Identifying sustainable land management practices within smallholder agriculture is a challenge. This is partly driven by the challenge of documenting farmers' perspectives and practices in an integrated manner with site-specific scientific soil assessment. Smartphone applications such as LandPKS provide new approaches to quantify site-specific soil degradation and fertility but are untested with African farm management. We surveyed 578 households in rain-fed maize (Zea mays) production areas of Tanzania using a stratified sampling frame to encompass a wide range of soils and agroecologies. A socio-economic survey and simultaneous sampling in focal plots documented farmer characteristics, perspectives, and management practices, along with soil properties and crop yields. For a subsample of 58 households, we additionally assessed site-specific field status with the LandPKS application. Farmer perceptions of change in soil fertility status were consistent with soil properties, for example, a field perceived to be declining in fertility was also likely to have low SOC (1.8% relative to 2.7% for increasing fertility). LandPKS provided additional novel insights into soil limitations such as identifying poor water infiltration areas consistently associated with farmer use of erosion control practices (water infiltration of 4 mm hr−1 vs 20 all other plots). This charts a way forward to address soil fertility and land degradation challenges through the use of smartphone applications to capture site-specific conditions and farmer concerns as the basis for land management recommendations that are highly relevant and address local conditions.