Sweet potatoes are a key staple and nutritious crop in Southern Africa. The orange-fleshed varieties help combat vitamin A deficiency (which causes many African children to go blind), while the plant’s adaptability makes it suitable for both household and commercial farming. With proper care, sweet potatoes provide food security and income opportunities.

By Agricultural Research Council

The sweet potato performs well in sandy loam, loam or clay loam soil. It needs good drainage and is sensitive to waterlogging, salinity and alkalinity. Stony or clay soils are not suitable for developing good storage roots.

The optimum pH of irrigation water should be 5,6 to 6,5.

Required Climate

As a warm-season crop, sweet potato is sensitive to low temperatures, especially frost. It performs best in subtropical areas but can be grown in all South African provinces, provided supplementary irrigation is available. The plant develops a degree of drought tolerance after the storage roots have formed.

Crop Rotation

Crop rotation is crucial in preventing the buildup of diseases and pests. Sweet potatoes should only be cultivated in the same soil once every three years.

Propagation

Propagation is by stem cuttings. Always cut shoots from a healthy plant. Do not use vines from volunteer sweet potatoes, as they may carry pests and diseases. Pieces of stem, 20-30cm long, should be taken for cuttings. The leaves can be removed from the cuttings before planting, but this is not necessary.

Top cuttings are more vigorous than cuttings from other parts of the vine. Plant them in a prepared bed by covering half their length in moist soil.

Press the surrounding soil down firmly so that it makes proper contact with the stem, promoting quick root development. Water the plants immediately after planting

Soil Preparation

Loosen the soil to a depth of 25-30cm to allow for good root aeration, root penetration and drainage by either ploughing or using a fork, hoe or spade. Remove stones and break down hard soil layers. All clods should be smashed until a deep, fine bed is obtained.

Planting Method

Ridging. Sweet potato plants are grown on ridges or mounds. The ridges are especially beneficial in areas prone to flooding, and also ease harvesting. Cuttings are laid 30cm apart on the ridge, with the basal end planted in the soil. Holes of suitable size are made, the vines are placed in the holes and the soil is pressed down firmly around the cutting. Cuttings are planted vertically with three to four buds (nodes) under the soil surface.

Flat bed. Some farmers grow sweet potatoes on flat beds in sandy soil with good results.

Spacing

Sweet potatoes are more commonly regarded as a field crop rather than a home-garden vegetable because their runners can cover a large area. For a home garden with limited space, just a row or two is sufficient.

Place stem cuttings 25-35cm apart (or the length of a spade blade) within the row.

Typically, ridges are made one metre apart, but they can also be spaced 90–150cm apart for field production or 80–90cm apart for home gardens, with a height of approximately 30-40cm.

Planting Time

Areas with light to mild frost: November to mid-December.
Areas with heavy frost: mid-November to the beginning of December.
Frost-free areas: August to March.
Cooler regions: September to February.
Winter rainfall areas: mid-November to the start of December; November is the optimal time for rainfall.

Sweet Potatoes_Photo: Landbouweekblad Archive — Photo: *Landbouweekblad* archive

Fertilisation

Soil samples should be taken a few months before planting to rectify soil fertility problems. General recommendations are the following:

Sweet potatoes require a high proportion of nitrogen (N) and potassium (K). About 500-750kg/ha (75g/m²) of fertiliser mixture such as 2:3:4 (30) + 0,5% zinc (Zn), 2:3:2 (22) + 0,5% Zn, or 3:2:1 (25) + 0,5% Zn can be applied directly before planting. It should be lightly worked into the soil before forming the ridges. For sandy soils, the amount can be increased to 1 000kg/ha. Apply a top dressing of 120-150kg/ha LAN (12g/m of row) or 200kg/ha (20g/m²) ammonium sulphate three and six weeks after planting, if necessary. Sandy soils will need at least two top dressings. Water thoroughly after applying the fertiliser.

Well-matured (six months) compost can be used to supplement some of the chemical fertiliser, but it must be dug into the soil some time before planting.

Water Requirements

Sweet potatoes are moderately drought-tolerant. Water stress during the first few weeks after planting and the period of tuber formation (30–60 days after planting) will cause low yields.

Sweet potatoes are thirsty plants! They need 450-600 litres per square meter spread evenly over 3-4 months.

Harvesting

Sweet potatoes are ready for harvesting four months after planting in warm areas and five months after planting in moderate areas. The soil should be soft during harvest to prevent breakage and skin damage. Watering should be withheld about 30 days before harvesting as a method of field curing. In warm areas, cut the vines four to seven days before harvesting to allow the tubers to cure.

Use a hand fork to lift the tubers and remove them by hand. Ensure you do not damage them. If the season is allowed to be too long, the tubers will become too large. Rub the soil from the sweet potatoes, wash them and leave them to dry in the sun for one to two hours when temperatures are moderately high. At 32°C, harvested tubers can suffer sun scald within 30 minutes. If left in the field overnight at temperatures below 5°C, chilling injury may occur. Store the tubers in a cool, dry place at a temperature of around 15°C.

Storage

Sweet potatoes can be stored fresh for three to six weeks after harvesting. Do not store damaged tubers. They can be left in the ground until they are needed.

It is better to extend harvesting through planting at fortnightly intervals during the planting season.

Sweet potatoes can be used in many different ways

Storage roots can be eaten raw, boiled, baked or cooked.
The tips of shoots (petioles) and young leaves serve as a vegetable. Crop residues are useful as feedstock for livestock.
Storage roots can be used to produce starch, alcohol, flour, jam and juice.
The high carbohydrate content makes sweet potatoes a key energy source.
The orange-fleshed sweet potato is a good source of vitamin A.
Green leaves provide additional protein, vitamins and minerals.