Mashed, fried, or roasted; there is nothing quite like the versatility of the potato. These starchy vegetables are a staple for many people across the globe, ranking as the world’s fourth most important food crop, and it can even be grown in a backyard vegetable patch. The plant flourishes in Southern Africa, producing two crops per year in some areas. It is, however, considered to be a high-risk enterprise because of the high production cost, fluctuating market prices, as well as pests and diseases that can make potato cultivation a gamble.
Potatoes are balanced in minerals and vitamins, making them a good source of nutrients for humans and livestock alike. They contain vitamins such as vitamin B1 to 6, C and E, to name a few. Minerals in potatoes include potassium (P), calcium (Ca), magnesium (Mg), iron (Fe) and folate.
Climatic and soil requirements
Potatoes are a cool weather crop, meaning that the best quality yields are derived from regions that have a mild climate — not extreme hot or cold temperatures — and a long average day length during potato growing season.
Soil texture can affect potato production. Potatoes need loose, fine soil.
Always test soil before planting — to plant a potato crop, it’s vital to understand the soil’s chemical status. Although potatoes can grow in a wide pH range, it can influence the yield.
The soil needs to be fertile. To determine this, extensive sampling on all soil can be done to understand soil fertility better. Between 20 to 30 topsoil samples of 500 g at 20 cm deep should be taken for every two to five hectares. For the same field size, five subsoil samples should be taken at 30 to 60 cm depth, also consisting of 500 g each.
The 20 to 30 topsoil samples per section can be mixed, and a smaller composite sample of 1 kg can be taken from the mixture. Do not mix the soil samples of different sections as that defeats the purpose. For subsoil samples, a composite sample of 500 g can also be taken.
Write down where the samples were taken such as which field and in which section. Also add information about irrigation practices, planting date, cultivar and target yield when submitting samples.
Uses of potatoes
Potatoes are mainly used as a food source for humans and livestock, but there are also instances where it has industrial uses. For humans, we often see potatoes in our favourite forms such as boiled, baked, or mashed potatoes, fried in the form of chips and crisps, or a key ingredient in other dishes such as stews, boiled beans and potatoes, or as potato salad. Potatoes are also used to produce starch and spirits.
To prepare the soil, first, compacted soil needs to be broken. The slightly moist soil can be tilled with a cross-ripper at the depth of at least 50 cm. This is important to make sure that the tubers and their weak root system have enough space to grow in. After ripping, further ploughing up to 30 cm deep is also needed. A disk can also be used to break any remaining clods.
Ensure that all crop residue has decomposed before planting any new potatoes.
Seedbeds need to be lightly irrigated before the final steps of preparation. At the end, you should have a seedbed that has a 15 cm deep layer of loose and finely crumbled, moist soil.
Potatoes are planted in moist soil. Dig furrows of approximately 20 cm deep and apply fertiliser to the soil. Place the tubers in the furrows, approximately 15 cm apart for smaller seed or “chats” (small potatoes of poor quality), but 30 cm apart for larger sizes. Cover them with the soil removed from the furrow. You can now lightly irrigate the planted potatoes.
Under irrigation, rows can be spaced between 75 cm to 100 cm apart, depending on the size of the tractor and or other equipment. If the potatoes are rainfed, rows can be 1,25 m apart.
You will need to choose a cultivar that meets the following criteria:
- Most suitable for your area,
- Average temperatures during the growing season,
- Day length,
- Soil type,
- Purpose of producing potatoes
- Diseases in the area, and
- Length of the growing period.
Ridging (or hilling) is a cultivation practice during which the soil surrounding the young potato plants is used to build a ridge or hill in which the tubers can develop. This can be done mechanically or with hand implements, depending on the size of the planting and the availability of implements.
Potato plants are ridged (or hilled) after they are well established — usually at a hight between 20 and 25 cm. This helps protect the tubers from sun damage and pests. You can either do this by hand or use implements at your disposal, irrigating lightly before ridging or immediately afterwards.
Potatoes are known to have a weak root system. For that reason, any fertiliser should be applied close to the seed tubers. However, the seed tubers and fertiliser should not be in direct contact. Cover the seed in a layer of soil before applying the fertiliser all around. Continue to cover with a top layer of soil and irrigate lightly.
Ridging may protect potatoes from pests and heat, but it can cause the soil to dry out quickly too. That is why it is necessary to ensure that there is enough moisture in the ridges.
Light irrigation on a daily basis may be necessary, but take care not to overwater plants. Good moisture levels help protect against potato scab.
Pest and disease control
This disease is responsible for scablike lesions. Scabs differ in size but are generally circular, raised, and brown. The disease is found all over Southern Africa, but the worst cases are when sandy soils have a pH above 5,5. The lesions are worsened by warm, dry soil conditions.
Soil treatments with quintozene and tuber treatments can aid in controlling the disease. Resistant cultivars or planting scab free tubers are also helpful. Crop rotation with non-host crops offer good results as well. Alternatively, green manuring with Brassica crops can be done.
PVYNTN (Potato virus Y-strain Necrotic Tuber Necrosis)
PVYNTN stands for Potato Virus Y-strain Necrotic Tuber Necrosis. It causes potato tuber necrotic ringspot disease (PTNRD) where the tubers show ring spots that affect the cosmetic value of the vegetables. Potatoes with these symptoms are rejected at the market, causing a financial loss to farmers.
Some weeds may serve as hosts for the virus which may infect the potatoes, but it can also be transmitted through grafting from infected plants instead of seeds.
Many aphid species also transmit the virus, such as the green peach aphid (Myzus persicae).
Farmers can prevent the spread of the virus by avoiding planting any solanaceous (plants from the potato and nightshade genus) crops near potato fields as the aphids spread this virus from the solanaceous plants. Keep an eye out for any symptoms in the fields. Where possible, infected plants should be removed and destroyed. Applying registered aphicides regularly during the growing season can aid in controlling aphid populations and prevent spreading of the virus. Weed regularly, and destroy all weeds near and in the fields.
Potato tuber moth
These grey-winged moths are nearly 1 cm long and have dark grey spots. The moth is nocturnal and hides between the leaves during the day, laying up to 250 eggs. The larvae mine through the leaves and tubers. For this reason, they can cause a severe loss in yield. Sprout development is impacted first because the eyes of the potatoes are attacked first.
Applying registered insecticides can help control these pests. Ridging potatoes and frequent inspection to ensure plants remain covered and thus protected may also help. Always use seed that is not infected. Discard old potatoes responsibly and remove volunteer plants from harvested fields.
Potato leaf miner
This small fly, in particular the female, pierces the leaves with her ovipositor, causing “stippling” that places the plant under stress. Often the eggs are also deposited in these scars. The small cream coloured maggot hatches and mine between the leaf surfaces. Leaves and stems are left with a burnt appearance and usually die off. Fewer leaves directly impact on the yield, even if the larvae do not attack the tubers.
Various insecticides are registered to control this pest.
These small insects have sucking mouthparts, however, they usually do not damage plants by their feeding only. After feeding, they spread viruses on plants elsewhere. The virus can spread to other plants in the field and cause a major break-out of Potato leafroll virus (PLRV) and potato virus Y (PVY). Infected tubers may be downgraded under the Seed Certifying Scheme.
Some insecticides are registered to control this pest. Only seed producers need to control this pest.
These microscopic roundworms live in the soil, and in Southern Africa, the root knot nematodes, lesion nematodes and potato cyst nematodes are the most significant. When a host plant, such as potatoes in this case, start to grow close to the eggs that have been deposited in the soil, they start to hatch and make their way to the roots. Nematodes damage tubers and roots, thus limiting the quantity of water or nutrients that the plant absorbs during growth. Infected tubers produce small galls where the female worms and egg sacks are found.
Tubers that are infected by lesion nematodes look unhealthy, showing purple-brown pimples, pustules, or wart-like protuberances.
Should your crop be infected with potato cyst nematodes, report it to the Department of Agriculture immediately, as it is a quarantine pest.
The two control actions that farmers can effectively apply are nematicides (applied at planting time) and crop rotation with cereals or grasses. Avoid planting infected seed.