Many thousand separate varieties of potatoes are grown, and the description
below is only a rather brief synopsis of the vast array of literature available on
potatoes.
Although plants can propagate from true seed the planting material used by food
producers is the swollen underground stem, or tuber (which for planting purposes is
known as seed or, occasionally, setts). Thus each new potato plant formed exactly
resembles its parent in its genetic makeup and, therefore, characteristics.
As a general rule, plants with white flowers have tubers with white skin, while
plants with pink, red, blue or purple flowers have tubers with coloured skin,
normally pinkish.
In the tropics potato plants do not normally flower, and are normally only grown
there as a cool season crop or at high altitudes.
The tubers contain about 80% water, 2% protein, 18% carbohydrate (mostly
starch), and a number of minerals; freshly dug tubers contain about 20 mg/100 g
Vitamin C though this decreases during storage.
PLANTING
Propagation: by means of the whole tubers (“seed potatoes” or “setts”) or parts of
tubers, cut to include at least one bud or “eye”. If they are cut, the tubers should be
cut at right angles to the main axis so as to avoid apical dominance (the inhibition of
lateral buds to form branches). Ideally, seed should be “chitted” before planting;
place the seed in single layers, “rose end” up—the end with the most eyes. Protect
from frost and/or direct sunlight. Before planting, remove all but 3–4 chits at the
rose end.
Soil: should be well drained. Optimum pH is 5.5–6. They do not normally need
lime, but do need a good supply of nutrients, manure being particularly beneficial.
They are moderately susceptible to saline soils.
Potash is also normally needed, but not always, and in smaller quantities than
Dormancy: potato tubers are normally dormant for at least six weeks, and up to
about 10 weeks, after the tuber is fully grown. Some time after this they may begin
to develop sprouts, or chits, indicating that they are ready for planting (see
“Propagation”, above).
Seed rate: for maincrop about 1. 5–3 MT/ha, for earlies about 3–4.5 MT/ha of
tubers, which for planting purposes become known as seed or setts.
Spacing: 20–30 cm between plants; 70–120 cm between rows for maincrop, 40–60
cm for earlies.
Depth: 5–15 cm. Potato plants should be earthed up or ridged up as they develop, to
improve the development of the tubers and to prevent the upper ones from turning
green ie developing solanine when exposed to light, and also to reduce the build-up
of blight disease. Solanine is an alkaloid which is toxic in high concentrations,
causing vomiting and other symptoms.
Intercropping: potatoes are normally grown in pure stand, but are occasionally
mixed together with maize or beans. A fast growing catch crop (1Gc, page 62) such
as lettuce can sometimes be grown on the ridges before the potato haulm takes over.
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The FAO estimate for global production in 2004 was 328 million MT.Fertiliser is commonly used, up to 110 kg/ha Nitrogen and 225 kg/ha Phosphate.nitrogen or phosphate.