Comparing farmers’ soil fertility knowledge systems and scientific assessment in Upper Eastern Kenya

•Farmers have a consistent soil fertility classification knowledge system.•Scientific and farmer soil fertility assessments are correlated.•Farmer-derived soil quality index (SQI) correlated better with Factor analysis SQI than with the simple additive SQI.•Indicators of soil quality can be used to guide land use practices.•Understanding farmer knowledge of soil fertility promotes better communication with soil specialists.