The objective of this paper is to compare soil fertility evaluation based on experience and knowledge of smallholder farmer communities with the evaluation by scientists based on soil analysis and model calculations. The role of the smallholder farmer community in soil fertility evaluation and management was examined from two `research for development¿ projects in northern Tanzania. These are the African Highlands Initiative (AHI) and the Soil Water Management Research Group (SWMRG). Participatory approaches were applied by both projects. Farmers¿ experience and knowledge of local indicators of soil quality were used in identifying soil fertility constraints and in generating resource flow maps. The farmers¿ evaluation of soil fertility was compared with soil analytical data and with calculations of maize yields by the model QUEFTS. Farmers¿ indigenous knowledge in soil fertility evaluation mostly agreed with laboratory analysis and model calculations by QUEFTS. Model calculations identified potassium as the most limiting nutrient in the highlands in northeastern Tanzania for yields less than 3 t ha¿1 and phosphorus for yields higher than 4 t ha¿1. In Maswa (Lake Victoria Basin) nitrogen was most limiting. Given that farmers¿ evaluation of soil fertility is relative to what they see around them, there is a need to verify their observations, but also the interpretation of laboratory data by models like QUEFTS requires continuous and critical validation. Both projects have shown that there is scope to reverse the trends of declining soil fertility in smallholder farms in northern Tanzania. Essential was that the interaction with scientists has built confidence in the farmers because their knowledge in addressing soil fertility constraints was recognized.