Once onion seedlings have been successfully established in the field, proper crop management becomes essential for achieving a profitable harvest. Fertilisation, irrigation, weed control and disease management all play a critical role in determining bulb quality and yield. This second part of the series focuses on the management practices that help farmers produce healthy onions and maintain quality after harvest.
Fertilising
During soil preparation, work in 100g of 2:3:2 (22) or 2:3:4 (30) per 1m². The crop is a heavy feeder, and needs nitrogen and potassium. But remember that too much nitrogen (N) late in the season can cause too vigorous leaf growth, delayed bulb development and thick-necked plants. Phosphorus (P) and potassium (K) are required throughout the growing season. Use 10g LAN per 1m2, as well as 10g potassium chloride (KCl) per 1m², three weeks after transplanting and again at six weeks after planting, especially if the soil is sandy. Apply them 5–15cm from the plants. Carefully work in the top dressing with a fork, but be careful not to damage the roots. Water immediately after applying the top dressing; this will allow the fertilizer to start working immediately.
Irrigating
Onions require approximately 400–600mm of water during the growing season. Onion roots are concentrated in the upper 30mm of the soil and for this reason the soil must be kept moist. Do not water onions for three weeks before harvesting.
Harvesting
In home gardens, onions can be harvested when 100% of the leaves have lodged. For commercial plantings, onions are usually harvested once 50% of the leaves have lodged. Do not leave onion plants in the soil for too long. Onions are lifted by loosening the soil with a fork or a harvester. Collect onions, make bundles and tie the leaves together. Hang from the ceiling of a storeroom to dry.
Plants can be dried on the field by placing them in wind-rows in such a way that the leaves protect the bulbs against sunburn. If it rains, turn the plants over. Once the neck of the bulb has dried completely, the leaves can be cut and the bulbs stored. For fresh marketing of short day onions, the onions are partially dried and then marketed immediately in order to realise high prices. The preference of the South African market is for small to medium bulbs with a mass of 80–100g.
Storage
After drying and cleaning, onions must be stored in a dry place. Onions for storage should be mature, thoroughly dried and not damaged. The storeroom should be well ventilated and have a low temperature and a dry atmosphere. Turn the bulbs often, so that all sides of the entire bulb are exposed to air and light at regular intervals. Store in layers no thicker than 10cm. Rotten bulbs must be removed immediately.
Yield
Under favourable conditions, yields of 30–40t/ha can be achieved. Top farmers can harvest up to 60t/ha. In South Africa the average yield is 25t/ha. Factors such as planting density, cultivar, planting date and growing period influence yield.
Crop management
Onion plants, particularly seedlings, do not offer much resistance to weeds, so weeds must be controlled to avoid competition for nutrients and water, particularly when the onion plants are small. Weed carefully to avoid damaging the roots. Stop working between the rows when the foliage becomes dense.
Also read: Cultivating onions | From soil requirements to transplanting
Pests and diseases
Thrips are very small insect pests, which feed on the leaves by sucking plant sap. Attacked plants become silvery and flecked. Farmers should practice crop rotation to combat this. Nematodes attack the roots, but can be controlled.
The most important diseases are: Downy mildew: Grey to purplish mounds on the leaves. Leaves turn pale green, then yellow and die off.
Downy mildew is favoured by low temperatures and high humidity. Pink root appears on the roots of seedlings and older plants. The root turns pink, shrivels and dies. More tolerant cultivars are available.
Purple leaf spot or Alternaria blotch.
Large brown lesions appear on the leaves and eventually kill the leaves.
White bulb rot: White fluffy fungal growth occurs on the bottom of the bulb. The bulb becomes rotten.
Basal rot: The leaves die from the tips and if you cut through the bulb you will see brown rot.
Storage disease:
Black mould: The bulb shows blackening just below the skin and later becomes rotten.
Control: Do not store damaged onions and those showing disease symptoms. Inspect bulbs in storage and remove infected ones and clean containers with a bleach solution.
Onion production requires consistent management throughout the growing season and careful handling after harvest. By applying the correct fertiliser programme, managing water efficiently, controlling pests and diseases, and ensuring proper curing and storage, farmers can maximise both yield and market value. With good management, onions remain one of the most rewarding vegetable crops for South African producers.
Source: Agricultural Research Council – Production Guideline for Winter Vegetable Production
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