Potassium sources, microorganisms and plant nutrition: Challenges and future research directions

Until recently, potassium (K) has not received considerable attention because of the general belief that soils contain ample amounts of this element. In addition, low rates of K fertilizer application in agriculture have led to rapid depletion of K in the rhizosphere soil in many underdeveloped countries. This results in various negative impacts, including preventing optimum utilization of applied nitrogen and phosphorus fertilizers. To compensate for these losses, massive use of K fertilizers in agriculture has been suggested. Potassium fertilizers are manufactured from rock minerals, particularly sylvite (KCl) and carnallite (KCl center dot MgCl2 center dot 6H2O). Unfortunately, to date, there is no cost-effective technology available for converting rock minerals into potassic fertilizers. Potassium-solubilizing microorganisms (KSMs) can release K from soil/minerals into plant-available forms, which could be a sustainable option. The possibility of using KSMs as efficient biofertilizers to improve crop production has been increasingly highlighted by researchers. In this review, the existing forms of K in soils and their availability and dynamic equilibrium are discussed. In addition, different K fertilizers and their advantages and disadvantages for crops are described. Furthermore, the microorganisms usually reported as K solubilizers, the research progress on KSMs, and future insights on the use of these KSMs in agriculture are reviewed. Screening and analyses of the published literature show that organic acid production is the common mechanism of K solubilization by bacteria and fungi. This review may serve as a proposal for the future research avenues identified here.