Potassium or potash (K) is one of the macronutrients, needed by plants in relatively large amounts. It is important in plant growth and many of the plant’s internal metabolic processes, including water uptake. Potassium also helps promote flowering and fruiting, as well as ensuring good cold weather and disease resistance and hardiness.
Potassium deficiency normally starts on the older, lower leaves.
Potassium deficiency is caused when there are insufficient suitable potassium-containing materials in the growing medium (soil or compost). It can also be caused when plants are growing in dry soil, which prevents the roots taking up the potassium from the soil.
As potassium is very water soluble, it is easily washed out of the soil or compost by excessive rainfall or overwatering, leading to a deficiency. Shortages are worse on light, sandy and chalky soils that drain quickly and contain little organic matter to hold on to the potassium.
To check if your soil is short of potassium, you can use a simple soil-testing kit.
