Development projects of the International Fund for Agricultural Development (IFAD) often target areas where populations are extremely poor and suffer from seasonal food shortages. In these areas soils are generally infertile and long droughts are a common occurrence, causing crop failure and famine. Under those conditions, cassava (Manihot esculenta Crantz) is often a food of last resort, as the crop is very tolerant of poor soils and drought. Cassava is often grown in fragile environments, such as on slopes and in the forest margins. It has the reputation of depleting soil nutrient reserves and causing severe erosion when planted on slopes.
A large proportion of harvested cassava roots is processed into food, animal feed and various industrial products. The processing of some of these products requires large amounts of water and produces equally large amounts of waste water. This water may be high in organic constituents and cyanide, which can pollute the ground water or the lakes, streams or rivers into which it is released. Other waste products resulting from cassava processing are often inadequately disposed of, causing a foul smell and unattractive sight, and giving the cassava processing industry a reputation of polluting the environment.
As part of an intensive effort to develop a Global Cassava Research and Development Strategy, IFAD commissioned CIAT, in collaboration with other institutions, to review the existing information and make a comprehensive assessment of the effect of smallholder cassava production and processing on the environment and biodiversity.
Most cassava is produced by smallholder farmers living in marginal and fragile environments in Africa, Asia and Latin America. The crop is usually grown on very acid and infertile soils (often prone to erosion), where other crops would not grow well. But few farmers apply animal manures or fertilizers to their cassava crop, and those who do usually use only low rates. This study reviews available data in the literature on nutrient absorption, distribution and removal by cassava when either the roots alone or whole plants are harvested and removed from the field.
Nutrient removal is a function of dry matter production and yield, but as yields increase, so does the nutrient concentration of the plant tissues, resulting in a non-linear relationship between nutrient removal and root yields. Thus, at high yields, nutrient removal is indeed quite high, but removal of nitrogen (N) and phosphorus (P) is still lower than, and potassium (K) removal is similar to that of other crops. At low root yields (below 15 t/ha) cassava removes much less N, P and K than most other crops. This is because most nutrients (except K) are mainly present in leaves (intact and fallen) and stems, and if these are returned to the soil, nutrient removal is minimal. However, in areas where leaves and stems are also utilized and removed from the field, nutrient removal can double or triple, depending on each particular nutrient. In this case nutrient depletion can become of serious concern. Thus, returning leaves and stems to the soil is an essential first step in preventing nutrient depletion and maintaining soil fertility.
As for any other crop, to maintain soil productivity and sustain high yields, nutrients removed in the harvested products of cassava should be replaced in the form of chemical fertilizers, animal manures, ash or compost. In most soils cassava does not respond to high rates of P application because: 1) P removal in the root harvest is very low (much lower than that of N and K); and 2) the fibrous roots of cassava become naturally infected with mycorrhizal fungi present in all natural soils. The symbiosis between cassava and mycorrhiza enables the crop to absorb P even from soils with very low levels of available P. This, and cassava's tolerance to high levels of aluminum (Al), allow the crop to grow well in acid and low-P soils. Moreover, in soils that are low in nutrients, cassava reduces its growth rate and nutrient absorption, producing low but stable yields without seriously depleting the soil's nutrient supply.
For production of a moderate fresh root yield of 15 t/ha it is recommended to apply about 80 kg of N, 10-20 kg P2O5 and 50 kg K2O/ha. To maintain a high yield of 30 t/ha, annual application of 150 kg N, 20-30 kg P2O5 and 150 kg K2O/ha may be required. Compound fertilizers that are high in N and K but low in P are the most suitable for cassava. However, in some soils that are extremely deficient in available P (<2mg P/kg dry soil), mostly found in Brazil and Colombia, the crop responds mainly to the application of P. Initial applications of 100-200 kg P2O5/ha may be necessary to obtain maximum yields, but with continuous cropping on the same land these annual applications can be reduced in a few years to less than 50 kg P2O5/ha.
Where animal manures are available it is recommended to apply about 5 t/ha of manure together with chemical fertilizers high in K. Where chemical fertilizers are not available or too costly, it is recommended to apply 7-10 t/ha of manure in combination with wood ash. If neither manures or fertilizers are readily available, soil fertility can be maintained by planting a green manure crop and mulching or incorporating the green manure before planting cassava; alternatively, cassava can be rotated annually with green manures or grain legumes. Intercropping cassava with grain legumes may also improve soil fertility, especially if the legumes are well fertilized and the crop residues are returned to the soil. However, high yields cannot be achieved or maintained for very long if no outside nutrient sources (fertilizers or manures) are applied to compensate for nutrient removal in the harvested products, as well as losses by leaching, volatilization or erosion.
Cassava has the reputation of causing serious erosion when grown on slopes. Some people argue that this reputation is undeserved, since cassava is often grown on already-eroded soils where few other crops can survive and be productive. Nonetheless, a review of the literature indicates that production of cassava on slopes generally causes more erosion on an annual basis than other crops grown under the same circumstances. Cassava, together with castor bean, common bean (Phaseolus vulgaris), upland rice and cotton, seems to cause considerably more erosion than maize, peanut, sugarcane, pineapple or sweetpotato. This is mainly due to the fact that cassava needs to be planted at a relatively wide spacing. Initial growth and canopy formation are slow, leaving soil exposed to the direct impact of rainfall during 3-4 months after planting. On the other hand, once the crop canopy is closed, erosion is usually minimal during the remainder of the crop cycle.
Once the topsoil is eroded away, it is very difficult to restore the soil's productivity. For that reason erosion should be minimized. Many experiments have been conducted to develop effective ways to reduce erosion in cassava fields. Good agronomic practices that increase yields, such as adequate fertilization, closer plant spacing and planting on contour ridges, are very effective in reducing erosion. Intercropping, reduced tillage and planting contour hedgerows of grasses, such as vetiver or lemon grass, are very effective in reducing erosion and may also increase cassava yield or total income. These practices, used alone or in combination, can reduce erosion by 50-90%. Thus, when properly managed, cassava production on slopes does not necessarily cause serious erosion.
Most erosion-control practices will require some additional inputs of capital (e.g. fertilizers or seed of intercrops), labor (e.g. establishing and maintaining contour hedgerows) or may reduce cassava yields by competition (e.g. intercrops and hedgerows) or by reducing the area available for cropping (e.g. hedgerows). Moreover, some of these practices may not be feasible or do not fit well in the current production practices (e.g. fertilizer use in much of Africa or minimum tillage in Thailand). The choice of most suitable practices is very site-specific and often involves trade-offs between advantages and disadvantages. The most appropriate practices depend on the biophysical and socio-economic conditions of the region, and can best be determined by the farmers themselves. For farmers to adopt better soil-conserving practices it is essential that they are aware that soil erosion is a problem, and that they themselves are involved in the development and testing of better production practices that reduce erosion. The adoption of more sustainable cassava production practices is limited less by a lack of basic knowledge than by a lack of understanding of farmers' needs and limitations. The development and dissemination of sustainable practices require a holistic and farmer participatory approach. Governments can mandate certain regulations about land use and management of sloping land, but these can seldom be enforced, as poor farmers living in these areas have few other alternatives.
There is no documented evidence that cassava production has had a significant effect on the biodiversity of other species. But it has sometimes led to serious deforestation, such as in the northeast of Thailand, which has probably contributed to a loss of biodiversity. There is also no clear evidence that cassava production in Brazil or Mexico, the two centers of origin of Manihot sp., has contributed to the loss of biodiversity in that genus. However, continuous monocropping of cassava, accompanied by annual burning of the sparse native vegetation in northeastern Brazil is threatening the survival of seven native wild Manihot species. It is recommended to collect and conserve these species ex-sitio. It is also recommended to collect and conserve six other species which are closely related to Manihot esculenta, and which may be used in the future for interspecific breeding to incorporate favorable characteristics not presently found in M. esculenta.
Most forms of cassava processing produce large amounts of waste, the type and composition of which are governed by the processing method and sophistication of the technology used. Cassava processing is generally considered to contribute significantly to environmental pollution and to depletion of water resources, due to the strong and unpleasant odor and the visual display of waste products. Some forms, especially starch extraction, also require large volumes of water.
A review of the literature suggests that in most areas water depletion is minimized by the adoption of processing technologies suitable for the water resources available. This balance is maintained as long as traditional processing methods are used, or volume of processed product is not increased substantially. However, technology changes can increase water demand. The recent adoption of more automated processing technologies in parts of Africa is especially noteworthy. This will allow the rapid processing of larger volumes of roots, which could create a serious strain on limited water resources. In other areas, even those with a large concentration of cassava processors (such as Tamil Nadu, India), the impact on water supply is minimal compared to that of other users, such as agriculture or domestic. At a site-specific level, starch processing may exacerbate an already existing water shortage problem, but is unlikely to be the prime cause.
Many scales of cassava processing exist in the world; each will affect the environment differently. Large-scale processing, if left unchecked, will have the largest impact on the environment. However, such processors usually are closely regulated by governments - given their size and relatively few number, this is easily achieved. The larger enterprises also have resources, financial and technological, that are not available to small-scale processors to deal with their waste. Small-scale processors, who generate only small amounts of waste individually, are usually clustered together, hence magnifying their impact.
This study was not able to find conclusive evidence that cassava processing contaminated groundwater supply. It is possible that groundwater supply near cassava processors is contaminated, but it is often difficult to disaggregate cassava processing from other forms of waste-generating activities. The proportion of contaminated sites in cassava processing areas is likely to be small, with most wells and bore holes remaining unaffected. However, studies are few and other processing sites may have ground geology that is not capable of protecting the groundwater supply from being polluted.
Surface water presents a different picture. Given sufficient volume of discharge, either from a large processor or from a high concentration of small processors, eutrophication of slow moving water systems (ditches, lakes, etc.) can occur. This is especially problematic during the dry season.
Cassava processing may also produce solid waste, either in the form of peels or fibrous by-product (pulp). The solid waste does not seem to be a problem; usually it is sold soon after its generation or it is stored before sale. Potential problems occur when the waste is improperly stored and left exposed to the rain. In this situation, leachates can be generated that filter through to the groundwater supply. However, reports of this are few.
Cyanide is liberated during cassava processing; this can be a problem, especially in processes that create large amounts of "squeezed juice". Care should be exercised that cyanide-containing waste is either diluted or stored in such a manner that the cyanide concentration is reduced. This is usually the case, even if the waste is stored for a short time.
Cassava production can have some negative effect on soil fertility through crop removal of nutrients, but it is likely to have a more serious and long-lasting effect on the environment as a result of erosion. At current yield levels, soil nutrient depletion by cassava is generally far less than by that of other crops. But due to the low value of cassava products, application of manures and chemical fertilizers may not be economically justified, or farmers may not be able to afford the purchase of fertilizer. Once the nutrient supply in the soil is depleted this can easily be corrected by application of fertilizers. Cassava production, however, does seem to cause serious erosion when the crop is grown on slopes. Soil degradation due to erosion is not easily corrected. Farmers should be encouraged and materially supported to plant cassava on less steep land, and to use appropriate measures to reduce erosion. With these practices, erosion can be reduced by 50 to 90%.
Cassava production does not seem to have had broad effects on biodiversity, either of other Manihot species or of those of other genera. There are, however, localized situations that merit attention, as well as the need for plans to minimize future genetic erosion.
Cassava processing can have negative, mainly site-specific, effects on the environment, by producing unpleasant odors and an unsightly display of waste. However, the long-term and broad-based impact on the environment is generally minimal and can be corrected by proper waste treatment, with technologies which are either presently available or under development.
Recommendations are made for planners and policymakers, both of a technical and general nature, that will, (1) help producers reduce erosion and maintain soil productivity, (2) counter deforestation leading to a loss of biodiversity, and (3) establish guidelines and regulations for cassava processors to make more efficient use of water resources, to reduce the negative impact of processing waste on the environment, and support research and development on value-adding of by-products.