Why getting our soil facts straight is key to increasing cassava production in Southeast Asia
This blog is part of the Agriculture and Ecosystems Blog’s month-long series on Restoring Landscapes.
Rumour has it that planting cassava is inherently bad for the soil. The crop is capable of widespread soil erosion, damaging the environment and reducing the productivity of other crops by removing too many nutrients from the soil, so the popular mantra goes. Yet at the same time, cassava can apparently grow well in poor, degraded soils with no or very little fertiliser application at all.
Cassava’s low demand for - and highly efficient use of - water and most nutrients has earned it a reputation as an ideal crop to cultivate on poor soil. But the association between degraded soil and the crop itself seems to have stuck, and the stigma is difficult to shake off.
As with most rumours, you have to dig a bit deeper to get to the roots of the truth.
We can quell the rumour if we can share the information that we as researchers have about how to prevent soil erosion and better manage cassava production systems with the people who need to know. Proper erosion control and soil fertility management is at the heart of sustainable cassava production, and has the potential to boot smallholder livelihoods.
In Southeast Asia, cassava production has increased rapidly and many people want to be part of the boom. Rapid population growth and economic development have driven many people, especially the poorest, to grow cassava on sloping land. But farming on the slopes is usually deterred and sometimes banned – which does not stop farmers from planting cassava on slopes because it grows well, at least for a number of years. Consequently, harvests go unreported, and best soil management practices and adoption of tried and tested sustainable soil management techniques are not as broadly adopted as they should be.
Concerns that cassava exacerbates soil erosion arise from the fact that growing it on steep slopes without intercropping or contour planting will cause erosion. This is because cassava is a large plant that needs to be planted at wide spacing to get high root yields. During the two to three-month period before cassava leaves grow enough to close the crop canopy, the soil between plants is left exposed to direct rainfall, which in turn results in soil run-off and loss. The steeper the slope, the greater the soil loss.
It is important to both protect the soil surface from the direct impact of raindrops and slow down the runoff and soil loss by reducing the length of the slope. The International Center for Tropical Agriculture's (CIAT) research has highlighted great opportunities for mitigating soil erosion in Southeast Asia. Intercropping cassava with fast ground-covering, short-term crops like peanut is recommended because it protects the soil surface, provides a quick harvest and income, and helps control weeds. Mulching with crop residues or grass on the soil surface protects soil from direct raindrop impact, greatly improves water infiltration and can further reduce erosion.
Planting strips of forage grasses like Paspalum atratum or shrubs such as Tephrosia candida along slope contours will act as a barrier, breaking the slope length and preventing the loss of topsoil – the most fertile part of the soil – while inducing the formation of natural terraces to capture moisture, reduce the angle of the slope and increase yields. The forages can also feed animals in integrated crop-livestock smallholdings.
The nutrient myth
As a poor man’s crop, smallholders practicing shifting cultivation often plant cassava as the final species in a sequence of crops on cleared land before allowing the land to return to fallow. They do this for two main reasons.
First, after three to four-year crop sequences with inadequate replacement of nutrients removed during harvests, the soil is usually depleted. Hence, only cassava will produce well on the impoverished soils of the plot. After the cassava, on the other hand, few crops at all will grow adequately. This reinforces the myth that cassava induces soil depletion, when in reality the soil has already been starved of nutrients before the cassava cycle has even started. In contrast to most other crops, cassava is capable of growing and providing some root yield even when soil fertility is low.
The second reason cassava is an ideal final crop in a shifting cultivation sequence is that it does not have a fixed harvest period. It can be stored in the soil for up to two years and harvested when other food has run out, often doubling in yield during that time. If soils are not tilled prior to planting and the crop remains in the ground for two years, the soil erosion hazard from planting on sloping lands is less than from an annual crop like maize that is commonly tilled before planting each year.
Although not a particularly “nutrient hungry” crop when compared to say maize, the nutrients cassava does take up still need to be replaced. Combined soil type-dependent soil fertility management is needed, using both organic and inorganic fertilisers as well as legume intercrops that can enrich the soils through biological nitrogen fixation, and a range of techniques that increase soil organic matter.
All of the above techniques can be combined to increase productivity, restore soil fertility and reduce soil erosion. At the same time, they can contribute additional income, food or livestock fodder for smallholder households.
The facts remain that growing any crops on sloping land requires proper soil management, and whatever you take out of the soil, you have to put back. These are facts of which more development workers, farmers and policy makers need to be aware.
Cassava is a largely ignored crop in terms of research, extension and policy agendas. While there is no doubt that things are changing, there is still concern, particularly at policy level, about perceived environmental damage caused by cultivating cassava - in particular soil degradation - and governments are reticent to promote expansion of the crop.
This lack of incentive inspires less than optimal cultivation and has a negative impact on potential industrial supply. At the same time as educating farmers about soil fertility and erosion management, we need to educate policy makers that, with the right management, growing cassava on slopes is not only possible, but can also be environmentally sustainable and provide a good income for poor smallholder farmers. Indeed, at a time when demand for cassava starch in food processing and biofuel industries looks set to grow - both domestically and in export markets - cassava offers more potential than ever to boost smallholder livelihoods.
To avoid concerns about degradation by cassava production and pollution from cassava processing, this insight must be extended to the private sector. Private companies need to promote sustainable production techniques as they source their processing feedstock. They need to make informed decisions about the appropriate location of processing plants and ensure that the processing industry manages waste in an efficient and environmentally responsible way. Using waste as a source of biogas or animal feed, for example, would be a step in the right direction.
Digging up the truth
Informed decisions about where and how to grow cassava should take into account site-specific information such as soil fertility requirements, the risk of soil degradation, bioclimatic conditions, on-farm food and animal feed needs, as well as market access and demand. Then the myths will be truly debunked and cassava can be recognised as a hardy, flexible crop with promising economic potential for the poor of the region.