Debates on soils and their management have too often been unnecessarily polarised between promoters of ‘organic’, ‘sustainable’ or ‘agroecological’ agriculture and those who argue that only large supplies of mineral fertiliser are the answer. These are often pitched in ideological terms, with little reference to technical understandings of soils. What can we learn from the decades of technical research on soils in Africa about what makes sense, where and for whom?
Experience across Africa demonstrates that a ‘one size fits all’ solution is inappropriate. An integrated approach to soils management is required, mixing different inputs in different amounts for different places. Deriving from extensive research we have learned that:
- Radical technological solutions to soil nutrient problems, such as through genetic modification for increasing nitrogen fixation, are unlikely given the complexity of the plant genetic/physiological processes involved, except through boosting nutrient utilisation at the margins. Similar gains may be realised by much simpler techniques, such as micro-dosing (see below).
- Fallowing remains an important strategy for long-term soil restoration in some places where land pressures are not intense. Improved fallows, using legumes and trees have been shown to have positive impacts. These approaches however take time and require extensive land areas.
- Conservation tillage approaches can work well, but reduce the availability of crop residues, often a critical source of fodder in mixed crop-livestock systems. They may be too labour intensive to apply beyond a small garden area. Herbicide based no-till systems developed for large-scale farm+s are usually not appropriate in African farming smallholder farming systems.
- While essential, there are distinct limits to biological soil fertility options, particularly in already nutrient-poor soils. Rotation, manuring, composting and other ‘sustainable agriculture’ and ‘low external input’ techniques are valuable, but often require considerable labour and skill inputs, as well as large volumes of biomass.
- Inorganic fertilizer use is low across Africa, averaging around 9 kg/ha (outside South Africa) according to the FAO. It is highest in southern Africa and lowest in the Sahel and Central Africa. Constraints to fertilizer use include: high prices, high import tariffs, market power of few suppliers, poor supply infrastructure, inappropriate bag sizes, inappropriate blend/mixes, poor labelling, adulteration, lack of enforceable regulatory systems, low rainfall, low agronomic efficiency.
- African soils are highly variable – they respond to inputs in radically different ways. Crops on poor sandy soils with low clay/soil organic matter content, for example, respond poorly to mineral fertilizer applications. This means that fertilizer focused programmes are inappropriate in large areas of the continent, unless complementary biological measures are taken.
- Home fields, gardens and old settlement sites respond better to mineral fertilizers, as soil organic matter has built up over time. Distinct variations in input responsiveness can be seen across and between farms. Application of inorganic fertilizer makes sense in some farms – and parts of farms – but not in others.
- Micro-nutrient deficiencies (e.g. Zn) may be as important as N, P, K and S. Getting the right composition, based on local soil testing and blend management, may result in major increases in production.
- Increasing the agronomic efficiency (i.e. the marginal increase in production per unit of input) of inorganic fertilizer use requires a) soil moisture, b) organic matter/clay fraction, c) efficient application. Measures to deal with water control and soil structure/organic content, take time and long-term investment. Efficient application can be enhanced through ‘micro-dosing’ – applying small amounts to plants in ways that maximises nutrient uptake.
A critical lesson from all this work is that a highly context-specific approach is required that takes into account the fertility status of the soil, the availability of organic inputs and the ability to access and pay for mineral fertilizers. Making soil fertilisation pay also depends on output markets and the value of farm products. This varies enormously across Africa, within regions and even within villages and fields.
As discussed in the opening blog in this series, simple diagnoses based on generalised country or region-wide estimates of ‘land degradation’ or ‘soil mining’, based on often wildly inconsistent extrapolations from micro-data, are often rather meaningless. While the narrative of a seemingly universal soil depletion may raise the profile of the issue, the prescriptions that sometimes follow are often inappropriate. Simplistic accounting approaches based on ‘nutrient balances’ do not do justice to the complex soil biology and chemistry, and site-specific dynamics, that affect soil fertility problems in different places.
This is not to say that soil nutrient deficits are not a problem. They are; and often are the major constraint to production, particularly in relatively wetter agro-ecosystems in Africa. Identifying where these challenges lie is an important task, but one that requires site-specific diagnostic techniques, with participatory field assessment tools showing much promise.
However, just adding nutrients is not enough. Given resource constraints – of both fertility inputs, labour and cash – maximising the agro-economic efficiency of input use must be a critical objective of any soil fertility management strategy. Without such an approach at the heart of any programme, resources will be wasted and the much needed production boosts will be inadequate.