Potassium is the most important nutrient plants need for healthy growth. They absorb potassium in even larger amounts than they do other macronutrients. Unlike other nutrients, potassium does not wind up in a plant's chemical structure. Instead, plants require it for ongoing processes important for their growth and health.
Plants feed themselves by drawing nutritional elements from the soil. Of those elements, six are considered "macronutrients" because plants need them in very large quantity. Potassium is one of these. Further, it is one of the three macronutrients considered "primary," the other two being nitrogen and phosphorus. Plants remove these nutrients from the soil relatively quickly, requiring you to replenish them by fertilising.
Potassium helps to activate more than 60 enzymes needed for healthy plant growth, while at the same time optimising the plant's pH for enzyme reactions. Another process potassium regulates is the opening and closing of the stomates, those pores through which plants exchange oxygen, carbon dioxide and water vapour with the atmosphere. Potassium also plays a complex role in photosynthesis, the process by which plants synthesise energy from sunlight, carbon dioxide and water. Further, a sufficient supply of potassium helps increase root growth, crop quality and reduce vulnerability to crop diseases.
Some plants, most notably legumes, act as important nitrogen fixers: they are particularly apt at converting atmospheric nitrogen into a soil-bound form that they or other plants can use. Nitrogen fixation occurs with the help of the bacteria genus Rhizobium. The bacteria takes up residence in root nodules, feeds on sugars the plant produce, and derives ammonium ions from atmospheric nitrogen. Ammonium ions represent a form of nitrogen which plants can readily use.
Nitrogen fixation relies on the healthy plant growth processes in which potassium plays important roles. For instance, potassium is required for photosynthesis, and photosynthesis produces the sugars that feed the Rhizobium bacteria. Potassium contributes to healthy root growth and robust activity in root nodules, the "factories" in which nitrogen fixation occurs. And nitrogenase, the catalyst for Rhizobium's ammonium production, is one of the many enzymes which potassium activates.
Symptoms of Deficiency
Potassium deficiency produces many symptoms depending on the species of plant. In many, the first signs include discolouration along the outer edge of the leaves. In some plants such as corn and soybeans this manifests as a consistent firing or scorching. In others, such as alfalfa, it may be a series of small white or yellowish dots. Leaves that curl, crinkle or turn leathery at the edges may also indicate potassium deficiency. Stunted growth, smaller leaves and thinner stems may also point to a lack of potassium in the soil.
Most commercial fertilisers come labelled in terms of the amounts of nitrogen, phosphorus and potassium they contain. The information is commonly referred to as N-P-K levels after the scientific abbreviations for those three elements respectively. N-P-K levels may be expressed in terms of percentage, such that 5-10-5 indicates 5 per cent nitrogen, 10 per cent phosphorus and 5 per cent potassium in the fertiliser. N-P-K levels may also be expressed in terms of ratio, such that 1-2-1 means that there's one part nitrogen, two parts phosphorus and one part potassium.