Potassium is the seventh most abundant element in the earth’s crust, making up 2.6% of its total mass. It is absorbed by plants more than any other element, except for Nitrogen and in some cases, Calcium. Potassium is taken up by plants in the form of Potassium ions (K+). It is not synthesized into compounds as with Nitrogen and Phosphorus but tends to remain in ionic form within cells and tissues.
Potassium is essential for translocation of sugars and for starch formation. It is required in the opening and closing of stomata by guard cells. Potassium encourages root growth and increases crop resistance to disease. It produces larger, more uniformly distributed xylem vessels throughout the root system. Potassium increases size and quality of fruit and grains and is essential for high quality forage crops. About 90 to 98 % of the Potassium occurs in primary minerals and is unavailable to crops, unless there is an abundant microbial population present.
Potassium has been found to be the element most required by tree crops such as prunes with a very high carbohydrate production. The most responsive vegetable has been the potato, which again is a high producer of carbohydrate as starch in the tubers. Potassium is mobile in the plant tissues.
Potassium in Soils
The following are general elemental relationships. The most overlooked element in agriculture other than carbon is potassium. Potassium levels have rarely been associated with nitrogen levels, but they should be. Potassium is directly involved in:
1) The synthesis of carbohydrates, proteins, oils and certain organic acids.
2) Acceleration of certain enzymatic actions.
3) The reductase reduction of nitrates which are fundamental for the synthesis of protein.
4) Increasing photosynthetic activity under low light conditions.
5) Facilitating the transport of carbohydrates inside the plant.
6) Cellular division.
7) Regulation of nitrogen absorption by the plant.
8) Increasing the resistance of plants to diseases.
We have found in corn, potatoes and grasses that when excess Nitrogen fertilizer is added to the crop during the growing season, there is a direct decline in the level of potassium present in the plant. Also, as the Potassium level declines, the Manganese level increases to a dangerously high amount in the plant. The same elemental relationship seems to exist whether soil applied or foliar applied Nitrogen applications are made.
Potassium strengthens plant vascular tissue and under adequate levels, increases the cell wall thickness up to three times. Potassium uptake is inhibited in most plants grown in soils with high magnesium and low oxygen levels. When Potassium and Magnesium levels are too low, use of the fertilizer form Sulfate of Potash Magnesia which is 22% K and 11 % Mg is about the ideal balance between these two elements. Potassium is the plants cellular liquid antifreeze system because it remains in the liquid, increases the salt level and decreases the liquid freezing point.
Remember, the more nitrogen that is added to a crop, the busier the Potassium becomes and the greater the need.