By George Munene

An acre of land under sweet potatoes has a yielding potential of 14-8 tons in four to six months. Once shunned as a food crop for the poor, the sweet potato with its myriad of health benefits and its ability to be grown with little or no pesticide application has gained a pride of place at the breakfast tables of many Kenyan households. Farmgate prices are now Sh70 per kilogram or Sh6000 for 100kg. Sweet potato production costs stand at less than half their selling price. However, to get such a bumper harvest and return, farmers have to practice the right crop establishment practices.

Planting Material

This begins with the selection of certified high yielding, disease and pest resistant propagation materials. Sweet potato planting material is not available in agrovets this makes sourcing for the right seeds/ vines difficult for most farmers. Certified sweet potato seeds or vines can be got from KEPHIS, KARI regional research center in your region and from The International Potato Centre (CIP). Vegetative material can be got from farmers accredited with localised propagation of sweet potato planting material from any of the aforementioned bodies. Vine cuttings are preferred to roots as they are free from soil-borne diseases. They are also higher-yielding and offer bigger and better-formed tubers. Vines used for planting should be no more than three generations removed from the original certified seedlings to get the best results.

Sweet Potato varieties

Popular varieties in Kenya

• Bungoma- 40-50 100 kg bags/acre
• Gikuyu- 40-50 100 kg bags/acre
• Kalam Nyerere- 40-50 100 kg bags/acre
• Mugande- 40-50 100 kg bags/acre
• Namaswakhe-40-50 100 kg bags/acre
• Nyakabondo- 40-50 100 kg bags/acre
• Nyakathuri-40-50 100 kg bags/acre
• Nyawo/Amina/Misambi- 40-50 100 kg bags/acre
• Polista- 40-50 100 kg bags/acre
• Odinga- 40-50 100 kg bags/acre

Fodder

• K117- 23 tons/ha foliage vine yield
• Mwavuli- 20 t/ha foliage vine yield

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Improved high yielding varieties

• SPK 013-Recommended for the western zone including the Lake basin
• Kemb 10 Maturity period: 3-4 months; Yield: 70 100 kg bags/acre; White skin colour. Cream flesh colour
• Kemb 20-Maturity period: 4-5 months; Yield: 75 100 kg bags/acre; Red skin colour. White flesh colour
• Kemb 23-Suitable for Central and coastal lowlands

Maturity period: 5-6 months; Yield: 90 100 kg bags/acre; White skin colour, Cream flesh colour

• Kemb 36-Maturity period: 5-6 months; Yield: 35 100 kg bags/acre; Red skin colour, White flesh colour    
• KSP11-Good performance in dry areas
• KSP20-Good performance in dry areas

KSP 20- Maturity period: 3-4 months; Yield: 95 100 kg bags/acre; Red skin colour, White flesh colour    

• KSP 47- Maturity period: 3-4 months; Yield: 70 100 kg bags/acre; Red skin colour, Light Orange flesh colour
• KSP 72- Maturity period: 3-4 months; Yield: 70 100 kg bags/acre; Red skin colour, Light Orange flesh colour
• KSP 84- Maturity period: 3-4 months; Yield: 70 100 kg bags/acre; Red skin colour, Orange flesh colour
• KSP 154- Maturity period: 3-4 months; Yield: 70 100 kg bags/acre; Red skin colour, Orange flesh colour Simama
• Japaneese
• Mugande
• Muibai ("Kemb 36")
• NASPOT 1-Popular in Ukambani; High dry matter content in tubers.

• Ex-Diani-Suitable for central and coastal lowlands
• Mafuta-Best for foliage production. Good for all areas.
• CIP 420009-Good performance in dry areas
• Mtwapa 8-Suitable for coastal and eastern lowland areas
• Enaironi- Maturity period: 3-4 months; Yield: 65-70 100 kg bags/acre; White, Orange flesh colour
• Ex-Simba- Maturity period: 3-4 months; Yield: 70 100 kg bags/acre; Red skin colour. White flesh colour
• Kanchwere -Maturity period: 6-7 months; Yield: 40-50 100 kg bags/acre: Red skin colour. Orange flesh colour
• SPK 004- Maturity period: 3-4 months; Yield: 80 100 kg bags/acre: Light brown skin colour. Orange flesh colour
• Tainung - Maturity period: 3-4 months; Yield: 80-90 100 kg bags/acre: Light brown skin colour. Orange flesh colour

Fodder

• Ex-Mukurweini- Maturity period: 3 months; Yeild:60-65 tons/ha foliage
• Musinya - Maturity period: 3 months; Yield: 90-95 tons/ha foliage

Source: https://www.infonet-biovision.org

An average of 10-20 tons/ha can be gotten from improved sweet potato varieties.

Other factors to consider when picking the sweet potato variety to grow:

• Target Market- The orange-fleshed Sumaiya has little fiber content making it favored for chips, crisps, purees

Processers prefer even shaped potatoes while regular consumers go for bigger sized ones.

Red skinned and yellow-fleshed sweet potato variety is the most grown in the country because of its equally high consumer demand.

• Nutritional variance- Orange fleshed sweet potatoes are rich in beta carotene, a precursor for vitamin A. This is similar to nutrients you can get from carrots that helps build eyesight

Purple fleshed sweet potatoes contain anthocyanin, an antioxidant helpful in preventing cancers

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Growing conditions

Different sweet potato varieties are suited to regions with varied rainfall patterns but on average they thrive in areas with 600-1600 mm per year of rain, some do well in more dry conditions. Irrigation should be done on a frequent basis to keep moisture levels consistent.

Ideal growing altitudes are 2100m above sea level with warm temperatures not below 10C and averaging 24C. Soil PH should be 5.6-6.6 Ph.

Sweet potatoes require well-drained soils with plenty of sunshine. Most soils do not need fertilisers while requiring little manure for their growth. Manuring is recommended to get healthier vines and tubers.

In soils that are very poor, a basal compound 17:17:17 fertiliser can be applied at a rate of 100kg/ha.

Planting

Being fairly hardy crops, sweet potatoes can be planted at any time in regions with no critical dry season. Planting should be done during the onset of short rains, and at the tail end of long rains.

An acre will need 16,000 seedlings, each costing between Sh4-7 per piece. You will need about Sh80,000 for an acre of good quality planting vines. Operation costs will round out at about Sh40,000.

All soils other than clay-soils which have a high bulk density and poor aeration which retard tuber formation are suitable for the growing of sweet potatoes.

Planting vines can be stored for a day or two in humid conditions to promote rooting at the nodes which enables for easy establishment once moved to the field. The roots however should be managed carefully whilst planting to avoid potential damage.

The cultivar should be healthy as well as being free from pests and diseases. The tip of the sweet potato vine is better than the middle portion for propagation as it is less of a pest and disease carrier. Using vines also leaves the entire tuber for consumption or selling compared to the use of roots for propagation requiring some roots be left for the next season’s planting.

Planting vines should ideally be 25-30cm long this avoids wastage while shorter vines take longer to establish and are poor yielders. More important, the vine should have at least five to eight nodes. At a depth of 4-6 cm a minimum of two to three nodes or one third to two thirds. Up to eight nodes can be buried in the soil.

Watering should be done regularly at the first 1½ months when the cultivar is young and weak; once vines germinate it’s easy for them to store food. Irrigation can be done twice a week thereafter; you ought to wet the root zone without causing run-off; overwatering causes leeching (loss of nutrients). Sandy soils require more watering than clay-based soils or soils with high organic matter. Excessive rains/ watering causes exponential growth of vegetative material but reduced tuber sizes.

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Sweet potatoes act as a cover crop suppressing the growth of weeds as well as enabling the soil to retain moisture and prevent erosion. A farmer need only practice weeding on the first two months before the crawling vines smother the weeds. Within two weeks of planting the first weeding is done after the second weeding two weeks after that when earthing up.

Once mature, sweet potatoes are harvestable for up to 12-18 months at four-week harvesting intervals. A prolonged stay in the field means roots have to mobilise assimilates for vines to continue growing, affecting the quality of tubers harvested. Diseases and pests such as moles and weevils also buildup if your crop is not rotated.

Ridging

Early thorough tillage is necessary to create a deep loose bed.

Sweet potatoes should ideally be grown on ridges, with 30 cm spacing between each vine, and one meter of spacing between furrow and ridge to allow for easy root multiplication and development as well as offer space for field management.

Ridges offer enough Space and loose soil for the rooting system to develop enabling tubers to extend and become bigger

Ridges can be dug along the contours of sloping land to minimize erosion and capture runoff water, they should be 30-45 cm, higher in wet areas to minimize the wash off of topsoil and

Potato residues and other farm refuse should be placed between ridges for moisture retention, weeds suppression and reduced soil erosion. Once they rot they also offer additional nutrients to the crop.