2. Farming systems
Many smallholder cassava growers already practise three key “Save and Grow” recommendations: reduced or zero tillage, protecting the soil surface with organic cover, and crop diversification.
FAO’s “Save and Grow” farming model seeks to limit mechanical disturbance of the soil by minimizing the ploughing, harrowing or hoeing of land. Continuous conventional tillage with tractor-mounted ploughs, harrows and rototillers buries the soil’s protective cover, kills soil biota, causes the rapid decomposition of organic matter, and degrades soil structure by pulverizing soil aggregates.
Zero tillage with mulching produced the highest yields
Crop yields are a function not of tillage, but of soil conditions. Cassava stakes can be planted, and can produce good yields, in soil that has not been tilled, provided that the soil is healthy, well-structured and free of compaction. In degraded soils, growing cassava without tillage may produce lower yields in the initial years. In the longer term, however – by reducing mineralization, erosion and water loss, helping to build up organic matter and maintaining soil aggregate stability and internal drainage – eliminating tillage promotes root functioning to the maximum possible extent. Once soil health is restored, untilled land can produce high yields and do so at a lower cost – to both the farmer and the farming system’s natural resource base.
Cassava growers should be encouraged to adopt minimum tillage and, ideally, zero tillage, especially on well-aggregated, friable soils with adequate levels of organic matter. Even where conservation tillage produces lower yields, it offers farmers economic advantages: reduced spending on the fuel and equipment needed for conventional tillage, and the opportunity to produce cassava more intensively and sustainably, without the need for high levels of external inputs.
Reduced or zero tillage will also be important as an alternative to conventional tillage in cassava-growing areas affected by climate change. Where rainfall is reduced, it will help to conserve soil moisture; where rainfall increases, it will help reduce soil erosion and improve soil structure, allowing better internal drainage.
Along with reduced or zero tillage, FAO recommends maintaining a protective organic cover on the soil, using crop residues and mulches, in order to protect the surface, reduce runoff and erosion, and suppress weeds.
Ground cover is especially important in cassava production: because the initial growth of cassava is slow, the soil is exposed to the direct impact of rain during the first 2 to 3 months of its growth cycle, and the wide spacing between plants favours the emergence of weeds. Fast-growing legumes smother many unwanted weeds that normally proliferate during cassava establishment and after the cassava harvest, thus providing weed control that is less demanding than manual weeding and less expensive than spraying with herbicides.
Mulch cover also serves as an insulating layer that reduces diurnal temperature variations and water evaporation. It increases the soil organic matter content and provides a favourable environment for soil micro-organisms and below-ground fauna. By creating physical soil conditions – reduced soil temperatures, higher levels of moisture, increased water infiltration capacity and lower evaporation – mulching favours higher yields.
In “Save and Grow”, farmers are encouraged to cultivate a wider range of plant species in associations, sequences and rotations that may include trees, shrubs and pastures. Mixed cropping diversifies production, which helps farmers to reduce risk, respond to changes in market demand and adapt to external shocks, including climate change. Rotating or associating nutrient-demanding crops with soil-enriching legumes, and shallow-rooting crops with deep-rooting ones, maintains soil fertility and crop productivity and interrupts the transmission of crop-specific pests and diseases.
Smallholder cassava farmers in many parts of the tropics practise intercropping with early maturing crops, such as maize, upland rice and various grain legumes. Intercropping protects the soil from the direct impact of rain, reduces soil erosion, and limits weed growth. It also produces crops that can be harvested at different times during the year, increases total net income per unit area of land, and reduces the risk of total crop failure. Growing cassava with short-duration legumes also supplies both carbohydrates and protein, the foundation of a healthy diet for the farming household.
In marginal areas where cassava is the main crop, it can be rotated with grain legumes, such as groundnuts, mungbeans, cowpeas and soybeans, which fix atmospheric nitrogen and make it available to the successive cassava crop. Sequential cropping of cassava and cowpeas improves soil fertility to the point where applications of mineral fertilizer can be reduced, with no loss of yield.
Save and Grow: Cassava (FAO, 2013) can be purchased from firstname.lastname@example.org