|Better Farming Series 16 - Roots and Tubers (FAO - INADES, 1977, 58 p.)|
The sweet potato is a climbing herbaceous plant. It may live for several years, but often it is harvested after 3 months, without waiting for it to flower.
The stems may grow to 2 or 3 metres in length; they are thin and climbing or creeping. They have nodes at varying distances apart. It is thought that the varieties with short inter- nodes yield more heavily than those with long inter- nodes.
The leaves vary greatly in size and shape. Depending on the variety, they may be entire, heart- shaped or deeply divided with three, five or seven lobes. The leaf veins and the leaf- stalks are green or red.
The flowers, usually violet, sometimes white, are clustered in the leaf axils. Many varieties of sweet potato in cultivation do not have time to produce flowers and fruits before the harvest.
The most important part of the plant is the roots, because they can develop into tubers.
The tubers are parts of the creeping roots that have built up food reserves.
These tubers are produced at points where the roots cease to spread out near the surface and turn downward into the soil. By making mounds that are not too wide, the formation of tubers is helped.
Leaves and tubers of sweet potato
Varieties of sweet potato differ greatly in the number, shape and size of their tubers and in the color of the peel and flesh of the tubers.
Sweet potatoes may be round or elongated. In colour they are white, yellow, red or violet, with soft or firm flesh. They may weigh between 0.3 and 3 kilogrammes.
Sweet potatoes grow well in warm, sunny and humid regions. At the same time, they withstand drought very well. For that reason, they are suited to dry savanna country.
Sweet potatoes need regular rain to grow, especially when the leaves are coming into growth. But if there is too much rain at harvesting time, the tubers rot.
The sweet potato will grow in poor soils. The most suitable soil is a light, well- drained, sandy loam. If the soil is too rich in organic matter and nitrogen, the plant produces a great deal of useless stem and leaves, and only after a long time produces a very few tubers.
As a rule, sweet potatoes are grown on ridges or mounds after deep tilling.
This way is better than growing them on the flat.
The mounds and ridges protect them from too much moisture. The ridges are made about 75 centimetres apart.
But it is still better to plant sweet potatoes on round mounds 30 to 40 centimetres high and 1 metre apart. The mounds should be made as narrow as possible.
This forces the plant to bend its roots downward quickly. In bending, the roots build up food reserves and develop tubers.
The farmer must know his varieties well. He must know how long they take to form tubers, and see to it that the harvest will be in the dry season.
It is better to plant several times, at intervals, so that the whole plantation does not become ready for harvest at the same time. By doing this, you can lift the sweet potatoes as and when you need them.
Sweet potatoes are propagated from cuttings or from tubers.
- Propagation from cuttings
Propagation from cuttings is possible only when the sweet potatoes remain in the field all through the year. The cuttings should be 20 to 40 centimetres long, with three to five growth buds. It is best to take them from the tips of young stems. Take the cuttings only when you are ready to plant them, and keep them in the shade until they are inserted in the soil. Propagation from cuttings is the most economic way of increasing your plants.
Plant cuttings at a slant, leaving 3 or 4 centimetres above ground, and press the soil down firmly. If you plant them on mounds, you can put four or five cuttings in a circle on each mound. This will give you a planting density of between 15 000 and 30 000 plants to the hectare.
- Propagation from tubers
If you do not have any plants of sweet potatoes with enough leafy growth to provide cuttings, you can propagate from tubers.
In this case, the tubers must be made to sprout in a cool nursery bed. If the tubers are large, cut them into several pieces. After about a month, remove from the tubers the young shoots that are 15 to 20 centimetres long and plant them.
This method of propagation from tubers is usually done only on a part, say one third, of the area on which sweet potatoes are to be grown. Later, cuttings from the plants thus obtained can be used to enlarge the plantation.
CONTROL OF WEEDS
One or two cultivations in the early stages of growth are enough. In 4 to 6 weeks after planting, the plant's own leafy growth will closely cover the soil.
When cultivating, remake the mounds at the same time.
CONTROL OF DISEASES AND PESTS
Sweet potatoes attacked by diseases and insects yield only a small harvest of poor quality.
You must wait 3 to 5 years before growing sweet potatoes again on the same field.
- Rot and fungi
Diseases that kill the growing plants are caused chiefly by various fungi. Some fungi make the leaves turn yellow and wither. Other fungi make the stems or tubers rot. Signs of the disease are yellow leaves and black marks inside the stems and tubers.
Other fungi cause the young plant to rot. It stops growing. The roots and the tubers already formed turn black. It is not long before the whole plant withers and dies.
To control most forms of rot, you must choose resistant varieties. Do not use for propagation cuttings or tubers taken from plantations attacked by rot.
Do not grow sweet potatoes on the same soil 2 years in succession.
- Insect pests
Sweet potatoes may be attacked by certain insects, especially by weevils.
The adult insects eat the leaves, stems and tubers. The female insects lay their eggs in the stems or roots; the larvae tunnel into the tubers. Serious damage is caused by weevils.
To control the weevils, use insecticides. Before planting tubers and cuttings, dip them in a solution of Dieldrin.
In places where harvested sweet potatoes are stored, they can be fumigated with phostoxin in tablets.
Depending on the varieties of sweet potato and on the way they are grown, yields vary from 4 to 7 tons per hectare on average. On a modern and well- cared- for plantation, yields may be much higher, and may even be more than 20 tons per hectare.
The length of time for which sweet potatoes can be kept differs with the varieties and the harvesting season. If they are harvested in dry weather, the tubers may be stored for 2 or 3 months.
But part of the harvest may be destroyed by rot during storage. Damaged tubers are most quickly attacked. Damp conditions encourage rot.
To prevent rot, dry the tubers in the sun for a time after harvesting.
For good keeping, the tubers of sweet potatoes should be harvested when they are quite ripe, when the stems and leaves have turned yellow. Take care not to damage the tubers. Remove all diseased and damaged tubers. Dry the tubers in the sun. Store them under cover in a dark, dry, cool, well- aired place. Put them on dry ground or on boards supported on posts, and do not heap them up too much.
Sweet potatoes are of great value as an energy food.
The sweet potato, especially the coloured varieties, contains vitamins. The yellow ones are the richest in vitamins.
The tubers contain much starch, and this can be extracted from the tubers in factories.
The sweet potato can also be used for making alcohol. The leaves of the plant are used for food, both for people and animals.