Most of our ideas about soils ignore the millions of years since mankind started farming. What happened during the 99.9% of soil’s history contains very important lessons. Let us celebrate the International Year of Soils by looking at what that history can tell us – and build on those lessons for the future.
Soil restoration can provide the basis for the sustainable, long-term development of an entire farm.
In the tropical world, fallowing kept farmers’ soils fertile for thousands of years by providing 70 to 95% of their soil organic matter. But today, since most smallholder farmers possess less than 2 hectares of land, in a large part because of population growth, fallowing is dying. As a result, the developing world is experiencing a severe soil organic matter crisis.
The soil organic matter crisis is critical because soils are being so rapidly damaged and depleted, becoming the primary limiting factor for the world’s smallholder farmers. If a farmer adopts a new cassava variety, it may improve his or her cassava production, but it will do almost nothing for the farmer’s maize, bean, vegetable or animal production. But if the farmer successfully improves her or his soil, it will have a major impact on everything else, too. ‘Foundational technologies’ such as soil restoration, can therefore provide the basis for the sustainable, long-term development of an entire farm.
Looking at soil history will debunk three commonly held myths about soil restoration. The first myth is that productive soils will inevitably deteriorate over time. For instance, in all long-term experiments carried out in Africa, even those including chemical fertilizer, decreasing fertility was found. This loss of fertility correlated with decreasing soil organic matter levels and the resulting availability of nutrients. But humid tropical forests the world over, by maintaining the soil organic matter content, have maintained impressively high levels of biomass productivity for millions of years, with no fertilizers and often on very infertile soils.
The second common belief to go out the window is that soils need to be ploughed to stay friable and productive. Tropical forest soils were never ploughed, yet after millions of years they are far more friable and naturally productive than most agricultural soils. In fact, family farmers who convert forest land rarely plough it during the first year. Doing so would be ‘like ploughing the sea’, as Simón Bolívar once remarked. Rarely do we need to plough land unless we have previously degraded it.
The third myth is that good modern farmers must use monocrops. But tropical forests maintain biodiversity and thereby increase soil quality and productivity. And the oft-repeated claim that phosphorus will limit productivity because of the phosphorus lost in grain harvests is based on seriously faulty nutrient assessment studies. Furthermore, crops grown with a biodiverse mulch will feed directly from the mulch, just as tropical forests do. In this situation, most phosphorus in annual crops spends 1-8 months in the mulch before being taken up by the crop, and after less than a year, will once again return to the mulch. In contrast, only 10% of chemical phosphorus applied to soils is used the first year, about 5% the second year, and less each year thereafter. Therefore, with a biodiverse mulch, each atom of phosphorus can produce about 15 times more biomass than it can from fertilizer.
A movement that transformed agriculture
Farmers using green manure/cover crops can often produce a centimetre of topsoil every 3 to 4 years.
Interestingly, and not at all by chance, three of these lessons from history coincide with the three principles of the Conservation Agriculture movement that began in Brazil in the early 1980s. These are:
1. plough the soil as little as possible,
2. keep the soil covered, and
3. maintain biodiversity.
In 35 years, this movement has transformed the agriculture of 3 million farmers on 30 million hectares in Brazil and Paraguay, and has spread to some 30 other nations. Farmers’ yields have doubled or tripled, reaching up to 8 tonnes per hectare of maize. Between 1992 and 2012, the same 1 litre of diesel came to produce seven times more grain. Over a 22- year period, Conservation Agriculture has resulted in soils with higher levels of organic matter and available nitrogen, phosphorus, potassium, calcium and magnesium, and with lower acidity. In the meantime, the per-hectare use of nitrogenous chemical fertilizer has fallen. In long term experiments, Conservation Agriculture produced a 64% increase in organic carbon in the top 10 cm of the soil. Needless to say, the world desperately needs more such successes.
Conservation Agriculture’s increasing yields also show that we do not need to resort to subsidised chemical fertilizer – subsidies that are tremendously expensive. The current President of Zambia told me that with what the government spent on fertilizer subsidies in the last few years, they could have built a school in every village across the country. Furthermore, cheap fertilizer reduces the incentives of farmers to produce the biomass that will improve their soil in the long-term. That is, all this wasted money not only cannot solve the basic, underlying problem of soil depletion, instead, it makes it worse.
Legumes as green manure/cover crops
The three principles of Conservation Agriculture
Plough the soil as little as possible. This is also known as no-till, zero tillage or minimum tillage. This practice maintains soil structure, reduces damage to soil organisms, reduces soil losses to erosion, reduces loss of organic matter and nitrogen and saves labor and expenses. On the other hand, weed control will suffer without ploughing, and farmers using animal traction may need to start using new equipment.
Keep the soil covered. Mulching prevents erosion, provides a constant, well-balanced source of nutrients, protects the soil from the hot sun, greatly reduces soil moisture losses, and helps control weeds. The main problem in maintaining year-round soil cover is that crop residues are seldom sufficient.
Maintain biodiversity and use green manure/cover crops. In Conservation Agriculture, farmers use rotations and intercropping to maintain biodiversity. These practices reduce the risk of pests and diseases, support soil micro-organisms and use water and nutrients in the whole soil profile more effectively. An essential component of such a system are green manure/cover crops. These are defined as any plant, whether a tree, bush, vine or crawler, that fertilizes the soil or controls weeds. They include multi-purpose grain legumes and can often provide high-protein food for sale or consumption. Unlike traditional green manures, they are rarely cut down in the flowering stage and are rarely ploughed into the soil. They can thereby control the increased weed problem caused by lack of tillage and produce plenty of in situ biomass to keep the soil covered.
Green manure/cover crops are crucial. It is often said that nature can only produce a few centimetres of topsoil in 100 years, but experience in country after country has shown that farmers using green manure/cover crops can produce a centimetre of topsoil every three to four years.
In fact, when using edible legume species, the value of the grain often exceeds the costs of production, so the net cost of restoring soil fertility over decades is actually negative. Chemical fertilizer will never compete with that price! But fertilizer can supplement green manure/cover crops. When smallholder soils reach a productivity of about 3 tonnes per hectare, fertilizers can be profitably used. At this level of soil productivity, the fertilizer will produce a greater yield response with lower risks.
Experience around the world shows that it takes about 20 to 25 tonnes per hectare per year (green weight) of leguminous biomass to maintain soil fertility over time. Never in 40 years have I heard of a smallholder farmer using 20 tonnes of fresh compost or animal manure each year. Most smallholder farmers don’t have enough animals to produce this amount of manure, and composting requires too much labour to be cost effective for most subsistence crops. But dozens of legumes can produce double or triple this amount of biomass.
Runner beans (Phaseolus coccineus) and mucuna (Mucuna spp.) can easily produce 70 tonnes per hectare per year, lablab beans (Dolichos lablab) and jackbeans (Canavalia ensiformis) 50 to 60 tonnes per hectare per year, and pigeon peas (Cajanus cajan), densely planted, can produce about 30 tonnes.
Some farmers are adding trees as ‘dispersed shade’ to their Conservation Agriculture. The trees’ light shade reduces the excessive midday heat that decreases crop productivity in the lowland tropics. Trees are also extremely drought resistant because of their deep root systems; the fertilizing leaves are out of reach of free-grazing animals; trees preserve moisture in the soil through lowered soil surface temperatures and reduced wind velocity; and they can provide firewood and fodder. Furthermore, as climate change occurs, farmers can merely cut fewer branches off their trees, so the crops underneath will continue to enjoy optimum ambient temperatures. Two important species are mother of cacao (Gliricidia sepium) and Faidherbia albida.
Interestingly, Conservation Agriculture with trees is ecologically about as close as one can get to producing food in a forest. In 35 years of intensive learning, we’ve travelled right back to where mankind started thousands of years ago.
Roland Bunch () is an independent consultant and the author of Restoring the Soil, A Guide for Using Green Manure/Cover Crops to Improve the Food Security of Smallholder Farmers (Winnipeg: Canadian Foodgrains Bank, 2012).