Assessing soil system changes under climate-smart agriculture via farmers’ observations and conventional soil testing

Soil degradation remains a challenge in African highlands, where land management lacks a strong context-specific evidence base. We investigated the impacts of recently implemented soil and water conservation (SWC) practices – farmyard manure addition, incorporation of crop residues in soil and Fanya juu terracing under agroforestry system on soil health indicators in the East Usambara Mountains of Tanzania. Farmers’ observations of soil changes were combined with conventional soil testing to assess the initial impacts of SWC practices relative to conventional non-SWC practice. Majority of farmers (66-83%) reported that combining Fanya juu terracing with organic amendments led to soil colour change from red to black and an increase in crop yield. Despite the observed darkening of the soil, there was no significant increase in soil organic carbon stock and the contents of N, P, K. There were important changes in soil physical properties, including greater aggregate stability (mean weight diameter of 1.51-1.71 mm) in the SWC plots, a greater volume of transmission pores (>60 μm) and coarse storage pores (10-60 μm) in the surface soil layer (0-15 cm) and greater volume of fine storage pores (0.2-10 μm) and residual pores (0.2 μm) in the sub-surface layer (15-30 cm) of the SWC plots compared to the conventional plots. These changes indicate that SWC rapidly enhances infiltration and retention of water within the root zone, which are important for increasing crop yields and improving the resilience of the agro-ecosystem to environmental stress. Combining SWC with effective soil fertility management is needed for sustainable highland agriculture.