Throughout our history as farmers, we have relied on organic matter in the form of burned ashes, animal manure, seashells, and plant residues to maintain soil fertility. And although farming systems have undergone tremendous change over the past 10,000 years, organic matter is still the key to long-term soil fertility. Decomposing organic matter provides all essential nutrients to plants, creates the basis for the biological food web in the soil, improves the structure of soil and its capacity to retain air and water, and increases the retention of nutrients from mineral fertilizer.
Here in Rwanda, as the population has grown, more and more land has been taken under the hoe, at the expense of forests, fallow land, and grazing land. In such an intensive system of cultivation, organic matter is continually in short supply. This shortage presents a big challenge for One Acre Fund’s Rwandan farmers.
One strategy to address the shortage is integrated soil fertility management (ISFM), which combines the use of mineral fertilizers with organic matter. These two types of inputs build off one another, substantially improving the efficiency of both inputs as compared to applying either alone. The macronutrients in fertilizer are immediately available to plant roots and they are better conserved due to the retention power of organic matter. At the same time, the large quantities of nutrients in mineral fertilizers lessen the dependence on the already overstretched supply of organic matter, meaning that nutrient reserves in soil are not depleted.
One Acre Fund supplies farmers with mineral fertilizer, and we provide training on composting to produce more organic matter. But even in this context, it is a challenge for many farmers to get their hands on sufficient quantities of plant and animal remains.
One potential solution is cows. Cows are a popular way to produce organic matter in Rwanda, but few of our farmers own cows. There is a common belief, however, that owning a cow is an economical way to create organic matter. Unfortunately, cows are labor- and land-intensive for farmers. They are almost all raised in zero-grazing systems in which farmers must cut and carry grasses directly to their animals. Many farmers cannot grow enough grass for their cows, which makes it necessary for them to collect grass from the small amount of commons that are left, including roadsides and woodlot floors. The cow’s diet is poor in protein and energy, and cows generally underproduce milk.
Another possible solution is agroforestry. Tree roots dig deep into lower soil layers using nutrients that would otherwise be lost, and recycle them back to the soil surface in the form of leaves. One Acre Fund is beginning to trial nitrogen-fixing fodder shrubs, which can be used to produce rich fodder, mulch, and bean poles, while at the same time restoring nitrogen in the soil. Trees can be difficult for many farmers to adopt because of land shortages, but if they plant along field borders and hill contour lines they can work efficiently, especially if maintained at shrub level.
A final potential source of organic matter is human manure. Most waste streams are effectively recycled, except for human manure (humanure), which is deposited in deep pit latrines. While most people prefer to forget about this waste stream, it can be used safely provided proper composting techniques. While on average each Rwandan family does not produce enough humanure to completely cover its organic matter needs, this resource has the potential to save at least a few ares of fertility for per family per year, which is certainly not insignificant.
Integrated soil fertility management, cows, agroforestry, and human manure are imperfect solutions to supply more organic matter to Rwandan soils. One Acre Fund is exploring all four options because even a small increase in organic matter makes a difference for our farmers. In the long term, however, there must be a reduction in the population growth rate and increased opportunities for off-farm income to significantly reduce the strain on organic matter in Rwanda.
Many One Acre Fund farmers must rent oxen-pulled plows in order to prepare their fields for planting. The cost can be $30 USD or more per acre each year. For our farmers, this is a huge investment, and it is one they choose to make because plowing helps with weed control. However, some agriculture researchers say that plowing has long-term negative consequences for soil health.