Soil fertility refers to the ability of a soil to sustain agricultural plant growth, to provide plant habitat and result in sustained and consistent yields of high quality. One indicator of soil fertility is C:N ratio –the carbon-to-nitrogen ratio.
When microorganisms consume residue too fast, soil is covered less, which results in less protection from agents of erosion. It also means that soils are less protected from sunshine and scorching winds; or there are fewer habitats for arthropods which reside in residues and perform a key function in breaking down crop residue and eating weed seeds. On the other hand, these same residues need to eventually decompose to release plant nutrients and build soil organic matter. That’s why it’s important to maintain a balance of soil cover that ultimately breaks down.
Under optimum conditions of C:N ratio of 24:1 (24 parts carbon to 1 part nitrogen), soil microbes spur release of nutrients like N, phosphorous and zinc to crops. This ratio also influences the amount of soil-protecting residue cover that remains on the soil. The 24:1 ratio therefore strikes a balance between the two.
Analysis of Vital Signs soils data found that all sampled soils fell below the optimal C:N range for microbial activity (Fig 1). This signifies that most soils in Rwanda are depleted and would benefit from corrective measures like fertilizers and sustainable agricultural practices.
Fig 1: Measured and Optimal Soil fertility as depicted by C:N Ratio across Rwanda Soils
This finding can be attributed to Rwanda’s steep terrains and vulnerability to soil erosion, as well as widespread unsustainable agricultural practices.
Fig 2: Spatial distribution of top soil C:N ratio across Rwanda
None of the plots on the map had a topsoil or subsoil C:N ratio that was 24:1 or greater, and most of the C:N ratios were much less, suggesting lower fertility (Fig 2, 3).
Fig 3:Spatial distribution of sub soil C:N ratio across Rwanda
The results suggest that Rwanda needs to implement extensive sustainable agriculture policies and interventions to increase soil fertility for improved agricultural yields and livelihood. These practices could include mulching, intercropping or fallowing using crops with high C:N ratio. Practicing agroforestry especially on steep slopes, and terracing to reduce soil erosion could help conserve and enhance soil fertility if agroforestry tree species are selected carefully. These species should have high C:N ratio or be leguminous to increase soil microbial activity.
An integrated approach will be necessary to promote interventions at all levels, from the community level with focus on daily agricultural practices, to agricultural investments and national land use policies. It is likely that most of the bad agricultural practices are out of lack of capacity and awareness. Awareness and capacity building can therefore help strengthen adoption of these interventions.