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Oil palm agroforestry systems associated with conservation management practices have been proposed as a more sustainable alternative to monoculture. We examined whether oil palm agroforestry systems (less and more diversified) under conservation management contribute to improving soil fertility (an indicator of soil sustainability), using a secondary forest fragment as a reference. We analyzed the effects of the agroforestry systems and the secondary forest on the following variables in the arable layer (0–20 cm) and in the layers along the soil profile (0–5, 5–10, 10–20, and 20–30 cm): active acidity (pH), calcium (Ca), magnesium (Mg), potassium (K), phosphorus (P), aluminum (Al), organic matter, potential acidity (H + Al), potential cation-exchange capacity (CEC), effective cation-exchange capacity (ECEC), Al saturation (AS), and base saturation (BS). Overall, soil acidity was lower in the agroforestry systems than in the forest, in all layers. In the arable layer, the pH, Ca, Mg, K, P, and BS values were higher in the agroforestry systems than in the forest. Liming, inorganic and organic fertilization, and the maintenance of organic matter on the soil may have contributed to improving soil fertility in the oil palm agroforestry systems. Our results suggest that, in the eastern Amazon, oil palm growing in agroforestry systems associated with conservation practices can contribute to the maintenance of soil fertility at adequate levels for the species cultivated in these systems. [ABSTRACT FROM AUTHOR]