As Nile and Brady’s classic text, The Nature and Properties of Soils notes, the slash-and-burn system is not inherently unsustainable. In theory, natural vegetation is cut and burned from a plot of land and crops are grown for several years until the nutrients, built up over years, have been used and yields begin to decline. The farmer then moves on to the next plot to continue the cycle. Only after an undefined period of time, after the original ‘slash’ plot has re-established natural growth and replenished soil organic matter and nutrient pools, does the farmer return to cultivate the plot. With an adequate land base and rotational cycle, new deforestation is avoided.
But this system breaks down when farmers are forced to return to regrowth plots (for various reasons – population increases, food insecurity, changing land tenure, etc) before they’ve had an opportunity to re-initialize some level of stability. In a closed system, external inputs are unnecessary because the essential nutrients/inputs to sustain the system are available and continuously recycled in a perpetual cycle of growth and rejuvenation. By definition, agriculture necessitates an ‘opening’ of the system. Farmers grow crops, which leave the field to be sold at the marketplace or consumed directly. Contained within these products are a multitude of nutrients, carbon and water that also leaves the ecosystem with the commodity. This alteration of the ecosystem also implies that to maintain the same level of productivity, external inputs are now necessary to replenish inputs leaving the system or new woodlands must be cut down, which furthers the damage of deforestation.
Farmer-to-Farmer (F2F) is a USAID-funded program that ‘provides technical assistance from U.S. volunteers in developing and transitional countries with the goal of promoting sustainable improvements in food security and agricultural processing, production and marketing.’ As a F2F volunteer working with local ACDI-VOCA staff in the Bandundu District in (central) DR Congo, I found the slash-and-burn system commonly practiced in the region to be particularly concerning. In discussions with farmers, they noted that increasing land pressure has forced them to fallow fields for only a few years before they must be cropped again. A lack of capital and access also means that the majority of farming is done without improved seed or external fertilizer inputs. All of this occurs on fairly sandy soils, which under the best conditions creates a naturally leaky system with inherently low organic matter. Therefore solutions for improving yields and sustainability of farming in this region must include pathways to return nutrients not just at replacement levels, but at levels exceeding the amount of nutrients leaving the system.
It appears likely that stagnant or even declining yields in this region are the result of negative nutrient balances and a lack of organic matter input is driving soil degradation. The Bandundu farmers understand that their most fertile soils are those just coming out of forest. The higher organic matter in these plots can vastly increase the water holding capacity of the soil while supplying much-needed nutrients. Prior to burning, it is a relatively closed system in that the trees’ extensive root system incorporate nutrients and water that have leached down into the soil profile into new biomass, which then falls to the ground, decomposes and releases the nutrients to start the cycle again. When they burn the forest to produce grain, the system is open. The root system no longer reaches the depths required to retrieve leaching nutrients and additional nutrients leave the field in the grain. In these degraded soils, the yields are low and so too is the biomass/residue production, generally the only source of carbon and nutrients being returned to the soil. The biomass added back to the soil is not nearly enough to balance the losses (let alone increase) so the soil is degraded further.
Through our farmer discussions and townhall-style debates, we devised a series of approaches, which draw heavily on the framework provided through the practices of integrated soil fertility management (ISFM). For example, the seasonal farming pattern in Bandundu is governed by bimodal rains distributed by a longer ‘A Season’ and a shorter rainy period in the ‘B Season.’ A primary tenet of ISFM is that to increase both short- and long-term soil fertility, a combination of organic and inorganic nutrient sources is necessary because neither nutrient source is available in sufficient quantities. Currently farmers do add back crop residues, but these sources are generally of lower quality (high C:N) and lack adequate tonnage. To be successful, however, re-aligning the two seasons to adhere to the principles of ISFM is critical. Historically, due to a longer rainy period, the A Season is the most likely period to produce a profitable crop. During this season, along with the organic inputs, farmers will begin small-scale experiments incorporating inorganic nutrients to offset any nutrient deficiencies, nutrient immobilization by organic sources and/or to improve synchronicity of nutrient release from organic sources.
In this manner, the combination of the two nutrient sources complement each other by providing an immediate nutrient supply along with a slow-release across the growing season. The more stable/recalcitrant carbon from the organic matter will begin the rebuild the soil organic matter pool for long-term soil resilience. The farmers will target fertilizers to specific plots, identified as those of medium to higher fertility where organic matter is present, which will likely provide the greatest benefit.
Additionally, matching crops to cropping seasons is critical. Maize is likely a riskier option in the B season. Farmers noted that the varieties they have available are generally longer maturity varieties while the rains are limited in the B season and less predictable. Even in the best conditions during this time, maize production is likely to have severe yield reductions due to nutrient limitations. When combined with the higher probability of water stress, farmers may be better off growing shorter season legumes or improved fallow. For the former, our discussions illustrated that soybeans and cowpeas are potentially viable and available options provided they can produce high biomass and adequate yields. With promiscuous varieties or access to inoculation, this substitute may be far less risky under the prevailing short season variability. Where this inoculation can be procured is less clear. Their most likely option is in Kinshasa. Moreover, despite not producing a crop, legume fallows can still be incorporated with the full season cassava that is the other staple crop of this region. Even when planted alone, green manures may increase yields of the subsequent maize crop in the A season and reduce overall risk to crop failure over the long-term. In my discussions with local farmers, I found that they were aware of this practice, but had not tried it in their fields.
In concluding, the sense I got from the farmers in Bandundu is that they felt the slash-and-burn system as it exists locally is fundamentally unsustainable. If slash-and-burn agriculture is to remain the dominant form of agriculture in the region, farmers must have the ability/flexibility to rest and improve severely degraded fields. Access to adequate capital, fertilizers, supplemental organic inputs, rotated (inoculated) legumes and improved seeds can help to stem some of the ‘leakiness’ of the system. Any future investment by the farmers or any other group interested in agriculture in the region must acknowledge the constraints and limitations placed upon farmers to provide immediate food and/or income, and devise interventions that reduce the need for long fallow periods. It may not be possible to ever fully close this system once it is open, but an integrated approach that matches inputs and crops to both fields and cropping seasons may be a viable path toward closing the loop.
Acknowledgement: This paper stems from work funded by a joint effort between USAID and ACDI-VOCA through a program called Farmer-to-Farmer
Chris Graham, Assistant Professor, South Dakota State University