cyanide producing sugar derivative:
Sweet types—HCN (potentially 30–100 mg HCN/kg) is present only in the two
outer layers of the tuber, which can be eaten raw after peeling. The tubers have a
soft, white flesh. They normally mature in about six months, and can then be left
stored in the ground for almost a year.
Bitter types—HCN (potentially 1350 mg HCN/kg) is present throughout the tuber,
which must be cut up and boiled, the HCN squeezed out, and then reboiled in clean
water; or it can be roasted or fermented. The tubers have a firm, yellow flesh which
normally mature in twelve months or more and can be left in the ground for up to
four years.
PLANTING
Propagation: by stem cuttings, 20–30 cm long, often cut from the middle of the
parent stem to avoid Mosaic Virus which is often present on the lower parts. About
half the length is pushed into the soil, where it sprouts readily within about 7–14
days.
Soil: best on sandy or sandy loam soils, but cassava can grow almost anywhere if
the soil is not waterlogged, saline or too shallow or stony. Some cassava varieties
are adapted to dry areas with alkaline soil, others to acidic mud banks along rivers.
Plant spacing: about 12,000 per hectare, 1–1.5 m apart. Sometimes two or more
stems are planted together, either on the flat or on ridges or mounds. They are sun-
loving plants.
Intercropping: very common, with a wide range of annual food crops. Legumes
such as peanuts are commonly used. Trials have shown that intercropping with trees
such as the acid tolerant leguminous shrub Flemingia macrophylla increases overall
yields per hectare, improves the soil fertility and provides fuel wood.
Rotation: since cassava can grow on exhausted soils which are unsuitable for many
other food crops, it is often grown at the end of a long period of monocropping
(monoculture), or as the last crop planted in shifting farming systems.
T here are two main types of cassava sweet and bitter. Most varieties of both —
types contain the glycoside linamarin which breaks down under enzyme action to
give the poison Hydrocyanic glucoside (HCN, hydrocyanic, or prussic, acid), a
T he HCN content also varies with the soil type, for example increasing in soil
which is deficient in potash, and also with the climate. Changes in toxicity can also
occur when varieties are introduced from one country to another. Old, stale tubers
are more toxic than freshly harvested ones.
B oth types of cassava are virtually immune to attack by the African Migratory
Locust. The bitter types are also only rarely eaten by wild game such as baboons,
pigs, rats and hippos, and they are sometimes the only crop that can be cultivated
in areas with this problem.
I n heavy clays or very fertile soils, or with high Nitrogen levels, vegetative growth
can be excessive and at the expense of tuber growth. Potassium is the most important
element, although Nitrogen, and to some extent Phosphates, also normally produce a
good response. However fertiliser is rarely used on this crop.