Small farmers in Sub-Saharan Africa exhibit low adoption rates for mineral fertilizers. A promising hypothesis explaining these puzzlingly low rates remains untested: a perception among farmers that fertilizer in the market has been compromised in ways that raise concerns about its effectiveness. Information about fertilizer quality problems is anecdotal rather than backed by reliable evidence. A challenge: little research to date has focused on understanding the relationships between input supply chains and product quality. To achieve a clearer understanding of this problem, this research links results from tests of the quality of 661 samples of fertilizers for sale in the markets of the Morogoro Region of Tanzania with data from a survey of the region’s 225 input dealers. Fertilizer nutrient and moisture content tests are performed on the same samples in multiple laboratories located in East Africa and in the United States. Results from our research provide the first assessments of market-available fertilizer quality in the region, as well as the first analysis of relationships between fertilizer quality and mineral fertilizer supplier characteristics.