MOST soya bean crops are grown without the application of top-dressing nitrogen fertilisers like ammonium nitrate or urea.
And yet the soya crops look dark green and healthy like any other crop receiving mineral nitrogen fertiliser,
Instead of expensive mineral N fertilisers, soya bean farmers inoculate their soya seed with a special bacterial preparation that contains live bacteria called rhizobia (one is a rhizobium). Inoculation involves mixing the seeds with the soya bean rhizobium inoculant just before planting.
The rhizobium inoculant works as the ‘top dressing’ in place of ammonium nitrate/urea.
The inoculant can be regarded as a ‘bio-fertiliser’; (bio- = living things, that is, fertiliser from living things, the rhizobia bacteria).
The difference between other ‘top dressing’ fertilisers is that rhizobium inoculant contains living bacteria mixed with the seed at the time of planting.
Rhizobium inoculants are produced by the Ministry of Agriculture at a factory located at the Soil Productivity Research Laboratory, Grasslands Research Station near Marondera.
Orders can be placed at the factory or through the Department of Research and Specialist Services (DRSS) headquarters in Harare or through your supplier of agricultural inputs. Rhizobium inoculants can also be sourced from provincial AGRITEX offices.
Farmers are also encouraged to check with their local AGRITEX officers.
Storing rhizobium inoculants
The rhizobium inoculants must be stored away from heat and away from direct sunlight.
Heat and light will kill the rhizobium bacteria.
The inoculant sachets (packets) must be kept in a shaded place or in a refrigerator, if one is available at all times.
For a long time, soya bean could not be grown in rural areas because there were no refrigerators to keep the rhizobium inoculants cool.
More recent research by Zimbabwean scientists has shown that rhizobium inoculants can keep for up to four months at room temperature without the need for a refrigerator.
Inoculant packets can also be kept in a closed clay pot or Wrapped in newspapers in a wardrobe away from heat and light.
Rats and mice will eat up the inoculants if not properly stored.
Shelf-life of rhizobium inoculants
Shelf-life is the length of time the inoculant remains viable. Shelf-life is longest when inoculants are stored at low (not freezing temperatures) like in a refrigerator, and shortest when inoculant is stored under very warm or hot conditions. Inoculants can remain viable for up to four months when stored in the dark at room temperature or up to six months in a refrigerator.
After the period indicated, the rhizobium bacteria will have used up all the food (sugar) in the inoculant packet and died. It is not recommended to use inoculant packs left over from the last season.
The bacteria are long dead and no nodules will form on the roots.
The plants will look yellow and require the farmer to buy and apply commercial nitrogen top-dressing fertiliser.
Farmers are therefore advised to throw away all inoculant sachets left over from the previous season.
They should buy fresh rhizobium inoculant each cropping season.
Once the sachet is opened, the inoculant must be used the same day.
This is because the bacteria die quickly when exposed to the air and to light.
Cost of inoculants
Each packet of inoculant weighs about 80g.
One sachet is enough for 1 ha and costs US$5 at the factory gate.
This price is subsidised by Government who own the Rhizobium Inoculant Factory at Grasslands Research Station.
If a soya bean farmer fails to buy rhizobium inoculants, s/he must buy the expensive mineral nitrogen fertlisers such as urea and ammonium nitrate in order to supply nitrogen to the crop.
The equivalent for each US$5-worth sachet of rhizobium inoculant is to apply commercial nitrogen fertiliser such as ammonium nitrate at a rate of 4 x 50kg bags per ha.
This will cost the farmer a total of US$140 at current (2017) prices.
Using inoculant saves the farmer up to U$135/ha.
Such savings make soya bean to be a very lucrative crop.
Using the rhizobium inoculant represents a huge saving on the cost of nitrogen top-dressing fertiliser.
While the rhizobium inoculant is applied at planting stage at little cost, application of top-dressing fertiliser has additional costs as it requires equipment such as fertiliser spreader, tractor, fuel and manpower.
If inoculant is ordered from middlemen, or other commercial outlets, the cost may be higher due to transport and profit mark-ups.
What is important to note is that rhizobium inoculants significantly lower the cost of N fertiliser/ha for soya crops.
Inoculants are not only cheaper, they are more efficient in supplying the plant with N as leakages are minimal and environmental safety is enhanced.
Inoculation rates
One sachet of inoculant contains enough rhizobia bacteria to inoculate 100kg of seed, enough to plant up to one hectare. Each packet should not be used on more than 100kg seed. However, the sachet of inoculant can be used on less seed without any harm to the seed.
Our research has shown that in sandy soils, soyabean yields actually increase if more rhizobium inoculant is used up to four times the recommended rate.
When planting by hand, with limited labour, it may not be possible to plant one hectare in one day.
In such cases, it is recommended to buy up to four packets of inoculant per hectare to allow a fresh packet to be used each day.
All inoculated seed should be planted the same day.
Rhizobium inoculant production
The inoculant consists of nitrogen fixing bacteria called rhizobium.
The bacteria are multiplied by growing them in a sugar solution to obtain billions of cells per millilitre.
Special plastic bags (sachets) are filled with bargasse (fine sugar cane waste) which acts as the carrier material.
The bargasse is soaked with sugar solution for the rhizobium bacteria to feed on.
A syringe needle is used to inject the suspension of rhizobium cells into the sachets.
The sachets are further incubated at optimum temperature so that the cells multiply further to give a population of more than a billion cells per gramme of inoculant.
The sachets containing the rhizobium inoculant are then packed in boxes ready to be shipped to farmers.
Each sachet is labelled with the name of the legume on which the particular inoculant is to be used.
Each legume has its own type of rhizobium inoculant.
For soya bean, each sachet contains enough rhizobium inoculant for 100kg of seed; to plant up to one hectare.
Each box of inoculant sachets comes with instructions on how to use the inoculant.
These instructions must be followed carefully, otherwise the inoculant will not work properly.