Soil phosphorus sorption capacity after three decades of intensive fertilization in Mato Grosso, Brazil

Soil phosphorus (P) availability commonly limits the productivity of tropical croplands. While large fertilizer inputs can alleviate P limitation, this strategy is costly and relies on finite phosphate rock resources subject to price volatility. Nevertheless, high-P-input agriculture on P-poor and P-fixing soils is spreading rapidly in some regions of the tropics, particularly in Brazil, where farmers on average add twice as much P to soils as they harvest to ensure high yields. Here we ask whether P fertilizer inputs to tropical soils in excess of harvested P outputs will eventually build up a residual pool of soil P that crops can tap into if fertilizer inputs are decreased - a phenomenon observed in the U.S. and Western Europe, albeit on very different soils. We pose this question in Mato Grosso, Brazil, where we quantified soil P input-output budgets, total P, Bray-extractable P, P sorption capacity, P saturation, and other characteristics from a chronosequence of 31 plots that had been in soybean production for 0-31 years. Farmer interviews revealed ongoing annual additions of P fertilizer greater than P removals in crops, with an average farm P balance in the most recent year, including soybeans and a second harvest, of +14 kg P ha−1 y−1. Soil total P and Bray-P1 have increased, and P sorption capacity has decreased, with time in production. However, clayey soils rich in iron- and aluminum-oxides still have high P sorption capacity and low P saturation, even after three decades of intensive fertilization and residual P build-up. Our findings suggest that commodity crop producers farming on this soil type in Mato Grosso and other tropical regions may need to add annual inorganic P fertilizer inputs greater than the quantity of P recovered in harvests for up to a century or more before soil P budgets can be balanced without endangering yields. This result has implications for the sustainability of agricultural intensification in the tropics.