Potassium Deficiency in Cucurbits
· Cucumber · Pumpkin · Zucchini · Melon
Symptoms of potassium deficiency appear first on older foliage. A mild chlorosis develops at the margins and tips of the leaves. The leaf blade turns somewhat paler but the main veins remain dark green (interveinal chlorosis). If not amended, the chlorotic patches turn into a dry, leathery tan or scorch (necrosis) that later progresses from the leaf edge to the midrib. Young leaves remain small and dull, assuming a cupped appearance. Fruits often have a brown or spotted appearance and may not develop uniformly. A typical symptom is their club-shaped appearance with a narrow stem end and swelled fruit end. The plant is stunted with short internodes and prone to wilt. Although the growth of deficient plants may not be seriously impaired, the yield and quality of the fruits are often greatly reduced.
Plants of the family of the cucurbits are unique among crops in that they require more potassium than nitrogen. Deficiencies may occur because of low reserves of potassium in the soil or limited availability to the plant. Soils with low pH and sandy or light soils with little organic content are prone to nutrient leaching and drought, and may therefore cause problems. At hot temperatures these symptoms can develop rapidly. Drought conditions block the transport of water and nutrients to the plants. Heavy irrigation and high rainfall wash the nutrients from the root zone and can also lead to deficiency. High levels of phosphorus, magnesium and iron can also compete with potassium. This nutrient plays an essential role in the transport of water, the firmness of tissues and the exchange of gases with the atmosphere. Potassium-deficient plants are more susceptible to drought, salinity, frost stress and disease.
Add organic matter in the form of animal manure or plant mulch to the soil at least once a year. Wood ash also has high potassium content. Liming acidic soils can increase potassium retention in some soils by reducing leaching.
A variety of potassium fertilizers are available in the market. The most widely used product is potassium nitrate. Other mineral fertilizers include potassium chloride, potassium sulfate, and monopotassium phosphate. Always keep in mind that only a balanced fertilization can lead to optimal plant health and increased yield. Potassium fertilizers should be incorporated in the soil before planting. A soil test can be used to determine the rate needed. Foliar sprays seem to be less effective and can burn leaves.