Potassium (K) is one of the essential nutrients and is taken up in significant amounts by crops. Potassium is vital to photosynthesis, protein synthesis and many other functions in plants. It’s classified as a macronutrient, as are nitrogen (N) and phosphorus (P). Plants take up K in its ionic form (K+).
While potassium (K) doesn’t constitute any plant structures or compounds, it plays a part in many important regulatory roles in the plant. It’s essential in nearly all processes needed to sustain plant growth and reproduction, including:
- translocation of photosynthates
- plant respiration
- protein synthesis
- control of ionic balance
- breakdown of carbohydrates, which provides energy for plant growth
- regulation of plant stomata and water use
- activation of plant enzymes
- disease resistance and recovery
- stress tolerance, including extreme weather conditions
Perhaps K’s most important function in the plant is that it can activate at least 80 enzymes that regulate the rates of major plant growth reactions. K also influences water-use efficiency. The process of opening and closing of plant leaf pores, called stomates, is regulated by K concentration in the guard cells, which surround the stomates. When stomates open, large quantities of K move from the surrounding cells into the guard cells. As K moves out of the guard cells into surrounding cells, stomates close, therefore K plays a key role in the process plants use to conserve water.
Unlike nitrogen (N) and phosphorus (P), the other primary nutrients, K doesn’t form organic compounds in the plant. Its primary function is related to ionic strength of solutions inside plant cells.
Potassium plays a key part in increasing yields and controlling disease because it improves a crop’s winter hardiness. It allows crops to get a quicker start in the spring and increases vigor so growth can continue throughout the growing season.
Even though the period of K uptake varies with different plants, it’s still vital to maintain adequate K fertility levels in the soil because soil K doesn’t move much, except in sand or organic soils. Unlike N and some other nutrients, K tends to remain where fertilization puts it. When it does move, it’s mainly by diffusion and is always slow.
Plants generally absorb the majority of their K at an earlier growth stage than they do N and P. Experiments on K uptake by corn have shown that 70 to 80 percent can absorb by silking time, and 100 percent at three to four weeks after silking. K translocation from the leaves and stems to the grain occurs in much smaller amounts than for P and N. The grain formation period is apparently not a critical one for the supply and uptake of K.
Plants deficient in K don’t grow as robustly and are less resistant to drought, excess water, and high and low temperatures. They’re also more vulnerable to pests, diseases and nematode attacks. Potassium is also known as the “quality nutrient” because of its important effects on factors such as size, shape, color, taste, shelf life, fiber quality and other qualitative measurements.
Potassium-deficiency symptoms show up in many ways. One of the most common K hunger signs is scorching or firing along leaf margins. Firing first appears on older leaves in most plants, especially grasses. Newer leaves will show hunger signs first on certain plants and under certain conditions. In addition to growing slowly, K-deficient plants’ root systems develop poorly and have weak stalks. Lodging is also common.
Symptoms of deficiency can vary across crop species, but similarities exist for how nutrient insufficiency impacts plant tissue color and appearance. Nutrient deficiencies are commonly associated with the physical location on the plant
(i.e., whether the symptoms are primarily observed on older versus newly formed plant tissue), but these symptoms can spread as the severity of the deficiency progresses.