LAND PREPARATION

Cassava is an important food crop in sub-Saharan Africa. The crop produces acceptable yields under sub-optimum conditions. Nevertheless, good production requires good land preparation. Land preparation practices depend on

climate

soil type

vegetation

topography

degree of mechanization

availability of labor

traditional cropping systems

Where farmers grow cassava traditionally as the first crop after clearing the forest, no land preparation is required other than the removal of shrubs and vines, and cutting off branches of large trees to admit sunlight. When the first rains have softened the ground, farmers loosen the soil with a hoe, planting stick, or sharp instrument.

Depending on soil type and drainage, the field may be prepared as mounds, ridges, flat-tilled, or zero-tilled (Figure 1). Drainage conditions determine the size of the ridges or mounds, and the placement of crops. In wet areas, water-loving crops such as rice may be planted between mounds or ridges. Upland crops such as cassava, maize, and legumes which require good drainage may be planted on the side or top of the mounds or ridges.

Land preparation for planting cassava on upland and in valleys differs (Figure 2). On upland, farmers plant on flat ground, or mounds and ridges. In valleys, farmers prepare ridges or mounds above ground level to control waterlogging.

Cassava cultivation on mounds is common in West Africa. Farmers gather the soil into heaps. Mounds range from 30 to 60 cm high. They are lower than mounds prepared for yam, but have broader bases. Cassava tuberous roots spread more widely and penetrate less deeply than yam tubers. The space between mounds varies from 60 to 200 cm.

Where mechanization is available, farmers plow and harrow the land down to a depth of 25 cm.

For planting on flat soil, farmers insert cuttings directly into the land. For planting on ridges or furrows, farmers ridge or furrow the land after harrowing.

PLANTING

Orientation of planting. Plant cassava cuttings vertically, at an angle, or horizontally (Figure 3).

Vertical planting: place two-thirds of the cutting in the soil.

Planting at an angle: place two-thirds of the cutting in the soil, with an angle ranging from slightly above horizontal to about 60°.

Horizontal planting: place the entire cutting horizontally in the soil at a depth of 5-20 cm; usually 10 cm.

The orientation of the cutting influences growth characteristics. Cuttings planted vertically sprout and develop foliage quicker than cuttings planted at an angle or horizontally.

Vertical planting produces deeper tuberous roots than angled planting, while horizontal planting produces the shallowest tuberous roots. However, tuberous roots from vertical or angled planting are arranged more compactly, and are more difficult to harvest than tuberous roots from horizontal planting.

Most modern mechanical planters are designed for horizontal planting. The planter opens a furrow and drops the cutting horizontally.

Experience in different cassava-growing areas shows that

in clay soils with sufficient rainfall (1000-2000 mm/year), planting may be horizontal, vertical, or angled, because the moisture is adequate for sprouting

in sandy soils and under erratic rainfall, plant vertically. A 20-30 cm cutting, planted vertically, reaches 20 cm deep into the soil, and may find enough contact with available moisture

Planting time. Plant as early as possible, just before the rains or after the rains begin. Delayed planting leads to reduced yield. When planted early, cuttings sprout, establish well, and receive sufficient moisture; plants withstand diseases and pests later in the season.

Planting depth. Regulate planting depth according to prevailing environmental conditions. Shallow planting at low soil moisture results in poor establishment and low yield. Therefore, in dry, sandy soils, plant cassava cuttings deep; in moist and heavy soils, plant shallow.

Remember that deep planting makes harvesting difficult and increases production costs. However, deep planting is advisable in areas prone to termite attacks.

Plant density. Plant density depends on

soil and climate

variety

soil fertility

cultural practices

end use of tuberous roots

Optimum plant density varies from upland to inland valleys and depends on whether cassava is an intercrop or a monocrop. Denser planting is practised in inland valleys with distance between cassava plants ranging from 50 to 100 cm.

In traditional farming, cassava often grows together with yams, maize, groundnut, banana, and melon. The distance between cassava plants in the upland and depending on the type of intercrop ranges from 100 to 400 cm. In a monocrop, space cassava 80-100 cm within and between rows. Although no universal recommendation exists, in Africa, a plant density of 10 000-15 000 plants/ha gives a good crop.

FERTILIZER APPLICATION

Cultivation of cassava is widespread due to its ability to grow in poor soils. Cassava has an extensive root system and uses plant nutrients which are not easily accessible to other crops. In traditional farming, without fertilizers, farmers can obtain yields of 5-6 t/ha on soils that would not support other crops.

However, for good growth and yields, cassava requires friable, light textured and well-drained soils containing sufficient moisture and a balanced amount of plant nutrients. Under such conditions, yields of 40-60 t/ha are possible.

Like all rapidly growing, carbohydrate-producing plants, cassava impoverishes the soil rapidly, unless nutrients are replaced. Depending on the soil fertility, amounts of nutrients removed in a monocrop and an intercrop differ (Table 2).

Table 2. Nutrients removed (kg/ha) by cassava and sweet potato after 4 months of monocropping and intercropping in a low-fertility soil (Kapinga et al. 1995).

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Cropping system

N

P

K

---

Monocrop
cassava	82	6	80
sweet potato	104	6	94
Intercrop (cassava + sweet potato)
cassava	56	4	45
sweet potato	100	6	60

---

Nitrogen. Nitrogen is readily available to plant roots in the form of nitrate nitrogen. Because nitrogen is easily leached into lower layers of the soil, consider postponing application until plants are well developed. Symptoms of nitrogen deficiency are

stunted growth

narrow, pale green leaves, with yellow (chlorotic) discoloration starting at leaf tips and margins

premature dropping of leaves

Sufficient nitrogen to develop foliage is necessary for the development of tuberous roots. Excessive application of nitrogen without the simultaneous application of potassium and phosphate may promote leaf and stem growth without increasing yield. Yield may even be reduced.

Phosphorus. Cassava requires modest amounts of phosphorus for the root system. Response to phosphorus application is low. Symptoms of phosphorus deficiency are stunted growth and violet discoloration of leaves.

Potassium. Cassava removes large quantities of potassium. Symptoms of potassium deficiency are

stunted growth

dark leaf color, which gradually becomes pale

dry, brown spots on tips and margins of the leaves

necrosis on the margins of leaves

Potassium deficiency not only affects yield and content, but also root quality.

HARVESTING

Harvest cassava as soon as tuberous roots have accumulated sufficient amount of starch, but not too late, when tuberous roots become woody or fibrous. Early-maturing varieties are ready for harvesting at 7 months, while late-maturing varieties are ready 12 months after planting.

Optimum time for harvesting cassava varies according to

time of planting

variety

climatic and soil factors

market conditions

Most cassava varieties attain optimum weight about 18 months after planting when starch accumulation is highest (Figure 5 and Figure 6).

Farmers do not usually harvest all the plants on a plot at the same time, because cassava remains in good condition for only a few days after harvest. Farmers harvest only the quantity required for immediate use.

In traditional farming, farmers harvest manually. Farmers cut the stems a few centimeters above the ground with a machete, then loosen the soil around the tuberous roots, and pull the stub of the stem to lift out the root.

Mechanical harvesters are available to uproot tuberous roots, which are then picked by hand.

Harvesting is easier when the soil is moist. Harvesting is also easier if planting is on ridges or in beds and in loose or sandy soils, rather than on flat ground and in clay or heavy soils.

SUGGESTIONS FOR TRAINERS

If you use this Research Guide in training

Generally

Distribute handouts (including this Research Guide) to trainees one or several days before your presentation, or distribute them at the end of the presentation.

Do not distribute handouts at the beginning of a presentation, otherwise trainees will read instead of listening to you.

Ask trainees not to take notes, but to pay full attention to the training activity. Assure them that your handouts (and this Research Guide) contain all relevant information.

Keep your training activities practical. Reduce theory to the minimum that is necessary to understand the practical exercises.

Use the questions on page 4 (or a selection of questions) for examinations (quizzes, periodical tests, and so on). Allow consultation of handouts and books during examinations.

Promote interaction of trainees. Allow questions, but do not deviate from the subject.

Respect the time allotted.

Specifically

Discuss with trainees about experiences and problems of agronomic managament of cassava (10 minutes).

Present and discuss the content of this Research Guide, using the study materials listed on page 3 (45 minutes).

Have plant and tuber samples, stem cuttings, weed specimens, herbicides, and so on available for each trainee.

You may photocopy the illustrations and tables of the Research Guide on transparencies for projection with an overhead projector.

Conduct the practicals suggested on page 3 in groups of 3-4 trainees per group (2 hours). Make sure that each trainee has the opportunity to practice. Have resource persons available for each group and practical.

Organize your practicals and demonstrations well. Keep trainees busy. Prevent trainees from scattering around the field.