Sweet potatoes are drought tolerant perennial vines, with one main season. It is widely grown on a small scale, mainly in subsistence farming.The root tuber is long and tapered with a smooth skin whose colour ranges between red, purple, brown and white. Its flesh ranges from white through yellow, orange and purple.They are used as food, boiled ore roasted and eaten either alone or with other foods such as milk, porridge, soups or grains.The sweet potato vines are useful and nutritious as fodder crop.Sweet potato is a food security crop that can be grown nearly in all parts of the country. It is an inexpensive crop because it does not need much labour or fertilizer and that can be grown by the rural poor. The orange-fleshed variety is a good source of vitamin A for nursing mothers and children.
Recommended varieties for low and high rainfall areas. New varieties are early maturing, high yielding, good virus resistance and contain increased levels of vitamin A. Examples include Zaparo, Naspato, Tainain, Kemp 36( a fodder),
Other Opportunities in the Sweet Potatoes Enterprise
Seed bulking – contract farmers to produce vines/cuttings
Livestock feed – growing of sweet potatoes as fodder also manufacturing livestock feeds
Industrial use – Starch is produced from sweet potatoes for industrial use (garment factories)
Production of composite flours
Key areas of policy concerns are:-
- Promote the production and utilization of food security crops
- Increasing agricultural productivity and incomes especially for smallholder farmers
- Encouraging diversification into non- traditional agricultural commodities and value addition to reduce vulnerability
- Enhancing the food security and reduction in the no. of those suffering from hunger and hence the achievement of the MDGs
- Encouraging private-sector led development of the sector
- Ensuring environmental sustainability
- 0 –2100 M ASL and occasionally found up to 2400M ASL. The crop prefers lower and mild elevation zones.
- Sweet potatoes is grown in a wide range of soil type, but does best on soils of friable/loose nature, which permit expansion of tubers.
- Sweet potatoes grow best in fertile sandy loams and do poorly in clay soils. The crop does poorly in water logged, too shallow or stony soils. Poorly aerated and bulky soils retard tuber formation and reduced yields.
- The crop is sensitive to saline and alkaline soils and they should be avoided
- Too high fertility may result in excessive vegetative growth at the expense of tuber and starch formation.
- It grows best at 24 0C, when temperatures fall below 120C or exceeds 350C growth is retarded
- 750 – 1000mm ideal but crop can withstand drought though under drought conditions, yield s are drastically reduced if drought occurs in the first 6 weeks after planting and also during root formation and development.
Source of Planting Materials
Certified seeds are available from the following KARI Station:-
KARI – MTWAPA, KARI EMBU, KARI KATUMANI, KARI KIBWEZI, KARI KISII, KARI KAKAMEGA
Sweet potato grows best and produces smooth, well-shaped storage roots in a well-prepared soil. Good land or soil preparation involves removal or incorporation of crop debris and any vegetation that may compete with the sweet potato crop, and deep manual or mechanical cultivation. Making ridges or mounds is recommended because of: sweetpotato
- Higher yields achieved
- Mechanization possible – during harvesting you can use a ridger
- Conserve soil moisture
- Reduces soil erosion
- Intercropping with other crops possible
- Planting on flat bed or fields should be discouraged as it reduces yields
- Propagated is by use of vines cuttings or storage roots.
- Storage roots
- Storage roots are used when there are insufficient stem cuttings available, or when the level of pest and disease infestation is high so that few healthy vines are left.
- The sets must be healthy, robust and avoid using the rinds. Cover with little soil 3 cm of soil to avoid rotting. The bed should be covered with straw to help retain moisture. This method is not recommended as the resulting yields are low.
Vines cuttings-The use of vine cuttings is the recommended practice. This is because:-
- They are free soil borne diseases
- Use of vines leaves tubers for consumption or selling
- Yields are higher
- They produces roots with better shape and size
Preparation of planting materials to achieve higher yields
- Select ‘clean’ planting material that are free of insects, soil, and any symptoms of viruses or fungal diseases.
- The apical (tip) portion of the vine is better than the middle or basal portions. This portion is less likely to carry sweet potato weevils and fungal pathogens, and has been found to establish faster than other portions.
- A length of 30 cm is recommended because cuttings of much greater length tend to wasteful of planting material while much shorter cuttings establish more slowly and give much lower yields.
- The delay between cutting and planting may affect yield depending on the storage conditions for sweetpthe cuttings. Storing cuttings for one to two days in humid conditions may be beneficial, promoting rooting at the nodes.
- Longer storage may adversely affect establishment by exhaustion of the cuttings’ energy reserves.
- To minimize losses, leaves should be stripped from the lower portion of the cutting, and bundles of cuttings wrapped in a wet cloth or sack and kept in a cool, shady place away from wind. If roots develop during storage, they should be planted carefully to minimize damage to the roots.
- In drier areas with only one main rainy season, leave some tubers in the soil over the dry season. When the rain comes the tuber sprout and the new vines are used for planting.
- If planting material is to be maintained in a multiplication plot before planting of the next crop, plant cuttings at approximately 15 x 20 cm spacing. New growth may be ready for cutting after 45 days.
- Sweet potato vine cuttings are planted at an angle with vine ends towards the centre of ridge or mound. 2/3 of the vine cutting is buried in the soil. Depth of planting is 4-6 cm deep
Planting on mounds
Make mounds 1M apart (i.e. centre of the mound to the next) each with a base diameter of 30-40cm.
Plant 3-4 vines in single stand at equal distance from each other.
Tender sweet potato crop grown on mounds
Planting on ridges
- Make ridges 1M apart (i.e. centre of the ridge to the next). The base of one ridge to the other is 10-15 cm.
- Tender sweet potato crop grown on ridges
- Planting is done either in single or double rows.
- Single row – Vines are planted at the middle of the ridge at 30cm between plants within the row. This is recommended in drier areas.
- Double rows – The vines are planted on left and right position of the ridges at 30cm between plants within rows and 50-60 cm between rows.
The recommended no. of cuttings is 27,000 per ha (11,000 per acre)
1 gunny bag of cuttings has approximately 2000 cuttings implying that 5 ½ bags – 6bags are needed for an acre
Time of planting
It is done at the onset of short and long rains when there is sufficient moisture to enhance establishment.
Field Management Practices
Fertilizer application, weeding
- Normally sweet potato does not require fertilizer application though in extremely poor soils a farmer can apply compound fertilizer e.g 17:17:17 at a 100 Kg per Ha in two splits, one at planting and the other after 2 weeks.
- Excessive nitrogen should be avoided since it encourages vegetative growth at the expense of tubers growth.
- Farm yard Manure is the only manure found to give a good response in sweet potatoes – it is spread over the bed just before ridging or mounding is done
- The 1st weeding must be done within 2 weeks after planting. The second one should come up two weeks later when earthing- up is being done
Pests have been shown to affect production of sweet potatoes.The most common pest of concern in the crop are the sweet potato weevil and moth. Sweet potato weevil. Adult female, body length 6-8 mm
Thickening and cracking of the vines due to feeding by the adult weevil
Larvae bores into the tuber leaving holes. This lead to bitter taste
Sweet potato weevil symptoms on tuber
- Integrated pest management for this insect is recommended, consisting of the following measures:
- Crop rotation
- Use of clean planting material, deep planting and regular hilling to fill soil cracks around plants
- Early planting and prompt harvesting
- Practice good field sanitation i.e. weeding, burn infested material
- Plant away from last year crop
- If seriously infested spray with dimethoate
- Organically incorporate a good amount of lantana camara before planting to repel the weevils.
Sweet potato moth (Omphisa anastomasalis)
With good management and depending on variety you can achieve up to 38 tons/Ha although average yields ranges between 15-30 tons/Ha.
Sweet potato can be stored in the field in fresh form and harvested as needed (piecemeal). Tuber stored for long periods in the field are usually attacked by weevils and large mammalian pests.
Fresh tubers can also be stored in covered pits under shade. Before being placed in the pit, the tubers are covered with banana leaves to avoid contact with soil. Sweet potatoes tubers can also be stored as dry chips.
Marketing and utilisation
Tubers should be free of surface wounds and bruises which reduces marketability and shelf life
The tubers are mainly sold fresh in local markets. They fetch better prices in major urban markets like Mombasa and Nairobi.
Utilisation and Value Addition
- Sweet potatoes can be consumed whole- boiled, roasted, deep fried or mixed with cooked beans, mashed and consumed as a complete meal. The tuber can be made into chips, crisps or dried and ground into flour.
- The flour can be composited with wheat flour to make bread, cakes, biscuits, mandazis, doughnut, chinchin, crackers and chapatis. Sweet potato flour can also be composited with sorghum or millets and used to prepare uji or ugali.
- The leaves can be used as fresh vegetables or preserved.
- The vines are used as livestock fodder
- Starch is produced from sweet potatoes in much the same way as from cassava, except that the solution is kept alkaline (pH 6.8) by using lime, which helps to flocculate impurities and dissolve the pigments
Notable nutritional attributes of sweet potatoes include-
Consumption of sweet potatoes protects against obesity. Although they are starch rich food with a good deal of calories, they produce a sensation of satiety reducing appetite.
They are rich in Beta carotene making them appropriate in cases of arteriosclerosis (thickening and hardening of arteries)
Recommended for individuals with increased need for energy e.g. athletes and convalescence.