The Sweet Potato originated in Central America and is now grown throughout the tropics. It has been grown for many generations in West Africa and has proved to be a popular crop.
The plant is a creeping annual with a trailing stem above ground, called a stolon. As the stem creeps along the soil surface roots are produced at the nodes and draw water and nutrients from the soil. The leaves have different sizes and shapes according to the variety. Certain roots thicken to form tubers which are the edible part of the plant. The tubers vary greatly in shape, size, colour and taste according to variety. The plants produce large, bell‐shaped flowers and these form a fruit containing 4 small black seeds. However, propagation is by vegetative means, using cuttings.
The edible tubers contain latex and are rich in starch. They contain some sugar, protein and fat, and the pigmented tubers are rich in Vitamin A and some Vitamin B and C. The green leaves and stems are useful stock‐feed when fed fresh, or can be cut and dried as hay which may yield up to 1 ton per hectare. For the purpose of stock feed, the following is the analysis:
Table 1: Analysis of sweet potatoes for stock feed purposes
|Dry Matter||Digestible Protein||Total Digestible Nutrients|
There are hundreds of different varieties of sweet potatoes, and can be divided into three groups, which are:
Plants with deeply lobed leaves that produce white tubers and are slow maturing; Plants with heart‐shaped leaves that produce red tubers; and
Plants that produce tubers, which are round with yellow flesh and are rich in Vitamin A, B and C.
Local named varieties are Ribbok, Blesbok, Brondal, Impala, Koedoe, Mafuta and Bosbok.
The sweet potato likes hot days and warm nights with a mean monthly temperature of between 21°C – 29°C and at least 300 mm of rainfall over the growing period of 4 ‐ 5 months. In high rainfall areas the crop is planted towards the end of the rainy period. The crop can grow on any well drained soil and normally planted onto ridges or mounds.
The planting material can be; stem cuttings from the growing ends of the stolons, which are the materials normally used, but include pieces of tuber and pieces of the leaf stalks. Pieces of stem about 300 mm – 400 mm long are used. The bottom leaves are removed and the cutting is pushed into the soil at an angle. It is possible to obtain planting shoots by planting small tubers close together in nursery beds and allowing them to sprout. After 4 ‐ 6 weeks the sprouts are removed and planted in the lands making sure that healthy, virus free material is used.
The cuttings are planted 300 mm – 400 mm apart on ridges which are 1 000 mm apart and 500 mm high to prevent waterlogging and allow the stolons plenty of room to spread out. Stolons which grow down into the furrows should be turned back onto the ridges. Material used for planting should be free from pests and diseases. Using the spacing given above will give a plant population of between 25 000 – 35 000 plants/ha. Cuttings should put out roots and start to grow a week after being planted.
Sweet potatoes respond well to organic manures and where possible organic matter should be dug into the soil before the ridges are made up. This can be in the form of crop residues, grass, kraal manure or pig and poultry manure. Wood ash can also be mixed with the soil. Compound fertilizers can be applied to the seed and mixed in with the soil before ridging up. On fertile soils 400 kg/ha of a 2:3:4(30) fertilizer can be applied at or before planting with a topdressing at 6 weeks of 250 kg/ha of LAN. On infertile lands 800 kg/ha of 2:3:4(30) can be applied at planting with a topdressing at 6 weeks of 150 kg/ha of LAN.
The crop should be kept free from weeds by hand hoeing in the early stages of growth, but once the stolons and leaves cover the ground weeds are kept down and further cultivation is not required.
5. PESTS AND DISEASES
The grubs of these insects bore through the stems and tubers of the growing plant, and the adult weevils feed on the leaves, stems and roots. Spraying with chloropyrifos at pre‐planting and spraying at 5 and 10 week intervals should control this pest.
This is a fungal disease which attacks the tubers, causing black spots and rotting. Control of the disease is by using good rotations and clean disease free planting material.
This causes rotting in tubers that are being stored after harvesting. Storage bins should be kept clean and any infected tubers should be removed and destroyed.
The tubers will be ready for harvesting 4 ‐ 5 months after planting. The leaves of the plants turn yellow and begin to drop off when the crop is ready. The tubers are normally dug up by hand, dried and stored either whole or after being sliced into smaller pieces. Since the tubers do not keep well after harvesting, they can be left in the ground and dug up when required, although if left in the ground too long they may be eaten by termites. If the soil temperatures drop below 12°C there will be a negative effect on quality.
The plants are good producers and a single plant can produce up to 50 tubers. Yields of up to 40 t/ha can be obtained with good fertilising and rainfall, but an average yield would be 20 t/ha – 30 t/ha of tubers and 4 ‐ 5t /ha of green material from the leaves and stolons. Although the crop is grown mainly for the edible tubers it is also used as filler in canning and sauces. The tubers have high starch content and used to make starch flour, glucose, syrup and can be fermented to produce alcohol.