IN the last episode in this series on soya bean production practices, we looked at land preparation and planting.
We emphasised the need for good land preparation that gives a fine tilth with few or no large soil clods (magadhi).
Planting by hand is viable where the area to be cropped is small and labour readily available.
Otherwise, for large areas exceeding 3-5 ha, machines such as tractor-drawn seed drills and precision planters are recommended.
This is one area where mechanisation needs urgent attention to expand the scale of soya production.
In this episode, we shall look at crop establishment including application of chemicals to control weeds and other diseases.
By the way, if after planting a soya bean crop, there is a heavy downpour followed by a dry spell, a crust can develop in some soils hampering soya seed germination.
This can result in a poor crop stand with low plant populations that result in uneconomic yields.
The soil crust can be broken using a spike harrow when the seeds begin to emerge, resulting in good germination.
A light irrigation cycle, if available, can also be applied to promote seedling emergence in case of a dry spell.
Soya bean is normally planted in soils with sufficient moisture to allow for good germination.
Dry planting soya should not be attempted except in cases where the farmer is able to immediately irrigate to an equivalent of 20-30 mm rainfall.
If dry-planted, soya seeds remain in the soil for too long, they will crack due to the heat in the soil and be attacked by insects or even moulds (fungi).
Germination will be very poor.
Farmers should wait for good soaking rains (20-30 mm) and move in quickly to plant.
Weed control can be achieved by reducing the space between rows to between 30-45 cm.
At that spacing the crop comes up and within two-to-three weeks, the canopy closes, shading out all the weeds.
Moisture is also conserved as the sun will no longer bake the soils.
Apart from close spacing to shade out weeds, hand weeding can also be used to control weeds.
This labour-intensive method is expensive, time-consuming and inefficient as many weeds close to the plants remain untouched.
A lot of soya plants are also cut down by the weeding team as they try to get at all the weeds.
Hand weeding is slow and unsuitable for large soya fields exceeding a hectare.
The use of crop chemicals called herbicides is recommended for controlling weeds in soya fields.
There are two categories or groups of herbicides for soya bean.
In each of the groups, there are herbicides that control grass-type weeds and those that control broad leaf weeds like blackjack (tsine/umhlabangubo).
Pre-emergent herbicides are applied immediately after planting (within 48 hours at the latest) but before the soya bean crop germinates.
They work best when applied to moist soil.
If the soil is dry, a light irrigation of 10-15 mm rainfall equivalent must be applied so the herbicide seeps into the soil.
Post-emergent herbicides are applied after the crop has germinated and grown to the three leaflet stage.
There are post-emergence herbicides for grass-type weeds and others for broad leaf weeds in soya bean.
Some can be applied as a mixture of the grass and broad leaf herbicides.
Others must be applied separately.
It is important to read the label of the container or ask your supplier or AGRITEX officer for advice.
Weeds should be still young, at three-to-four leaves stage at the oldest.
If the post-emergence herbicides are applied too early, they can easily kill the young soya bean plants.
If applied when the weeds are too old, the herbicide may fail to control the weeds.
Timing is critical.
Common post-emergent herbicides can be purchased from crop chemical suppliers nationwide.
Farmers are advised to only source chemicals from reputable suppliers.
It is also important to check expiry dates after which the chemical should not be used.
The expiry dates is usually stamped at the container bottom or on the side.
Expired chemicals will not work.
Application of too much, too little or expired herbicides can result in serious yield losses.
Instead of soya bean, one harvests ‘sora’ beans!
Farmers are strongly advised not to increase the dosage of any chemical beyond that recommended on the label attached to the container.
The recommended dosage is the one that scientists, through careful experimental tests, have found to control the weeds but not to kill the crop.
Using an over-dose of chemical just because the weeds have grown too big is very dangerous.
Several cases have been reported of farmers who ‘burnt’ their soya bean crops after applying too much chemical in the mistaken belief that a bigger dosage will control the disease.
In all cases, farmers and their staff must first read carefully all the instructions on the container label.
Those going into the field to apply the herbicide must wear appropriate clothing as recommended by the manufacturer of the particular chemical.
Those applying crop chemicals must also read the container label so as to know exactly what steps to take in case of accidental contact with the chemicals.
Purple label chemicals are the most dangerous followed by red-labelled and then those with orange labels.
Green label chemicals are the least harmful but can be dangerous if swallowed.
Once the crop has successfully germinated, the farmer should monitor the crop for weeds, pests and disease.
If the crop received adequate fertiliser and was inoculated with rhizobium, it should start light green but turn dark green from about four weeks onwards.
That is the time when the nitrogen-fixing rhizobium bacteria in the root nodules start to manufacture ammonium fertiliser for the soya bean crop.
If the roots are carefully exposed by digging on the side of the plant, the nodules can be seen on the roots as little round swellings!
The nodules are not disease swellings; they are small fertiliser factories full of nitrogen fixing bacteria.
Scouting of the field should be carried out by walking in a zigzag pattern across the field or crossing along a diagonal from one corner to the opposite.
Plants should be checked for leaf-eating larvae.
If detected various insecticides ranging from cabaryl, dimethoate and monocrotophos can be sprayed to control the infestation.
At the start of flowering, fungicide should be applied especially to those soya bean varieties that can be infected with the disease called soya bean rust.
This application of fungicide is pre-emptive.
It is done even if no symptoms are observed.
The fungicide will stay in the tissues of the soya bean plant and ward off any fungal infections that come.
Soya bean rust disease appears in the lower leaves as yellowish brown little spots on the underside of leaves.
These look like red spider mites.
Gradually, the lower leaves become damaged and little spots spread all over the undersides of the leaves.
Those farmers planting older varieties of soya bean must watch out for this rust disease.
The severity of the disease appears to have reduced over the last few years.
We shall dedicate a whole discussion to soya pests and diseases in a later instalment.
Some of the latest varieties from seed houses have been bred for resistance to soya bean rust disease.
In such cases the farmer may opt to omit fungicide application.
In all cases, farmers must consult their seed suppliers or their AGRITEX extension officers for advice on pest and disease control.