Sweet potatoes form a key staple and poverty-relief crop in southern Africa. Orange-fleshed types are useful in combating the vitamin A deficiency which causes many African children to go blind.
The sweet potato performs well in sandy-loam, loam or clay-loam soil. It needs good drainage and is sensitive to water-logging, 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.
As a warm season crop sweet potato is sensitive to low temperatures, especially frost. It performs best in subtropical areas, provided supplementary irrigation is available. The plant develops a degree of drought tolerance after the storage roots have formed.
Crop rotation is essential to prevent disease and pest build-up. Sweet potatoes should only be cultivated in the same soil once in three years.
Propagation is by stem cuttings. Always cut shoots from a healthy plant. Do not use vines from volunteer sweet potatoes because these might carry pests and diseases. Pieces of stem 20 cm to 30 cm 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 surrounding soil down firmly so that it makes proper contact with the stem to promote quick root development. Water the plants immediately
Loosen the soil to a depth of 25 cm to 30 cm 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.
Ridging. Sweet potato plants are grown on ridges or mounds. The ridges are especially beneficial in areas prone to flooding and ease harvesting. Cuttings are laid 30 cm apart on the ridge, with the basal end planted in the soil. Holes of suitable size are made, the vines placed in the holes and soil pressed down firmly around the cutting. Cuttings are planted vertically with 3 to 4 buds (nodes) under the soil surface.
Flat bed. Some farmers grow sweet potatoes on flat beds in sandy soil with good results.
Sweet potatoes can be regarded as a field crop rather than a home garden vegetable. This is because its runners take up a large area. For the home garden (where space is limited) a row or two should do.
Place stem cuttings 25 cm to 35 cm apart (or the length of a spade blade) in the row. Generally, ridges of 1 m apart are used, but they can also be 90 cm to 150 cm apart for field production or 80 cm to 90 cm for home gardens, and about 30 cm to 40 cm high.
- Areas with light, mild frost: beginning of November to mid-December.
- Areas with heavy frost: mid-November to beginning of December.
- Frost-free areas: August to March.
- Cooler areas: September to February.
- Winter rainfall areas: mid-November to beginning of December; November is optimal.
- Soil samples should be taken a few months before planting to rectify soil fertility problems. General recommendations are the following:
- Sweet potatoes need a high ration of nitrogen (N) and potassium (K). Approximately 500 – 750kg/ha (75g/m2) of a fertiliser mixture such as 2:3:4 (30) + 0,5% Zn or 2:3:2 (22) + 0,5% Zn, or 3:2:1 (25) + 0,5% Zn can be applied directly before planting. It must be worked into the soil lightly before making the ridges. For sandy soils the quantity can be increased to 1 000kg/ha. Apply a top dressing of 120 – 150kg/ha LAN (limestone ammonium nitrate) (12g/m of row) or 200kg/ha (20g/m2) ammonium sulphate three weeks and six weeks after planting, if necessary.
Sandy soil will require at least two top dressings. Water well after the fertiliser has been applied.
- Well-matured (six months) compost may be used to supplement part of the chemical fertiliser, but it must be worked into the soil some time before planting. Four big handfuls/m2 (15 – 20m3/ha) of compost can be applied and dug in before planting.
Sweet potatoes are moderately drought tolerant. Water stress during the first few weeks after planting and the period of tuber formation (30 to 60 days after planting) will cause low yields. As a general guideline, sweet potatoes require between 450 mm and 600 mm of water, well distributed throughout the growing season.
The sweet potatoes are ready for harvesting four months (warm areas) to five months (moderate areas) after planting. Soil should be soft during harvest to prevent breakage and skin damage. Withhold watering from about 30 days before harvesting as a way of field curing. In warm areas, cut vines four to seven days before harvesting for the tubers to cure.
Use a hand fork to lift the tubers and take them out by hand. Make sure you do not damage them. If too long a season is allowed, the tubers will become too large.
Rub the soil from the sweet potatoes, wash and leave them to dry in sun for one to two hours when temperatures are moderately high. At 32°C, harvested tubers can get sun scald within 30 minutes. If left on the field at night at temperatures below 5°C, chilling injury will occur. Store in a cool, dry place at about 15°C.
Sweet potatoes can be stored fresh for three to six weeks after harvesting. Do not store damaged tubers. They can also be left in the ground until they are needed. It is better to extend harvesting through planting at fortnightly (two-weekly) intervals during the planting season.
David Magoro, Sunette Laurie, André van den Berg and Alistair Thompson are staff of the Agricultural Research Council’s Vegetable and Ornamental Plant Institute (ARC-VOPI) in Pretoria.