Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Currently, potassium fertilizer prices are about one third of the 2022 peak price. However, expect prices to rise in the future. This is because potassium deposits that are mined (largely in Canada and Russia/Belarus) are a finite resource. US deposits are much smaller and are diminishing and most of our potash comes from Canada. Even with high prices, vegetable growers should not limit potassium fertilization because of the effects on vegetable quality.
For vegetable growers, potassium (K) is a nutrient that is essential for quality, especially in fruiting vegetables. Research by Jerry Brust at the University of Maryland has shown the critical role that potassium plays in tomato fruit ripening and quality (see https://sites.udel.edu/weepotassiumlycropupdate/?p=13902 for more information). My research has shown that potassium plays an important role in sugar levels in watermelons.
Potassium influences fruit quality through its effects on sugar accumulation (soluble solid levels), acidity, size, appearance, and color. Fruiting vegetables such as cantaloupes, watermelons, and tomatoes have high potassium requirements as do fruit crops such as grapes, peaches, and strawberries. Potassium is critical for improving quality of fruits by maintaining desirable sugar to acid ratio and ripening of fruit and is the most important nutrient regulating the quality of fruits. Potassium is also required for high levels of production in legume vegetables such as beans, peas, and edamame.
Unlike nitrogen, phosphorus, calcium and magnesium, potassium does not form part of the structure of the plant and plant compounds; rather, the role of potassium is to regulate vital physiological functions such as carbon assimilation, translocation of proteins and sugars, water balance in plants, turgor pressure in plant cells, and root development. Potassium is involved in many other aspects of plant physiology such as activation of many enzymes, photosynthesis, respiration, and regulation of stomatal opening.
Often potassium deficiency in vegetables is found in soils testing high in potassium due to issues with potassium uptake, root function, and source-sink issues with fruits (high fruit demand). Potassium deficiency symptoms occur when plants cannot extract adequate potassium from the surface soil. Remember that the majority of vegetable root systems are located in the top six inches of soil where the plant obtains much of its needed water. Inadequate potassium uptake can occur due to root restricting problems such as compaction and with reduced root function due to high soil temperatures, particularly in black plastic mulch. In addition, when the soil test potassium level is below optimum, potassium deficiencies will be likely. Grid sampling for soil nutrient evaluations has found that there is a large variability in soil test potassium levels. This means that certain areas of fields may be below critical potassium levels for good vegetable production.
Potassium uptake increases dramatically in vegetables during rapid vegetative growth just prior to flowering and during reproductive growth and uptake peaks just before flowering begins. A deficit of potassium during the late vegetative stage or during the reproductive stages will be reflected quality problems in many vegetables.
Potassium is taken up by plants primarily through the process of diffusion rather than the process of mass flow (how nitrogen is taken up). The slow rate of diffusion and limited distance that potassium can move by diffusion means that potassium is not as easily available to crop plants as nitrogen. Deficiency symptoms appear first on the oldest leaves because potassium is mobile in plants. Because potassium is taken up by diffusion, symptoms often occur during drought periods or when irrigation has been limited.
To manage potassium in vegetable crops, the keys are to maintain high levels in soils, manage irrigation so that water deficits do not occur and potassium diffusion to roots is not interrupted, add additional potassium though fertigation or side dressing prior to flowering, and to consider foliar potassium additions during peak use periods in high demand crops.
Potassium deficiency on cucumber leaves. Deficiency symptoms begin as yellowing of leaf edges on oldest leaves. As potassium is mobilized from older leaves and moved into growing points, symptoms will progress and necrotic areas will develop.