Potassium is one of the three macro elements essential to plant growth and development. The macro elements, which together with potassium include nitrogen and phosphorus, are so called, not because they are more important than the micro elements, but rather because they are needed and consumed in large quantities. It is important to take note of three points here.
*Firstly, by being required and consumed in large amounts, available potassium is liable to be lacking in certain circumstances.
*On the other hand, high concentrations of potassium are liable to reduce the availability to the plants’ roots of micro elements such as magnesium. This element by playing an essential role in photosynthesis is no less vital to plant physiology than potassium itself.
*Thirdly, while potassium is easily leached from light sandy soils, it becomes attached to mineral clay particles in heavy soils, by virtue of its positive electrical charge. This means that uncontrolled applications of potassium fertilizer, can significantly increase soil salinity, the gravity of which should not be underestimated both for the health of the plants in the short term, and the soil in the long term.
The question for us gardeners therefore is how to ensure adequate supplies of potassium without affecting either the chemical balance of the soil or its salinity. The safest method is to feed the plants by regularly adding well-rotted organic matter to the soil like compost or commercially prepared humus. Compost as it breaks down to a mineral state, slowly releases nutrients in the soil, including potassium. Furthermore by improving the soil’s aeration and expanding its micro biotic activity, mineral uptake of all essential elements is enhanced.
Before planting a new garden, carrying out a soil test will reveal the concentrations of potassium present. If these are below what’s required, then adding compost may not by itself be enough to cover the shortfall. When I set up a garden I normally add slow release fertilizer to the compost and then rely exclusively on organic matter after a year or so. Slow release fertilizer is more expensive than readily soluble fertilizer, but much safer and less environmentally damaging.
Lawns under an intensive growing regime may also require additions of potassium in chemical form. From the point of view of root development, it is important to fertilize both in the autumn in Mediterranean climates, and in the early spring. Again I recommend the use of slow release fertilizer. Make sure, particularly in the autumn that the potassium value, marked by the chemical symbol K, is high relative to the N for nitrogen, and the P for phosphorus.