Altitude range
Sweet potato requires altitudes from 0 – 2500 m above sea level, depending on variety but most of them do well in an altitude range of 1000 – 2100 masl

Soil type and conditions
Requires a well-drained, deep sandy loam and loamy clay soils, preferably with friable/loose nature. Sweet potato does poorly in clay, stony, rocky or waterlogged areas. Soils which are excessively acidic or alkaline encourage bacterial infection and negatively influence yield. The optimum pH should range between 4.5 and 7.0. Sweet potato nutrient requirements include: N, P, K, Mg, Ca, Fe, B

Temperature range
The temperatures should be average range of 15 – 30°C growth

Rainfall
It requires rainfall of 750 mm – 1500 mm p.a., with at least 500 mm during growing season

Steps during land preparation

1.    Ploughing can be done using hand hoe, ox-plough or tractor. Plough deep enough for root expansion. Harrowing may be necessary to ensure an appropriate tilth. In dry environment, sunken beds can be used for moisture retention
2.    Prepare ridges 30 - 45 cm high, 60 cm wide, and spaced 1.0 m apart manually, by oxen, or a tractor ridger along the contours. Ridges helps to minimize runoff and reduces soil erosion
3.    If preparing mounds/heaps space 60 - 100 cm wide and 30 - 60 cm high, prepare manually using a hoe or jembe. Mounds are better in flat areas that are infested with mole root rats or in areas experiencing problems of drainage. It is preferable to plant sweet potato on mounds in areas experiencing problems of drainage.

Ridges                        Mounds

Seedling emerging from the tuber

Planting materials should be obtained from accredited nurseries and institutions like: Private certified companies, JKUAT and KALRO

The length of cuttings/vines should be 25 - 40 cm long with at least 3 - 4 nodes

To select good planting material, consider length, age and health of planting material. Vine tips of old plants are weak and grows slowly while vine tips of young plants are vigorous and grow rapidly

Vine multiplication: can be done on nursery beds under rapid seed multiplication or conventional seed multiplication on the flat ground as an alternative to use of direct cuttings or vines

Rapid vine/seed multiplication: done when quality seed is limited but required in large quantities to ensure sufficient planting materials for timely planting/at the onset of rains

Locate rapid seed vine multiplication beds near a reliable source of water

Locate nursery beds away from where a similar crop was planted in the previous season

Keep isolation distance of 100 m between the old sweet potato field and the nursery

Raise nursery beds at least 20 cm above the ground, spaced 1.0 m wide and of convenient length to accommodate the anticipated planting material

Manure and fertilizer application

Apply well-rotten farmyard/poultry manure at the rate of one debe and 25-50 g of NPK (17:17:17) per m2 based on the analyzed and tested soil.

Where several varieties are planted, 5 m distance should be left between the beds/varieties and a space of 50cm to 1.0m between beds is sufficient where one variety is being multiplied.

Use mini stem cuttings of 3-4 nodes, where 2 nodes are inserted in the soil and 1 node ab

Reproductive stage

Keep the seedbeds weed-free and rogue any off types and infected plants. Rapid vine multiplication can also be done in a shed /fleece net to conserve materials for a longer time.

Conventional vine multiplication: should be done on the flat ground in an open field and near a source of water

Source planting materials from tip of the vine or younger parts

Plant cuttings of 3-4 nodes or 20-30 cm long at 50 cm x 30 cm on flat ground

Plant cuttings vertically (buds facing up) with two thirds of the cutting inserted into the soil.

Types of sweet potato include: white, yellow and orange fleshed

Each type has various varieties White fleshed; Mugande, SPK 013 Yellow fleshed; Kenspot 1, Namnyekera, Cuny and Bungoma Orange fleshed; Kabode, Vitaa, Jankaroti, kenspot 4, Kenspot 5 

Planting
Plant vines at an angle of 45 degrees, 30 cm apart, with vine ends (bases) towards the centre of the ridge, ⅔ covered with soil, leaving ⅓ above the soil. Where ridges are wider than 1.0 m, double rows at 30 cm apart can be planted on ridge. It can also be grown on terraces

Plant 3-4 vines of 20-30 cm length on each mound equidistantly in a triangular pattern

Plant on ridges or mounds/heaps, as relay in cereals or as a monocrop in the second season at onset of rains which also acts as a cover crop to reduce soil erosion. It can also be used as a cover crop in orchards and other plantations

Both ridges and mounds should give a plant population of 13,500 plants per acre. Too high population may result into many small roots. 

Sufficient water should be applied to wet the root zone, without causing deep drainage, flooding or run-off

The aim is to keep the soil moisture conditions as constant as possible. More frequent, light irrigations are preferred to larger water applications

Apart from being wasteful of water, over watering can cause leaching and loss of soil nutrients while contaminating the groundwater and streams with the nutrients. 

Keep the field weed-free within the first two months after planting

Two to three weeding is recommended

First weeding can be done using hand hoe and subsequent weeding by hand pulling of weeds

Uproot weeds and spread on the soil as mulch. Thereafter the vine spread should cover the ground to smother weeds 

Add manure and fertilizers to the soil in the right amounts to provide the required plant nutrients for vigorous crop growth 

Fill the gaps in case of poor sprouting, within the first 2 weeks after planting

Earth the roots to avoid greening of roots

Rogue out off-types and diseased plants 

Pests include;
Sweet potato weevil, sweet potato mites, whiteflies, aphids, rodents, livestock and wild game

Sweet potato weevil Cyclas spp

Sweet potato weevil are small black beetles. Adult weevils feed on the vines and roots, while the larvae make tunnels into the vines and roots. Damaged roots become bitter with bad smell, rendered unmarketable
Sweet potato weevil
Control
Earth the roots during to minimize weevil damage.

Rotate sweet potato with other crops to control pests

Integrated pest mangement including; using clean seed, avoid old stems-field sanitation, rotation avoiding old sweetpotato fields, earthing, planting on time and harvesting promptly. Some farmers use pheromone traps and repelants

Sweet potato mites
Sweet potato vines react by forming hairiness condition referred to as “erinose” and die back during dry season

Sweet potato mites

Control
Select mite-free planting materials and plant in clean, mite-free fields good field sanitation is required

Remove affected plants and destroy/burn them or clip the top part of the affected vine and burry deep to reduce mite population

Spray infected fields with miticides refer to cautious use of pesticides

Rotate after every season with non-root and tuber crops, preferably cereals and legumes – to break pest and disease cycle

Rodents
Rodents often burrow near the roots and chew on the plant roots

Sweet potato loss from rodents

Control
Integrated pest management (IPM) is the best control option

Destroying rodent burrows

Keeping the field and surrounding areas clean and clear of vegetation field sanitation

Digging a deep ditch around the perimeter of the field to prevent rodents from digging tunnels straight into the fields

Use of repellent materials inside their unblocked burrows e.g. cow dung mixed with chili or urine

Use traditional traps 

Major diseases include;
Sweet potato virus disease (SPVD), Alternaria leaf spot, sweet potato feathery Mottle Virus disease (SPFMV) and sweet potato sunken vein virus (SPSVV)

Sweet potato Virus Disease (SPVD)
Is caused by a synergistic combination of Sweet Potato Feathery Mottle Virus (SPFMV) and Sweet Potato Sunken Vein Virus (SPSVV). SPVD is also spread by use of diseased planting materials

Damage: Stunted growth, small and narrow leaves with a distorted edge, wrinkling, vein-clearing, mottling and the whole plant may appear yellowish

Sweet potato Virus Disease (SPVD)

Control
Use resistant varieties.

Clean planting materials; field sanitation and weed free

Plant new fields away from old fields and uproot off-types

Rogue diseased plants to minimize disease spread

Crop rotation

Materials conserved in a fleece net will remain disease-free for at least one year depending on the management level

Alternaria Leaf Spot
Causal agent – Alternaria spp
Causes blackened lesions on the stems, leaves
Damage: Leaves become shrivelled, blackened, die and fall off in severe cases

Alternaria Leaf Spot

Control
Use of resistant varieties
• Use health, clean disease-free planting materials
• Destroy or burn infected crop material
• Practice crop rotation
• Use fungicides for commercial vine production

Sweet Potato Feathery Mottle Virus (SPFMV) disease
Aphid transmitted

Sweet Potato Feathery Mottle Virus (SPFMV) 

Maturity indicators include: 

Vegetative stage

Yellowing and drying of the lower leaves, cracks forming on the soil surface of the ridge or mound

Sap exuded from the cut surface of mature roots and does not darken easily while the cut surface of immature storage roots gives a dark greyish colour 

Delayed harvesting leads to fibrous and pest-infested roots

Harvest piecemeal is done for specific needs; domestic, market, contractual arrangements or complete depending on purpose

The vines should be reserved for future planting

Harvest carefully not to damage remaining roots

Sweet potato will produce 35 – 95 bags per acre and 13,500 vines/acre

Harvested vines should be stored under shade

Tie vines in bundles of 50 or 100 pieces. Ensure every package is well labelled showing variety, contact of the vine multiplier and date of harvest. The vines are put in gunny bags 1000 vines each and should be aerated most of the time

Transport planting materials immediately after harvesting, possibly in the evening or at night

Untie and empty the bags once the destination is reached, to avoid accumulation of heat in the bags which can damage the vines. 

There are three storage methods that can used;

In-ground storage: harvesting of what is required for current use and leaving the rest of the roots in the ground

Check the field regularly for cracks or exposed roots and cover them with soil. This help to prevent root damage by weevils and exposure to the sun

Clamp storage: make a flat mound of dry soil 1x1m wide raised about 10cm above the ground

Cover the mound with dry grass for cushioning the roots and moisture absorption. Or instead of using grass for thatching, which is rare currently, iron sheets could be used.

Carefully pile sorted roots on the mound in layers to form a pyramid then cover the roots with more dry grass and a layer of dry soil 10-20 cm thick

Construct a grass roof about the mound to protect it from the sun or rain. Allow a gap between the roof and the mound for ventilation

Dig a channel around the clamp to allow drainage of water from the roof

Remove roots from the pile as required ensuring that the pile is covered again with grass and soil

Pit storage dig a pit in a dry place. The size of the pit will depend on the quantity of roots to store

Line the pit with dry grass. Prevent ants/termite’s moisture leakage to the stored roots and monitor to avoid root damage

Pile the sweet potato roots in the hole and cover with more grass and dry soil

Insert a bamboo pole or plastic pipe through the soil above the roots to act as a ventilation pipe

Cover the pit with a raised sloping grass roof to protect it from the sun and rain. 

Wash, sort, and grade the harvested roots, depending on the intended use

Washing is to remove soil from the roots without bruising the skin

Sorting is done to remove rotten, damaged, malformed and weevil infested roots, and debris

Grading is done to categorise the sorted roots to have uniform produce, based on size, shape, colour, weight, maturity, market demand

Keep harvested roots protected from direct sunshine to avoid shrinkage and shrivelling, and to meet buyer requirements for grade and size. 

Sweet potato can be baked, boiled fried or mixed with other foods