Multiple constraints prevent smallholders from adopting fertilizers even with regional supply of agricultural inputs expanding and soils being weared-out. Using comprehensive farm-level data from the eastern Brazilian Amazon, we found that market proximity had a significant positive correlation with fertilizer adoption, even after controlling for liquidity, land tenure, education, experience and access to rural extension services. Nevertheless, few smallholders completely replaced nutrients from vegetation with fertilizers. Instead, we found that a hybrid system that combines nutrients from vegetation and fertilizers was approximately twice as common as exclusive fertilizer use. We suggest that the option for this diversified “nutrient portfolio” may result not only from a lack of capital or knowledge regarding return on fertilizer use, but also from the need to adapt to the economic constraints facing smallholders and minimize risk. Results indicate that a rural extension program aimed at supporting a rapid and complete replacement of ashes from vegetation by fertilizers could prove unsuccessful for Amazonian smallholders.