5. Crop nutrition
Combining ecosystem processes with judicious use of mineral fertilizer forms the basis of a sustainable crop nutrition system that produces more while using fewer external inputs.
Alley cropping with leguminous trees can boost yields
Cassava is highly tolerant to acid soils, and has formed a symbiotic association with soil fungi that help its roots take up phosphorus and micronutrients. Since most of the absorbed nutrients are found in the stems and leaves, returning them to the soil helps maintain soil fertility for the next crop.
Its ability to produce reasonable yields on poor soils has given rise to the belief that cassava does not require, nor even respond to, mineral fertilizer.
The results of extensive trials reviewed by FAO have shown, instead, that many cassava varieties benefit from fertilization. Cassava’s need for fertilizer is increasing, as traditional means of maintaining soil fertility are abandoned under more intensive production systems.
Cassava yields could be increased markedly if farmers had access to mineral fertilizer at a reasonable price. In the Democratic Republic of the Congo, the use of improved varieties and mineral fertilizer, led to increases in cassava root yields of 30 to 160 percent.
Initially, cassava should be fertilized with about equal amounts of nitrogen, phosphorus and potassium. However, if the crop is grown continuously for many years, the N-P-K balance will need to be modified to compensate for the removal of nutrients, especially potassium, in the harvest. That can be done using compound fertilizers that are high in K and N, and relatively low in P.
To cut their input costs, farmers should reduce volatilization of nitrogen and losses of nutrients to runoff and erosion by always covering the applied fertilizers with soil. The supply of nitrogen fertilizer can also be optimized with urea compressed into supergranules or urea prills coated with cake made from neem seed oil. Both technologies slow considerably the nitrification of urea, reducing losses to the air and to surface water runoff.
While mineral fertilizer can help to boost yields, alone they cannot sustain crop production in the long-term on degraded land. Farmers can maintain and improve soil quality and health using a number of other “Save and Grow” measures:
- Intercropping with grain legumes make some nitrogen available to the cassava crop. In Nigeria, after two years of cassava-soybean intercropping, incorporation of soybean residues led to yield increases of 10 to 23 percent. Research at two locations in the Democratic Republic of the Congo found that planting four rows of groundnuts between widely spaced rows of cassava also boosted root yields.
- Alley cropping with fast-growing leguminous trees may also be an effective means of improving soil fertility. In Viet Nam, alley cropping with two leguminous tree species, Leucaena leucocephala and Gliricidia sepium, had a marked and consistent long-term beneficial effect on cassava, both when it was fertilized and when it was not fertilized.
- Green manuring – i.e. mulching legume crop residues prior to planting cassava – also improves soil fertility. Effective green manures include cowpeas, groundnuts, pigeon peas and velvet beans. In Colombia, mulching native legumes led to yield increases similar to those obtained with mineral fertilizer. In the Democratic Republic of the Congo, incorporating into the soil 2.5 tonnes per ha of dry matter of the wildflower Tithonia diversifolia produced a higher cost-benefit ratio than use of mineral fertilizer.
- Animal manure and compost are good sources of organic matter, which improves soil structure, enhances water holding and cation exchange capacity, supplies micronutrients, and promotes the below-ground activity of earthworms, bacteria and fungi. In trials in Indonesia, a combination of 5 tonnes of compost with judicious use of mineral fertilizer produced higher yields than fertilizer alone.
Control of soil erosion is essential for sustainable soil fertility management. Growing cassava tends to cause more soil losses to erosion than most other crops, especially where farmers do not use cover crops or mulches to protect the soil.
“Save and Grow” practices reduce runoff and erosion significantly. One option is minimum or zero tillage, which slows the decomposition of organic matter and maintains soil aggregate stability and internal drainage. If the land is prepared using conventional tillage, ploughing and ridging on slopes needs to be done along the contour, and contours should be planted with noncompeting hedgerows of grasses or shrub- or tree-legumes to slow water runoff and trap eroded sediments.
Research in Colombia and in several Asian countries has shown that all erosion control measures were enhanced by applying mineral fertilizer, which leads to faster soil coverage by the plant canopy.
Save and Grow: Cassava (FAO, 2013) can be purchased from firstname.lastname@example.org