Potassium in Plants and Soil

Potassium is the third primary nutrient. Like phosphorus, it must be added in many areas, but not usually in arid regions. It is highly soluble in water, but since it adheres tightly to clay particles in the soil, it is not easily leached. A single application will last for a growing season in good soil.

Potassium is essential for the growth of plants, and plants take large amounts of it from the soil. Potassium is often referred to as “potash.” The word potash originated in colonial days when wood and other organic materials were burned in pots for the manufacture of soap. The ashes were rinsed with water, which was collected and evaporated. The residue was largely potassium salts. Today, potassium is mined in a similar manner to rock phosphate.

The potassium content of U.S. soils depends on soil weathering, with arid climates having more potassium. Potassium is usually deficient in the heavily-weathered soils of the Southeast and East, but plentiful in the arid West.

Potassium functions within the plant in a number of specialized ways. It is involved in many of the basic chemical reactions in the plant, including the synthesis of protein, carbohydrates, and chlorophyll, and in the storage of carbohydrates. It increases the resistance of some plants to disease and it aids in the formation of oils in oil-bearing seeds. It improves the rigidity of stalks, and helps plants overcome the effects of adverse weather or soil conditions.

In a general way, potassium contributes to the overall vigor of plants. Under normal growing conditions with adequate nutrient supplies, many plants use as much potassium as they do nitrogen, which is three to four times the amount of phosphorus used.

A slight deficiency causes slowed growth and dull green leaves. The amount of starch in storage organs (such as potatoes) decreases, and stems become weaker, causing some plants to fall over easily. A more severe deficiency causes tips and edges of leaves to turn yellow and die. The yellowing moves inward between the veins, leaving the veins green. Because potassium is mobile within plants, it is transported to the new leaves, leaving older leaves deficient. Symptoms show up first in the oldest leaves.

In the soil, potassium leaches more quickly than phosphate, but more slowly than nitrogen. It may leach especially fast from a lightweight, fast-draining container mix. Because potassium may be leached from the soil, annual applications should be made in sufficient quantities to supply the plant’s needs.