1. Although cassava is often perceived to be a nutrient-depleting crop, this is normally not the case. Unless yields are very high and/or plant tops are also removed from the field, nutrient removal is considerably lower than that of most other crops, with a possible exception of K. For continuous cassava production on the same land it is recommended to apply chemical fertilizers in the ratio of 2-1-2, 3-1-2 or 2-1-3 of N, P2O5 and K2O, depending on the fertility conditions of the soil. For maximum efficiency, compound fertilizers with the above balance of nutrients should be available to farmers.
2. In very P-deficient soils, cassava responds well to relatively high applications of P in the form of triple- or simple superphosphate, DAP, MAP, fused Mg-phosphate or rock phosphate. With continuous cropping the rates of P application can be reduced over time.
3. In many regions where cassava has been grown for many years, farmers who apply animal manures and/or fertilizers tend to apply too much P and not enough K to obtain high and sustainable yields. This is partly due to the unavailability or high cost of K-fertilizers.
4. In areas of high-pH or calcareous soils, cassava yields are often limited by low availability of soil Zn and Fe. These problems are often incorrectly diagnosed. Simple and cost-effective solutions are either already available or can be developed.
5. In areas where animal manures are available, it is recommended to apply about 5 t/ha of manure together with high-K fertilizers.
6. In areas where chemical fertilizers are not available or are too costly, it is recommended to apply 7-10 t/ha of animal manure, to plant green manures about 3-4 months before cassava, or to rotate cassava on an annual basis with green manures or grain legume crops. In that case, application of ash to the soil is highly recommended. Maintaining soil fertility through long-term fallowing is an option only in sparsely populated areas.
7. Cassava has a reputation to cause erosion when cultivated on slopes. This was found to be a serious problem even on gentle but long slopes where large amounts of runoff can accumulate in natural drainage ways. Cassava should normally not be planted on slopes of more than 15-20%. If planted on steeper slopes, special measures should be taken to control erosion, such as minimum tillage, closer spacing, fertilizer application, intercropping and the planting of contour barriers of grasses (at 1 to 3 meter vertical distance between barriers).
8. While it may be impossible to prevent farmers from planting cassava on steep slopes, as this may be the only or most profitable crop that can be grown, it is recommended to prohibit the use of land preparation by tractor up-and-down the slope. On slopes of more than 15%, contour land preparation by animal traction or hand preparation of individual planting holes is recommended.
9. Mechanical terracing of land for cassava cultivation is seldom economically justified, but the planting of contour hedgerows or leaving 1 m wide contour strips unplowed and unweeded for the formation of natural grass strips, will result in the formation of natural terraces after several years. The prunings of grass hedgerows or grass strips can be mulched on the soil surface between plants. The barriers, the mulch and the naturally formed terraces all contribute to highly effective erosion control.
10. When cassava is grown rather extensively in mountainous areas, by rotating crop cultivation with several years of fallow vegetation, it might be better to recommend the continuous cropping of a small area of the flattest land (using chemical fertilizers and manures to maintain fertility) and leave the rest of the farm in pastures or fallow, or to plant fruit or timber trees. This will eliminate the arduous task of annual slashing and burning, facilitate land preparation, increase yields (by fertilizer application), and reduce erosion and CO2 emisions from burning. It may also reduce the need for P applications as the VA-mycorrhizal population builds up under cassava cultivation. Where possible, cassava should be rotated with other crops or green manures.
1. Farmers should be encouraged to plant a range of cassava cultivars in a particular region or country, so as to mitigate against the devastating effect of a sudden outbreak of diseases or pests. Cassava breeders should maintain a wide genetic variability in germplasm collections, and use materials of varied genetic background in their crossing programs.
2. The introduction of genetic material from Latin America (the center of origin of cassava) to Asia and Africa should be encouraged (with due quarantine precautions), in order to broader the limited genetic base of cassava cultivars in those two continents.
3. Large-scale deforestation to increase cassava growing areas should be prevented to safeguard the native biodiversity in those ecosystems.
4. Collection and conservation of wild Manihot species will allow a more detailed study of their characteristics, with a potential of incorporating certain favorable characters into commercial cassava varieties; it will also safeguard against their possible extinction.
1. Cassava processing is often perceived as consuming large amounts of water and causing serious pollution. But, water consumption is high only in large-scale or highly clustered small- or medium-scale cassava starch factories. In spite of this, water consumption seldom leads to significant depletion of groundwater resources.
2. Pollution, due to the disposal of inadequately treated solid and liquid wastes from cassava processing is a problem mainly when a large number of small- or medium-size processors are clustered together in a relatively small geographical area. These pollution "hotspots" should be identified and groundwater quality and public complaints in the area closely monitored. This is a potential problem area as the number of processors is large, hence difficult to control, and they generally do not have the financial resources and technical know-how to comply with existing pollution control laws.
3. The cassava starch industry should be encouraged to reduce their water consumption and waste water production by the installation of efficient water recycling systems.
4. Waste water should be either retained within the factory's premises or adequately treated before release to outside water sources. TNPCB tolerance limits for discharge of effluents into surface waters are 30 mg/l for BOD, 250 mg/l for COD and 0.2 mg/l for cyanide (Appendix 3). An alternative is to set standards as a proportion of total organic loading before treatment, e.g. in Brazil the BOD in waste water must be reduced by 80% before release to the environment.
5. Factory owners should keep records of water use and waste output, so as to facilitate environmental monitoring. The impact on biological systems should be closely observed.
6. Incentives need to be available for the processor to use additional technology. If there are no economic advantages for the use of a technology, it will never be adopted.
7. Regulations should be imposed to ensure that processors have sufficient land for disposal of waste, e.g. in India a starch factory must have access to 2 ha of land for disposal of waste water at an average rate of 86 m3/ha/day.
8. Processing facilities should be sited at some distance from key waterways, e.g. in India processors are banned from establishing a factory within one kilometer of important streams, rivers or municipal drinking water sources.
9. Full cost recovery for water could be imposed on the larger processors, charging water at the going rate even if abstracted from rivers or wells.
10. The formulation and enforcement of sensible legislation is the key to success. Implementation should be phased in slowly against agreed time scales. This will give the processor time to adjust to the change and new equipment.
11. Enforcing and monitoring of regulatory standards is critical for success and suitable systems for this purpose should be in place.
