PLANTING
Soil: sandy loams are ideal for sweet potatoes, though there are varieties that will
grow well in most soil types. Sweet potatoes are sensitive to waterlogging and
alkalinity—pH6 is optimum ie they are tolerant of acidity, and are moderately
susceptible to salinity.
nutrients sweet potatoes normally respond well to high Phosphate and Potash
fertilisers. Typical applications are, in kg/ha: N20–80, P80–150, K80–170. K is
generally more important than N or P, and too much N may encourage stem growth
at the expense of tuber growth. Organic manures are very beneficial.
Propagation: stem cuttings about 20–45 cm long are almost always used in the
tropics, as they are less valuable than tubers and are also free from soil-borne
diseases. The lower half is pushed into the soil at an angle—sometimes only the
central part of the cutting is buried.
are transplanted into the field after 4–6 weeks when about 30 cm long. With this
system only the best tubers from the least diseased and insect damaged plants should
be used.
Spacing: normally on ridges about 90 cm high with about 30 cm between plants.
Sometimes on mounds or hills about 60 cm high about one metre apart, with several
plants per hill. The optimum plant population is about 25–30,000 plants (vines) per
hectare.
GROWTH CONDITIONS
Day length: short-day; most varieties will only flower when the day length is less
than 11 or 12 hours per day, though flowering is not of any great relevance to
farmers.
Growth period: 3–8 months, depending mainly on variety and rainfall. Plants are
mature and ready for harvest when the leaves turn yellow, but the tubers are best left
“stored” in the ground until needed. In arid areas smaller yields are produced, in
about 3 months.
Crop care: the vines should be lifted and turned back on to the ridges from time to
time. Although this practice will allow more weeds to grow in the unshaded inter-
row areas, it does prevent the vines from forming roots, so the plant can produce
fewer—but more uniform and larger—tubers.
Temperature: 25
The maximum is about 32 C. Sweet potatoes need a 4–6 month frost-free growing
period; a slight frost may kill young plants of some varieties. The foliage of older
plants may be damaged, even by a light frost, but the plants normally recover.
Rainfall: the optimum is from about 750 to 1300 mm per year, minimum is about
500 mm. If the plants are well established they can tolerate long dry periods. They
will usually also tolerate very high rainfall, especially if planted at the end of the
rainy season; a mature crop covers the soil surface with vines and leaves, reducing
both soil erosion and weed growth.
°
°
C is optimum, but most varieties will still grow, slowly, at 10 C. °gether in a nursery; they are then sprouted, and become known as “slips” which
In some countries in temperate regions small tubers are first planted close to-213
Fertiliser is rarely used, and although the plants do not have a high demand for