Sweet potatoes are high in beta carotene, vitamin E, C and B6, as well as iron and potassium. Orange types are also high in vitamin A. The high carbohydrate content is a key source of energy. This vegetable is therefore a highly nutritious staple food for poverty-stricken areas in Africa.
Climatic and soil requirements
Sweet potatoes prefer the warmer temperatures of subtropical climates and don’t fare well in areas with frost.
To find the most suitable time to plant, the following guidelines can be used:
- Heavy frost climate: Mid November to beginning December
- Light-mild frost climate: Beginning November to mid December
- No frost climate: August to March
- Cooler climates: September to February
- Winter rainfall climates: Mid November to beginning December
The plant is sensitive to salinity, alkalinity, and waterlogging, and needs soil that allows water to flush salts and alkaline minerals through. The ideal soil conditions range from sandy-loam, to loam, to clay-loam to offer good drainage. Clay soil or rocky soil prevent sweet potatoes from developing storage roots (tubers). It also needs good irrigation to develop these roots, but once they have formed, the plant builds a degree of drought tolerance. For optimum results, the pH of irrigation water should be between 5,6 and 6,5.
These vegetables have many uses, mainly as a food source. The sweet potato as most people consume it, are the large storage roots that can be eaten raw, boiled, baked, cooked, or fried. It can be used to make starch, alcohol, flour, jam, and juice.
The tips of shoots and young leaves can also be consumed, making it useful as stock feed.
Begin by loosening the soil up to a depth of 25 to 30 cm. This creates good root aeration, root penetration and drainage. You can use a plough, hoe, fork, or spade. Take care to remove rocks and break down hard soil layers. Ultimately, a fine bed without clods should be created.
Create ridges or mounds (approximately 40 cm high) for the plants to grow. This eases harvesting and benefits the farmer in areas prone to floods. However, flat beds can also be used. Ridges can be between 90 to 150 cm apart, or 80 cm for home gardens.
Sweet potatoes are cultivated through propagating healthy stem cuttings.
These cuttings should be taken from the stems of plants, not the vines as they may carry pests and diseases. Cuttings should be between 20 and 30 cm in length, with or without leaves.
Place the cuttings 30 cm apart. Each cutting is planted vertically, with at least 3 buds under the soil. Press the soil down firmly so that the stem has good contact with the soil for root development.
A few months before planting season, take a soil sample to establish whether the soil fertility needs to be corrected.
Sweet potatoes need the following nutrients: Nitrogen (N), potassium (K), and zinc (Zn). Fields need approximately 500 to 750 kg/ha (75g/m2) in either quantities of 2:3:4 930) + 0,5% Zn, or 2:3:2 (22) + 0,5% Zn.
The fertiliser can be mixed into the soil before mounds and ridges are prepared.
If the soil is sandy, however, fields can require up to 1 000 kg/ha.
A top dressing of limestone ammonium nitrate (LAN), can be applied in the quantity of 120 to 150 kg/ha (12 g/m2). Alternatively, 200 kg/ha (20 g/m2) can be added three and six weeks after planting. Water it well after application. Remember, sandy soil needs two top dressings.
Well matured compost can supplement chemical fertiliser, but also needs to be thoroughly mixed into the soil before planting.
Sweet potatoes need between 450 and 600 mm of water. The water should be well distributed during the plant’s growing period. Water stress during the first 30 to 60 days after planting can lead to severe losses.
Sweet potatoes are less affected by pests compared to other crops, but they are not immune.
Sweet potato weevil
This insect can cause a lot of damage. The larvae tunnels into the tubers and stems, whereas the adults feed on the leaves. The good news is that there are preventative measures that can help:
- Crop rotation in order to reduce the weevil population
- Not planting cuttings soon after the previous crop
- Adjust planting season to ensure there are no tubers in the dry season
- Minimise soil cracking
- Spray the fields with registered chemicals
- Harvest at the correct time
- Harvest all sweet potato tubers at once• Burn all infested plants after harvest
- Never use infested cuttings
- Burn all infested plants after harvest
- Never use infested cuttings
Sweet potato hawk moth larvae
These large brown caterpillars have a horn on the back of their bodies and red dots along the sides. They feed on the leaves of plants. If numbers are small, they can be removed by hand. In other cases, registered chemicals can control them.
This causes a loss in yields. The leaves and stem turn yellow, wilt and die. It can be diagnosed by cutting an infected stem. The infected vascular tissue will be reddish-brown. Infected plants also infect the soil.
The following preventative measures can be used:
- Use wilt-resistant cultivars
- Practice field sanitation
- Remove all infected plant residues
- Rotate crops
- Limit stress caused by water deficiency
Alternaria leaf spot and scurf can also be prevented in this way.
Sweet potato feathery mottle virus
Symptoms of the virus are not always observed, but it also severely affects
yields. The following preventative measures can be used:
- Obtain virus-free cuttings from registered vine growers
- Control weeds such as the wild ipomoea species around the fields
- Use healthy, vigorous-looking material
- Renew plant material every two or three years
- Sanitise all cutting equipment with bleach
The information provided in this article is credited to the Agricultural Research Council (ARC)ÚRC Vegetable and Ornamental Plant Institute (ARC-VOPI). For more information, contact +27(0)12-841-9611 or visit www.arc.agric.za.