Potassium (K) is absorbed by plants in larger amounts than any other mineral nutrient except nitrogen, and is required for nutrient movement in the plant. It is essential for the makeup of over 40 different enzymes and is involved in more than 60 different enzyme systems in plants. Potassium is also important in the formation of sugars and starches in plants. Crops that produce a large amount of carbohydrates (sugars) such as cotton, almonds, alfalfa, grapes, cherries and peaches require large amounts of potassium.
Potassium is used by plants to regulate the process of opening and closing the stomatal openings of their leaves. That process influences water use efficiency and carbon dioxide use in the plant. Potassium’s influence on cell turgor pressure and water relations in the plant helps the plants resist the effects of drought and temperature extremes, and aids resistance to many plant diseases.
Depending on soil type, 90 to 98% of total soil potassium is unavailable. Feldspars and micas are clays that contain large amounts of potassium, but plants cannot use the nutrient if it is trapped between the layers of those clays. In persistently dry soils, potassium remains unavailable, as there is no water film surrounding the soil particles. Over time, these minerals break down, and the potassium is released. However, this process is too slow to provide crops the amount of potassium needed for optimal yield.
Typically only 1% to 2% conventional potassium fertilizers applied to the soil is available at one time.
Plants lacking in potassium often display various signs of deficiencies, the most common being discoloration of the older leaves on the plant as compared to the younger leaves on the plant. The stem on affected tissue usually appears weak and is slender in size compared to healthy tissue. Other deficiency signs include inward curling of leaves, discolored leaf tips and marginal scorching. Another common sign of potassium stress is abnormally short internode length. A crop deficient in potassium may also display signs of various crop diseases.
Providing available potassium when the plant needs it is critical in establishing a healthy, vigorous crop that is resistant to disease and pests. According to the USDA Yearbook of Agriculture, “More plant diseases have been retarded by the use of potassium fertilizer than any other substance.” Sufficient readily-available potassium is especially critical.
when soil conditions are dry, and water-soluble potassium in the soil is unavailable for plant uptake.
While specific management practices for potassium vary by crop, region and specific soil conditions, multiple applications of potassium during a growing season are often beneficial.
Soil and tissue sampling is critical to effective management of potassium. Growers should base management decisions on the potassium needed by the plant throughout the growing season and how much potassium is available in the soil. Dependent upon the situation, potassium application may be appropriate at planting time, sidedress, foliar applications, through fertigation, or a combination of those methods.
As an essential major nutrient for crop production, potassium needs to be available to the plant at all stages of growth. Most potassium found in soils is unavailable to the plant, therefore the nutrient may need to be supplied to the crop via fertilizer.
The need for potassium can and should be determined from soil and plant tissue analysis.
Management recommendations for potassium vary by crop, region and specific soil conditions. It is often beneficial to split applications of potassium to match the optimal times of plant uptake in order to increase yield potential and quality. Proper potassium management can increase disease and pest resistance. In addition, this management practice can improve soil quality for sustainability, resulting in an overall increase in productivity.