Industrial-scale soy agriculture has recently become a major landuse in the state of Mato Grosso, Brazil in southeastern Amazônia. The region’s soils are dominated by Oxisols with low native phosphorus (P) availability and a high capacity to bind P in biologically unavailable forms. Thus, P fertilizers are essential for high-yield agricultural production. The state’s soy fields produce ~40% of Brazilian soy and are fertilized with substantial P (~30 kg P·ha^-1·yr^-1), KCl, and lime. Resulting yields rival those anywhere in the world (~2.8 Mg·ha^-1). Understanding the fate of fertilizer P in these soils has implications for the sustainability of these yields and this input-intensive farming system. How much is exported in crops or bound irreversibly in soil? Does any accumulate in available forms over time? Will high annual inputs be required indefinitely? To answer these questions, we compared forest soils and soils along a chronosequence of soy fields (cleared for pasture in the 1970’s, converted to soy in 2003, 2004, 2007, and 2008) on a farm in Mato Grosso. Soil cores (2 m) were subsampled by depth and analyzed for texture, total P, P fractions (via modified Hedley fractionation), exchangeable cations, base saturation, and pH. 

Results/Conclusions

Forest soils contain ~40% clay, have a mean base saturation of ~17%, and pH of ~4.1. Soy soils have higher pH (~5.0) and higher base saturation (~39%). Not surprisingly, soy surface soils have significantly higher total P than forest, and older soy fields have significantly higher total P than younger fields (~45% of fertilizer P is removed as soybeans, leaving ~16.5 kg P·ha^-1·yr^-1 on fields). Preliminary analyses show that NaHCO₃-extractable P, assumed to be readily available, represents ~13% of total P in older soy fields, ~3% in new soy fields and ~7% in forest soils to 10 cm, and is undetectable below 50 cm across sites. NaOH-extractable P, assumed to be largely organic-bound P, accounts for >50% of P to 20 cm across landuses. Surprisingly, however, we find a substantial loss of NaOH-extractable P at depth: while total P is higher in soy, forest NaOH-extractable P is ~55% of P at 50 cm, but in soy it represents only ~10%, perhaps a legacy of pasture. Nevertheless, these data suggest that despite the high P-binding capacity, these soils won’t bind P indefinitely. This may mean that P inputs can be decreased with time, potentially increasing the economic viability of this farming system.