of them.


Soya, a legume, is a member of the same family as the Acacias, the African botanical iconic equivalent of the lion. Legumes are protein-rich and they possess a useful and unique ability to fix nitrogen (N), which is a building block of protein. Legumes include cowpeas, lucerne and beans.

Plants cannot use the nitrogen in the air around us, or atmospheric nitrogen. They must take up nitrogen from the soil, or mineral nitrogen, in the form of nitrates or ammonium.

Legumes are the one plant family that can take in atmospheric nitrogen, and ship it down to the roots. Below the surface, bacteria form nodules on the plant roots inside which they convert atmospheric nitrogen into the plant-friendly form.

Soya rotates successfully with other crops, because of the nitrogen it leaves behind.


The process is a collaboration between the plant and soil bacteria called Rhizobium. The relationship between bacteria and plant is mutually beneficial, much like a marriage, in which both partners are working hard to keep the system healthy and functional.

Once the bacteria have invaded the roots of the soya plant, they form small, round nodules that can be seen two to three weeks after planting. White or grey at first, the nodules change to a reddish or pink colour, a sign that nitrogen fixation up and running. One could think of this pink colour in the nodules as the lifeblood of the bacteria.

During the growing season nodules die-off and are replaced. Green nodules are past their sell-by date and the plant will discard them.
At the height of the growing season, mostly pink or reddish nodules should be seen. The presence of more white or grey nodules show that the N fixation factory is not working well.

This could be because of a poor strain of bacteria, low bacterial population numbers, undernourished plants, or plant stress, like flooding or drought.


In a natural (as opposed to an agricultural) ecosystem, legumes can contribute 20kg/ha to 65kg/ha of nitrogen to the soil. In a cultivated cropland legumes can add greater volumes of nitrogen, and figures of over 100kg of N/ha are not impossible.

Since legumes can fix their own nitrogen, farmers don’t have to use N-fertilisers such as urea, or ammonium nitrate, on the crop.

When nitrogen is plentiful in the soil, legumes will not invest much energy in fixing atmospheric nitrogen or nodule formation. Instead, the plant will use the nitrogen that is freely available in the soil.

N fixation is a process that uses up plant energy, as it ships sugars and carbohydrates to the bacteria nodules.

With quantities of N fertiliser in the soil, the plant will opportunistically switch to this ‘free’ nitrogen, before it takes in atmospheric nitrogen.

Legumes must have access to phosphorus (P) or they cannot fix N. P deficiency causes reduced plant growth and a reddish-purple colouring on the lower leaves.

Applying phosphate fertilisers helps with P deficiencies.

With enough P in the soil, soya can fix as much as 450kg of N/ha. If a soya crop needs 350kg N/ha, the crop can satisfy its own N requirements and leave behind residual N for follow-on crops. The plant residues must go back into the soil to get the bonus.

So, fertilising a crop, that is fixing enough N for its needs, will make no difference to the yield. As long as these plants are fixing nitrogen, they don’t respond to nitrogen fertiliser; alternatively, they will respond to the fertiliser and stop fixing N. Either way, there is no gain in plant production.

The farmer may give the crop a boost at start-up by incorporating small amounts of nitrogen (as little as 10kg/ha) at planting, before the nodules have formed.

Soya growers may boost the crop with an initial application of nitrogen (10kg/ha) to help emerging plants before nodulation.

Smallholder farmers are said to be resistant to using inoculant.

But farmers, big or small, generally have practical mind-sets and a certain degree of common-sense. No crop farmer in his right mind wants to buy fertiliser at exorbitant prices, when the plant will give it to him at a fraction of the cost.


Fertiliser is a high cost input and fertiliser prices in Africa are among the highest in the world. Inoculating soya seed with rhizobium bacteria, especially in lands which have never been planted to soya, seems to be a no-brainer.

At about 30% of the cost of fertiliser, inoculant is a cheap, effective resource.

Trials, in which seed was inoculated and P applied at 20kg/ha, have seen yields rise by between 102kg/ha to 172kg/ha (N2Africa, Bill and Melinda Gates Foundation).

Of course, nobody wants to inoculate seed if there are good indigenous populations of bacteria present in the soil, but current thinking is that there are not enough background bacteria to make the difference farmers are looking for in their crop yields.

There are a few reasons for weak nitrogen fixation in the field. The obvious one is nutritional stress which can be corrected by applying the right fertiliser; sulphur, iron, calcium, zinc, chlorine, molybdenum, manganese, boron, potassium, phosphorus and cobalt are important nutrients for soya.

The legume responds positively to nutrient inputs, and nitrogen fixation will improve as the plant gets healthier and more robust.

Where nodulation is feeble, expect low yields, even with N fertiliser applications. This is because nodules help the plant use fertiliser nitrogen efficiently.


Soya, a specific legume, needs specific rhizobia for the N fixing partnership. This is unlike the cowpea, which is a promiscuous legume. These rhizobia are not usually found in African soils, which makes inoculation of soya important.

Inoculate by measuring out a quantity of seed into a suitable container that holds the seed comfortably, in this case 15kg. The inoculant is packaged in 125g doses, the quantity required for 15kg of seed.

Pour 300ml of clean lukewarm water into a 500ml plastic bottle, add 2 tablespoons of sugar and mix properly. This solution is called the sticker.
Add the sticker to the seed and mix it well to coat the seed.

Mix the inoculant with the sticker-coated seed – gently – covering the seed in your container.

Rhizobia cannot survive in sunlight, so the container must be covered with paper or cloth and kept in the shade. Plant immediately to minimise the contact with the light and plant early or late in the day, for less sun.

If inoculant is stored in a cool, dry place in an earthen pot it may remain viable for as long as four months although a shorter period is probably advisable.

Taking the backache out of planting by hand, Michris Janse van Rensburg demonstrates his self-designed hand-held soya planter. The planter is a manual one-pass implement, ideal for small-scale soya growers.

Primary source : N2Africa, Bill and Melinda Gates Foundation

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