Every year ~32 million tons of potassium (K) are mined from Earth’s crust, where K is largely found in evaporate minerals. An essential nutrient for plants and animals, nearly all of this K is applied to agricultural crops worldwide. Although not overlooked by agronomists, K has garnered relatively little interest among biogeochemists, yet together with nitrogen (N) and phosphorus (P), K should be expected to exert a major control on terrestrial plant productivity. Moreover, as global food demand rises and agriculture expands further into highly weathered tropical soils, potassium is likely to become an increasingly critical - and perhaps economically constraining - resource for global agriculture. Such a shift would heighten demand for better potassium management strategies in which K losses could be minimized and the efficiency of applied K fertilizers optimized. Yet insights from work on N and P will not fully translate, as the K cycle has several unique features which present a conundrum for biogeochemists. In natural ecosystems, these include wasteful cycling at the plant level yet data suggesting reasonable conservation at the whole system level. Here, we explore some key points in the global K cycle as well as possible future trends, constraints and opportunities.
Results/Conclusions
Assuming current crop production of 5.25 x 1015 g C/yr, and a nitrogen demand of ~14.3 x 1012 moles, global crops require ~0.62 x 1012 moles of P and ~3.11 x 1012 moles of K. For P, this is roughly equivalent to annual phosphate rock mining (21 x 1012 g), but for K it is much more than supplied by potash (32.2 x 1012 g K). Clearly rock weathering supplies a considerable amount of crop K demand, but K is also very likely under-applied in some systems. And while possible P shortages and their economic impact have received considerable attention, historical prices for P and K reflect a roughly equal cost per mole, but higher price spikes for K in economic downturns combined with six-fold greater average costs to farmers to meet stoichiometric demands. As agriculture expands in the tropics, rock weathering is less able to meet demands and thus the relative cost to farmers is even greater. Fortunately, analyses of stream losses versus fertilizer application along with offsets in fertilizer timing versus demand suggest opportunities for improvements in K use efficiency. Finally, major global crops differ in their K requirements, providing opportunities to adapt to shifting resource constraints.