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

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

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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.