The phosphorus cost of agricultural intensification in the tropics

Abstract

Agricultural intensification in the tropics is one way to meet rising global food demand in coming decades(1,2). Although this strategy can potentially spare land from conversion to agriculture(3), it relies on large material inputs. Here we quantify one such material cost, the phosphorus fertilizer required to intensify global crop production atop phosphorus-fixing soils and achieve yields similar to productive temperate agriculture. Phosphorus-fixing soils occur mainly in the tropics, and render added phosphorus less available to crops(4,5). We estimate that intensification of the 8-12% of global croplands overlying phosphorus-fixing soils in 2005 would require 1-4 Tg P yr(-1) to overcome phosphorus fixation, equivalent to 8-25% of global inorganic phosphorus fertilizer consumption that year. This imposed phosphorus 'tax' is in addition to phosphorus added to soils and subsequently harvested in crops, and doubles (2-7 Tg P yr(-1)) for scenarios of cropland extent in 2050(6). Our estimates are informed by local-, state- and national-scale investigations in Brazil, where, more than any other tropical country, low-yielding agriculture has been replaced by intensive production. In the 11 major Brazilian agricultural states, the surplus of added inorganic fertilizer phosphorus retained by soils post harvest is strongly correlated with the fraction of cropland overlying phosphorus-fixing soils (r(2) = 0.84, p < 0.001). Our interviews with 49 farmers in the Brazilian state of Mato Grosso, which produces 8% of the world's soybeans mostly on phosphorus-fixing soils, suggest this phosphorus surplus is required even after three decades of high phosphorus inputs. Our findings in Brazil highlight the need for better understanding of long-term soil phosphorus fixation elsewhere in the tropics. Strategies beyond liming, which is currently widespread in Brazil, are needed to reduce phosphorus retention by phosphorus-fixing soils to better manage the Earth's finite phosphate rock supplies and move towards more sustainable agricultural production.

Phosphorus Over-Fertilization and Nutrient Misbalance of Irrigated Tomato Crops in Brazil

“…Phosphorus (P) is the nutrient that most limits crop productivity in tropical soils (Novais and Smyth, 1999; Salcedo, 2006), especially in Brazil (Roy et al, 2016). Recovery of P by crops is generally low, due to the strong sorption of P by iron (Fe) and aluminum (Al) oxyhydroxides (Fe ox and Al ox , respectively) (Novais, 1977; Baligar and Bennett, 1986a,b).…”

Alkalinized sewage sludge application improves fertility of acid soils

“…The indirect effect caused by the increase in soil pH was not very effective following the limestone application. Considering the concerns regarding fertilizer use, P recovery, and cost of phosphate fertilization in agricultural soils (Roy et al, 2016), alkalinized sewage sludge can be considered an alternative source of P. In addition, according Bittencourt et al (2014), 88,166 Mg of alkalinized sewage sludge destined for 2,288 ha of agricultural areas (corn, soybean, bean, oat, wheat, green manure, and stone fruit trees), provided 88% lime, 74% N, 73% P 2 O 5 , and 35% K 2 O for crop fertilization. In the same study, 80 farmers benefited from a reduced expense for fertilizers and limestone, saving an average of US$813.45 per ha.…”

Soil solution phosphorus turnover: derivation, interpretation, and insights from a global compilation of isotope exchange kinetic studies

“…This shows that soil solution P is more strongly buffered in some soils than others. Results from the fertilizer experiments thus confirm that in high P-sorbing soils, such as Ferralsols, additions of P fertilizers may lead to only incremental increases in solution P concentration (Roy et al, 2016). However, this does not necessarily translate to P availability (Pypers et al, 2006).…”

“…3b). In terms of agricultural management, in such a soil, P fertilization has to be higher than P output via crop removal to account for the buffering effect (Roy et al, 2016). However, once a soil reaches a certain P level and binding sites are saturated by phosphate and other anions, P exchange is less important and fertilizer inputs can be lowered to equal crop offtake (Syers et al, 2008).…”

“…First, P fertilizer is largely derived from rock phosphate, which is a non-renewable resource and major deposits are located in only a few countries (Elser and Bennett, 2011;Obersteiner et al, 2013). Second, applications of P fertilizers to soils with a high P sorption capacity can be inefficient because P largely accumulates in the soil in sparingly soluble forms (Roy et al, 2016). Third, leaching or runoff of P fertilizer from agricultural land to aquatic and marine ecosystems contributes to fish die-off and declining water quality (Carpenter et al, 1998).…”