For further information contact: Reinhardt Howeler

The NF Project also examines ways of maintaining or improving soil productivity. Nutrients lost by crop removal, volatilization, leaching, or erosion need to be replaced by applying fertilizers or organic manures, or through biological N2 fixation. Experiments are being conducted on:

Chemical Fertilizers

Most long-term fertilizer experiments have shown increasing responses to applications of N and K, whereas many short-term on-farm trials show initial response mainly to N. In very general terms, cassava should receive fertilizers with N, P2O5, and K2O at a ratio of either 2:1:2 or 2:1:3. However, optimal fertilizer rates depend on soil fertility, which can vary greatly from field to field. Thus, specific recommendations should be based on results of soil analyses, supplemented with analyses of the youngest, fully expanded, leaf (YFEL) blades taken 3 to 4 months after planting. Optimal fertilizer applications for cassava production in various locations, soils, and systems in Asia are shown in the following table.

Animal Manures

To maintain or improve soil fertility, cassava farmers in many countries apply farmyard manure (FYM), either alone or in combination with chemical fertilizers. Thus, in southern India, farmers may apply 12½ t/ha of manure in combination with N, P, and K fertilizers, whereas, in the Philippines, they apply 10 t/ha of chicken manure, and in North Vietnam, they apply 10 t/ha of pig manure in combination with 80 kg/ha each of N and K2O.

While animal manures may contribute to improving the soil’s physical conditions and are an important source of Ca, Mg, S, and micronutrients, they contain only low and highly variable amounts of N, P, and K. For example, one 50-kg bag of 15-15-15 chemical fertilizer contains roughly the same amount of N, P, and K as 1,000 kg  of wet pig manure. Large applications of manure are probably economical only in areas where manure is locally available. Otherwise, transport and application costs may be higher than the cost of chemical fertilizers. Where manure is available, a combination of 5-10 t/ha with 50-80 kg/ha each of N and K2O is probably adequate to maintain soil fertility and obtain high yields of cassava. However, if the crop’s leaves and stems are also removed from the field, then higher rates of N, P, and K are recommended.

Green Manures and Alley Cropping

Many experiments have been conducted on the use of green manures to maintain soil fertility by either incorporating forage or grain legumes as green manures before planting cassava, or planting shrub legumes in alley cropping systems. Yet, in Asia, these practices have been little adopted in cassava fields because most farmers cannot afford to use their limited land for an unproductive green manure crop. Exceptions are the use of Tephrosia candida as an erosion control barrier-cum-green manure in north Vietnam and of Gliricidia sepium or Leucaena leucocephala as either a plot border-cum-animal feed or green manure in some parts of Java, Indonesia. Most farmers prefer using animal manure or chemical fertilizers.

Lime, Mg, S, and micronutrients

Cassava is extremely tolerant of soil acidity. Thus, in most cassava-growing areas, the crop does not respond to lime applications. High applications of lime can even be detrimental by inducing Zn deficiency in soils with low available Zn content.

In many sandy soils, low in organic matter, cassava has shown symptoms of Mg deficiency, especially when only chemical fertilizers are applied. In that case, applications of 20-40 kg/ha of Mg as band-applied MgSO4.H2O or fused Mg-phosphate can eliminate the symptoms and increase yields. Symptoms of S deficiency have not been observed in cassava in Asia, so applications of this nutrient are unlikely to be necessary.

Symptoms of Fe and Zn deficiency are commonly observed in cassava grown in calcareous soils. Cassava is particularly susceptible to Zn deficiency, which can be controlled by either immersing stakes for 15 min in a solution of 2.0% ZnSO4.7H20, or by spraying plants with a solution of 1-2% ZnSO4.7H2O. No reports exist of significant responses to Fe applications, but foliar sprays or stake immersion  in 2% FeSO4.7H2O may solve Fe deficiencies problems. Large varietal differences in tolerance of Fe and Zn deficiencies have been observed, and changing the variety planted may be more practical than applying micronutrients.