Photo: Lindi Botha
Potatoes are a cool weather crop that realises its highest yields and best quality in regions that have a temperate climate and a long average day length during the growing season. While potatoes can be cultivated in most provinces in South Africa, there are seasonal guidelines.
The Agricultural Research Council (ARC) advises that tubers can be planted from August until June in frost-free areas. Where frost is experienced, as in the Free State and Mpumalanga, growers have a shorter planting period, and need to adjust planting dates based on frost prevalence.
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Potatoes prefer cooler growing conditions with temperatures ranging between 7°C to 20°C. Since moisture requirements are high – 800mm to 1200mm per year, areas with higher rainfall are better suited. It is however crucial that soils have good drainage, and good soil structure. Best results are obtained from light soils, with a pH of between 5.2 and 6.4.
Soil preparation
Since potatoes are a sub-surface crop, they are especially vulnerable to soil-borne diseases. Fields to which potatoes are planted should therefore be rotated, to prevent a build up of diseases and pests.
Justin Platt, managing director of RegenZ recommends a five year crop rotation, consisting of potatoes in the first year followed by a fallow period, then onions or butternut, a cover crop like sorghum in the fourth year, followed by potatoes once more in year five. The sorghum should be cut periodically to create a mulch layer and retain soil moisture.
Before planting potatoes, a soil analysis needs to be performed to understand how best to manage the land. A fertiliser programme should be formulated and followed accordingly.
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Soil preparation must then commence to break up compacted soil, incorporate crop residues, control weeds and prepare the seedbed. This starts with pre-planting tillage, cross-ripping the field to a depth of at least 50cm.
Deep ripping is important as the plant has a relatively weak root system and is sensitive to compacted soil layers. The ARC recommends that this be done when the soil is slightly moist to prevent deterioration of the soil structure.
After ripping, the field must be ploughed to a depth of at least 30cm. This should incorporate all crop residues to ensure complete decomposition of any plant material before the potatoes are planted. A disc may be used to break any clods and level the field.
The ARC states that a well-prepared seedbed consists of a 15cm deep layer of loose and finely crumbled moist soil. This can be obtained by tilling with either a spring-tined cultivator with a roller, a rotavator or a disc.
Planting and ridging
Timing is important for the planting process to ensure that temperatures are optimal during tuber formation. The ARC states that many cultivars don’t yield well if temperatures are too high during this period.
“Temperatures higher than 29°C during tuber formation hiders development, and subsequently the crop will yield more smaller sized potatoes. Potatoes are easily killed by frost, so do not plant a cultivar that has a long growing period if there are only three months from the date of planting to the first frost.”
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Potatoes should be planted when the soil is still moist, but not wet. Seed potatoes should be planted around 20cm deep in a furrow to which fertiliser has been applied. The ARC notes that the spacing between rows for potatoes grown under irrigation may vary between 75cm and 100cm, depending on the size of the tractor and the setting of the implements used.
For rainfed potatoes, the spacing between rows is usually at least 1,25m. The space between seed tubers in the furrow depends on the size of the tubers. Small seed or “chats” are usually spaced 15cm apart, while medium-sized seed may be spaced 30cm apart.
Once the furrows have been closed up, a light irrigation schedule should follow.
Keeping potatoes covered as they grow in the soil is vital, since sunlight will turn the potatoes green, and render them unmarketable. Since expanding tubers are prone to protrude from the soil, ridging needs to be done to ensure they remain covered. This can be done mechanically or with hand implements, depending on the size of the field and the availability of implements.
The ARC advises farmers to ridge after the potatoes have been well established, when plants are around 20cm to 25cm high. “Ridging when the soil temperature is high can damage the plant stems and cause lower yields. Avoid this by irrigating lightly before ridging or immediately afterwards.”
Protecting the harvest
Potatoes are sensitive to a range of pests and diseases, requiring intensive pest control. Platt notes that pest challenges most farmers will face include late blight, early blight, leaf roll virus, scab, mosaic virus, and nematodes. Regular scouting is essential to monitor the pest load and act timeously.
Young shoots start to appear from one week to three weeks after planting, with the total growth period ranging between 110 to 150 days from planting. New potatoes could be ready by day 60.
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The potatoes can be harvested as soon as the tubers have matured sufficiently to resist mechanical damage. The ARC says that in most cases, the skin of the tubers will be properly set, or matured, about two weeks after the plants have died.
“Readiness to harvest may be checked by digging up several plants from different areas of the field and rubbing the tubers with your fingers. If the skin rubs off, the tubers should be left in the soil for another seven to ten days to ensure that the skins have matured properly.”
The soil must be slightly moist when the potatoes are harvested to prevent excessive clod formation and subsequent damage to the tubers.
Potatoes must be removed from the fields as soon as they have been lifted as direct sunlight reduces their shelf life. Plastic bags should not be used to store potatoes as the tubers could sweat, and subsequently rot.
Email Justin Platt at [email protected], or Potatoes South Africa at h[email protected].
