Potassium is one of three major nutrients needed by all plants to grow and carry out basic metabolic functions. When you send a soil sample for testing, the test results will include information on the amount of potassium found, and a fertilization program to increase available potassium, if needed.
According to the University of Minnesota Extension, botanists don't completely understand the role of potassium in plant health. However, they know that plants need potassium for several important life functions, especially the movement of water, nutrients and carbohydrate molecules inside of the plant. Additionally, potassium promotes early, healthy growth, helps plants to form proteins, protects from water loss and provides resistance against pests and diseases.
Plants without adequate potassium in the soil show stunted growth and development. The leaves lose their green coloration, becoming yellowish, or striped and spotted, in some species. Eventually, the leaves turn brown and die. Symptoms begin in the lower leaves and move up the plant.
Soil tests measure the amount of potassium available for plant use. According to the University of Minnesota Extension, 90 to 98 percent of soil potassium is not in a form plants can use. Two types of soil tests, ammonium acetate and Melich-3, extract available potassium from soils and, from these values, calculate the potassium available in the soil in parts per million.
Potassium test results indicate different levels of available soil potassium. The exact range of numbers matching these levels depends on the soil test used. The University of Minnesota Extension characterizes soil potassium as very low, low, medium, high and very high, and notes that the lower the value, the greater the benefit you are likely to see in crop yields if you apply potassium as part of a fertilizer program.
Potassium occurs naturally in rocks found in the soil, where slow weathering makes it available to plants over long periods. However, this process alone is not enough to ensure adequate potassium for plant growth in most soils. Potassium, also known as potash, is an essential ingredient in most commercial fertilizers. The third number in the three-digit fertilizer number refers to the proportion of potassium. For organic gardeners, wood ash and manure both provide potassium.