The place of yams in a crop rotation

It is best to plant yams at the beginning of the rotation, as a first- year crop after clearing the land.

If yams are grown after a long fallow, they find plenty of mineral salts in the soil, and yield many good tubers.

How to prepare the soil for yams

Before planting yams, the soil must be well prepared.

- Clear the land before the rainy season. Cut down the trees, cut the branches. Stack the trees and branches and burn them.

Do not cut all the trees. Leave some of the little ones. They can be used as supports for the aerial stems of the yams. These natural supports will later be supplemented by stakes.

- Till the land to a depth of 20 to 40 centimetres.

- At this time add organic manures, well- rotted farmyard manure, compost or green manure, at 10 to 40 tons a hectare.

Inorganic fertilizers may be used to get a greater yield.

The amounts vary according to the country, region, or even the soils in the same field.

Research stations like IRAT' make a special study of food crops, and advise farmers.
In Liberia, it is known that the application of potassium (K) fertilizers is valuable in increasing yields.

In Nigeria and Ghana, the agricultural service advises that yams should be given the following fertilizers:

250 kg/ha ammonium sulphate;

65 kg/ha single superphosphate;

215 kg/ha potassium chloride.

The farmer who wants to make progress should all the time ask for advice from the agricultural service.

- In many African countries, yams are planted in mounds 30 to 40 centimetres high and 1 or 2 metres apart. These mounds are made at the beginning of the rainy season. The soil which has thus been well loosened holds plenty of water.

Sometimes the mounds are only made 2 or 3 months after planting. This earthing up encourages the development of tubers but takes a lot of work from the farmer.

If the soil is fairly deep and is deeply tilled, it is not always necessary to make mounds. In that case, more tubers can be planted and the density is greater.

Yams are planted at the beginning of the rainy season. Plant them 5 to 10 centimetres deep 1 metre apart in all directions or 90 centimetres by 1 metre. This gives the tubers plenty of room to fatten up, and the plant makes use of all the rainy season water.

How to propagate yams

Many kinds of yam bear flowers which fruit and produce seeds. So it is possible to obtain new yam plants by sowing these seeds.

But this way of propagating is no use to the farmer. The new plants grown from seed are not always like the parent plants. Often the yield is less, the tubers are much too small and of bad quality and contain a poison called dioscorine.

For all these reasons, it is better to propagate by cuttings. But here care is needed Take cuttings from ripe tubers, and not from the aerial stems, as is done with cassava. These root cuttings make plants which are like the parent plant, and give good yields.

For the cuttings use pieces of tuber or small whole tubers. To get regular sprouting and good yields, the cuttings "whether whole tubers or pieces) should weigh between 250 and 400 grammes.

The amount of yams planted represents a considerable part (about a quarter) of the harvest. That much of the harvest must be set aside and well stored for use in planting later.

Plant only fully ripe tubers. It is best to use the part of the tuber nearest the crown. This top of the tuber contains many growth buds and shoots more quickly than the rest of the tuber. For this reason, tops of tubers must all be planted in the same field.

The remaining yam tubers are planted in another field. They sprout less quickly.

With the Dioscorea bulbifera variety of yams, the bulbils can be planted in the same way as tubers. Wait until they are quite ripe, when they are easily removed from the stem.

Do not plant tubers or bulbils that are damaged, rotten or diseased.

Small yam tuber used for planting

How to plant yams

The bulbils, pieces of tuber or small tubers are planted in the top of the mound at a depth of 5 to 10 centimetres, and covered with soil. When there is too much sun or the light is too strong, cover the mound with grass, so that the sun will not dry out the young plant and the rain will not wash away the soil and the tubers

- In savanna country where there is a long dry period, stakes are not used.

The aerial stems trail on the ground. By covering it, they prevent weeds growing, and protect it against dryness.

A yam mound

Looking after the plantation

CONTROL OF WEEDS

For a good harvest, hoeing must be done two or three times during the early stages of growth.

When this cultivation is being done, the mounds are remade at the same time.

Later, the abundant vegetation of the yams prevents the growth of weeds.

It is then not necessary to hoe.

CONTROL OF DISEASES AND PESTS

Yams have few diseases.

However, rodents, some insects and fungi cause damage.

Damaged tubers rot quickly and cannot be kept for long.

Harvesting and storing yams

HARVESTING

Depending on the variety, yams are harvested 6 to 12 months after planting. Lift the tubers when the leaves and stems turn yellow and dry.

Do not leave the ripe tubers too long in the ground, otherwise they become bitter and may rot.

With some varieties, only one crop is harvested. Others are harvested twice.

At the first harvest, after 6 months, the biggest tubers are lifted.

The second harvest is taken 3 to 6 months after the first.

Or the crop may be harvested as and when needed.

STORING

Early varieties, such as lokpa, do not store well. These yams should be eaten immediately after lifting.

Late varieties, such as Dioscorea alata, may be stored for 5 or 6 months.

But they must be kept dry and protected from rats and other rodents.
They should be under a roof, on dry ground or on boards supported on posts.
To prevent rot, the tubers should not be heaped up too much.