Print PagePhosphorus (P) fertilizer has been an integral part of the rapid rise in global food production over the past half century. As input-intensive agriculture expands across the globe, however, the environmental costs associated with P management (soil degradation, eutrophication, and depletion of finite P reserves) have received increasing attention. Soybean is one such input-intensive crop that has expanded rapidly into both traditional and novel agricultural areas. While grain production doubled over the last 40 years, oil crop production, like soybean, quadrupled. The USA, Brazil, and Argentina now produce 80% of the world’s soybean, mostly for export. Here we use the lens of P fertilizer management to examine the limits to, and consequences of, intensive soybean agriculture in three of the world’s most productive regions: Iowa (USA), Mato Grosso (Brazil), and Buenos Aires (Argentina). The biophysical setting, landuse history, and current farming practices vary dramatically among these three regions, affecting the benefits and concerns associated with P fertilizer use. Using national and state level data on soils, fertilizer inputs, and production we asked how these differences affect 1) regional water quality, 2) soil degradation, and 3) the global P supply.
Results/Conclusions
In Buenos Aires and Iowa the dominant soils are Mollisols, with low phosphorus sorption capacity and relatively high native P concentrations (679 ppm, SD=263). Yet farms in Iowa apply an average of 50 kg P ha·-1·yr-1, while farms in Buenos Aires apply only ~8 kg P·ha-1·yr-1. Thus, Iowa farms incur annual P surpluses with an application:removal ratio >2, while Buenos Aires farms experience P deficits, with average application:removal ratios of <0.5. In Iowa, P-saturated soils contribute to eutrophication. In Buenos Aires, soil resources are being mined, such that without increased inputs, yields may decrease as pools of P are depleted. In Mato Grosso, the dominant soils are Oxisols, with high P sorption capacity and low P concentrations (245 ppm, SD=119). Here, fertilizer-intensive agriculture is new, so sorption capacity remains unsaturated and P losses to waterways are low. High inputs are needed to maintain yields and it may be decades or longer before sorption capacity is satisfied and fertilizer can be applied at replacement rates. Intensifying agriculture on Oxisols disproportionately increases P demand, potentially exacerbating pressure on global P supplies. These regions illustrate distinct approaches to intensive agriculture, each with benefits and concerns. Consideration of these consequences may inform how and where intensive soybean agriculture takes place now and into the future.
