SWEET POTATOES are generally an easy crop to farm as they are not capital intensive and high yields can be realised on small tracts of land. There are two broad categories of sweet potatoes:
l The staple type with white flesh and white or purple skin has a high starch and dry matter content.
l The desert flesh and orange skin with a high sugar and beta-carotene content.
Three distinct types of sweet potato
l Orange or copper skin with orange flesh for example Beauregard, Hernandez,Beerwah Gold, NC-3, LO-323, Centennial, Darby and Jewel.
Zimbabwean orange cultivators such as Beauregard have long, cylindrical to heavy elliptic tubers. They have high beta-carotene content and are fairly quick growers. It may become too big with long growing period.
l White or cream skin with white or cream flesh colour has a high yield and a good storage life.
It can produce good yield in a relatively short growing period (four months) which is important for cold regions. It produces some long, curved sweet potatoes, especially in sandy soils.
l Red or Purple skin with cream white flesh such as Northern Star, Red Abundance, Rojo Blanco.
Kodow is a very attractive and tasty cultivator when cooked also with appointed oval tuber. Its tips break of very easily. It requires growing periods of five months to produce a good yield.
Selection of a variety to grow should be based on market demand. Varieties are assessed on a number of parameters, including root shape and uniformity, marketable yield, skin and flesh attractiveness and plant vigour.
Climatic requirements
Because sweet potatoes are of a tropical origin, they adapt well to warm climates and grow best during summer. Sweet potatoes are cold-sensitive and should not be planted until danger of frost is past.
The optimum temperature to achieve the best growth of sweet potatoes is between 21 and 29 degrees, although they can tolerate low temperatures of 18 degree Celsius.
Storage roots are sensitive to changes in soil temperatures, depending on the stage of root development.
Soil Requirements
A well-drained sandy loam is preferred and heavy clay soils should be avoided as they can retard root development, resulting in growth cracks and poor root shape.
Soil pH should be adjusted to about 6,0 by applying lime ordolomite.
Rates of 240kg and 400 kg/ha respectively will raise the pH by 0,1 of a unit. The soil should be deep ripped and then disk cultivated to break up any large clods and provide loose soil for hilling of beds. A yearly soil test is recommended to assess soil properties, pH and nutrients levels before ground preparation.
Propagation
Sweet potatoes are propagated from sprouts or from slips (vine cuttings); sprouts are preferred.
Sprouts are grown from plant stock selected for its appearance, freedom from disease and off-types. Approximately 75kg of planting stock sweet potatoes are needed to produce enough sprouts to plant one hectare.
Cutting collection
Tip cuttings of about 30 to 40 cm long with approximately eight nodes are collected from the nursery bed, or the last established planting.
Tip cutting should be taken from crops that are old enough to provide material without excessive damage. Avoid ‘back cuts’ as these will have variable maturity and result in significant yield reduction.
The lower leaves should be cut away as tearing these off may damage the nodes that will produce the roots. Cuttings can be left under a moist cloth in the shade for a couple of days to promote nodal rooting before planting in the field. At the recommended plant spacing, 330 cuttings are required for a 100m row.
Seedbed production of cuttings
This involves the propagation of cuttings from harvested roots which are placed together in a seedbed. This is an alternative method of producing plant material which requires less labour but does sacrifice a percentage of marketable roots.
Planting cuttings
Cuttings should be planted at an about 45 degrees angle into heaps as this promotes good, even root development. Half of the cutting or three to four nodes should be buried at a spacing of 30cm between plants.
Mechanical planters are available and used on a large scale planting but manual planting is widely practised. This can be as easy as pushing the cutting into the heap with a forked stick. The labour requirement for hand planting is estimated a 32h/ha.
Cuttings need to be watered at or immediately after planting. Plantings should be scheduled to allow for progressive fortnightly harvest over the desired production period.
Sprout Production
Sprouts are produced from the conditioned roots in cold frames, heated beds, or field beds of clean sand or fumigated sandy soils. Conditioned roots are covered by more soil sand, though not too much. Four or five weeks are needed to develop strong plants if the soil in the plant has been kept at 23 to 26 degree Celsius.
Six to eight weeks may be needed if roots have not been ‘pre-conditioned’. Adequate moisture is especially critical to germination of the sprouts and proper root formation on the sprouts.
Planting the sprouts
Sprouts should be taken from the plant beds when 6-10 leaves and a strong root system have developed on each one. They are set out into the field as early as possible when the soil has warmed and the risk of frost or a cold weather period has passed. Plants should be spaced 30-38 cm apart in rows that are 1m apart. This requires approximately 14 520 plants per hectare. Management of water is critical to avoid transplant shock.
Bed formation
Sweet potato is grown on raised beds or mounds.
This provides the developing roots with loose, friable soil to expand to their potential size and shape without restriction.
It allows adequate drainage and provides easy harvesting with a mechanical digger. Mounds should be approximately 30 cm high and 40 cm wide at the base.
The main consideration is that developing roots remain under the soil within the heaps.
If using a mechanical digger at harvest time, it is important to match width of the mound with the width of the digger mouth. Spacing the mounds at 1,5-2,0m apart (depending on the tractor width) with a roadway every six rows allows access for boom spray.
Mounds are formed using hilling disks, and the base fertiliser can be incorporated during this operation.
Planting period
Planting time is mainly determined by the climate of a location. Sweet potato plants are damaged by light frost and the plants require high temperatures for a period of 4-5 months to yield well.
In areas with mild frost, mid-November to mid-December is the best time to plant, and usually the crops get ready for harvest from April to May. Mid-November to be the beginning of December is recommended in areas with heavy frost, with harvesting taking place from April to May.
It is common to plant from January to March in frost-free areas so that the growing season extends through winter.
Cold spells during winter can be a risk, depending on the climate of the specific area. In very hot areas, planting should be avoided from November to middle of February as storage root formation is reduced by high temperatures.