Potassium ions are found in every living cell, probably because they are necessary to activate enzymes controlling many crucial processes. In plants, these key processes include respiration, photosynthesis, and metabolism of proteins, carbohydrates, and nitrogen.
A role of particular interest that potassium plays is in regulating the opening and closing of the plant’s stomata, thus controlling its water balance. This in turn gives plants drought resistance, and probably accounts for the role potassium seems to play in cold tolerance and disease resistance.
Potassium is also involved in cellulose production.
Finally, potassium is important for maintaining all aspects of produce quality, including shape, color, flavor, and brix.
Mineral sources of potassium include granite dust and greensand, which is marine sediment composed of sand or sandstone.
But for most gardeners, hardwood ash is frequently the potassium amendment of choice. Compost is another excellent source of potassium, particularly if it contains fruit and vegetable waste. Kelp meal and alfalfa meal are also options.
Animal manure usually contains potassium, as well. Sheep manure seems to be a particularly good source of this mineral.
Plants can easily become deficient in potassium simply because their requirements for this mineral are so high. Root crops especially need a great deal of the K in NPK, but all crops use potassium at rates exceeded only by nitrogen.
Symptoms of potassium deficiency appear in older plant tissues first, but may eventually progress to younger tissues depending on the severity of the shortfall. These symptoms include:
- Inability to withstand extreme weather conditions.
- Increased susceptibility to disease.
- Stunted growth.
- Die-back of growing points.
- Lodging in grain crops.
- Scorched, curled leaf edges.
- Paling between leaf veins.
- Purple spots on undersides of leaves.
- Short, brown spruce needles that die and drop readily.
Should you suspect potassium deficiency in your garden, apply one of the natural sources listed above. Greensand and ash in particular are good for a quick boost in severe cases of deficiency.
Plants generally do not experience outright potassium toxicity. However, when the soil potassium levels are excessively high, plants are prone to what is termed “luxury consumption.” This in turn reduces the plant’s ability to take up other nutrients. The following deficiencies may result:
This problem can be avoided and corrected by limiting the amount of potassium applied to the garden or field to what is required.