Soil reserves of potassium are generally large, but most of it is not plant-available. On crop farms, negative potassium balance is common due to greater removal of potassium in hay, straw and grain than fertilizer potassium. Consequently, soil potassium depletion is increasing the prevalence of crop potassium deficiency. Crops require adequate levels of potassium for high yields, and global demand for potassium fertilisers is expected to increase significantly, particularly in developing countries. In this chapter, various aspects relating to the potassium cycle in crop production are discussed, including the use of potassium fertilizer in rain-fed and irrigated agriculture with the focus on soil potassium input and output, crop potassium residue and removal, soil potassium sorption and leaching. A good understanding of the potassium cycle in cropping systems would improve decision making for optimal use of soil potassium reserves and for better management of potassium fertiliser particularly on low potassium soils.