The phosphorus “tax” imposed by tropical soils is a barrier to increasing global food production
Agricultural intensification is one way to increase food production and meet global demand in coming decades. While this strategy has the potential to spare landscapes from conversion to agriculture and thus provide environmental benefits, it often relies on large inputs of materials (e.g., fertilizers, fuel, and other chemicals). We quantified one such material cost, the amount of phosphorus (P) fertilizer required to intensify agriculture on the 8-12% of global croplands overlying P-fixing soils in 2005, as well as for scenarios of potential cropland extent in 2050. P-fixing soils are common in the tropics, and can bind added P, rendering it less available to crops. Our assessment is based on a multi-scale analysis of Brazil, where low yielding agriculture has been replaced by high P input intensive production. Cropland P input-output budgets were developed at the national scale (1961-2012) and for all Brazilian states (2005-2010). We surveyed 47 owners or renters of soybean farms in Mato Grosso to learn how/if farm age influences fertilizer inputs or yields on the region’s P-fixing soils. Estimates of the global P “tax” imposed by soils in crop production were determined using two different classifications of P-fixing soils in the literature and observations from Brazil.
The boom in value of Brazil’s agriculture has been tied to increasing P fertilizer inputs (R2=0.89, P<0.001), and a substantial portion of added P is retained by the soil post-harvest. P input-output budgets for the 11 Brazilian states hosting substantial intensive agriculture show P surpluses of 5-18 kg P ha-1 yr-1. These surpluses were correlated to the percent of cropland overlying P-fixing soils (R2=0.84, P<0.001). In Mato Grosso, soybean producers added 40±7 kg P ha-1 yr-1, and harvested 20±2 kg P ha-1 yr-1 in soybeans, yielding an average P output:input ratio of 0.5. This ratio was independent of farm age (7-38 years, R2=0.003). Thus even after almost 40 years of fertilization, producers either cannot, or choose not to, reduce P inputs. Globally, intensification of all cropland with P-fixing soils in 2005 would require 1-4 Tg P yr-1 to overcome P fixation, which is 7-21% of global P consumption that year. This number is in addition to a similar amount that would be added to fields and harvested in crops. This sink in the global P cycle could double by 2050 to 2-7 Tg P yr-1 as intensification spreads across other P-fixing tropical soils.