Fertilizer: All About Potassium

N-P-K, Part III – Potassium by Steve Jones, Master Rosarian


Unknown-1The last of the “big three” nutrients for plants is Potassium. Like both nitrogen and phosphorous before it, potassium is vital to a plants development and growth. The third number, potassium is expressed as the percent of K2O, commonly called potash.


Potassium is a naturally occurring element in the soil, about 2.4% of the earth’s total. Feldspars, illite, muscovite, mica, and biotite are the principle mineral sources of potassium. Even though there is an abundance of potassium in the soil, it is mostly unusable. Weathering needs to occur to make the minerals available to plants.


Potassium makes up 1.0% of the plants total weight. It is vital for promoting root growth, carbohydrate metabolism, bloom size and color, offsets excess nitrogen, starch formation and breakdown, nitrogen metabolism and synthesis of proteins, adjustment of stomata movement which maintains adequate water relations, and the development of chlorophyll. Potassium is not a part of the compounds themselves, but rather acts as a catalyst in the reactions that occur. Potassium is absorbed into the plant as the ion, K+. Next to nitrogen, it is the next largest mineral element absorbed by plants.


Potassium leaches rather quickly so needs to be replenished. In areas with heavy rainfall, potassium may be completely leached from the soil. A potassium deficiency shows in the older leaves first. It shows a mottled chlorosis then the margins turn yellow, then brown (dies). It looks like the leaf was scorched. The leaves may turn purple. Young shoots become stunted, stalks are weak, stems are shorter, and blooms become distorted. It is claimed that a deficiency of potassium will increase a plants’ susceptibility to diseases, and produce blind shoots. In our area, it is unlikely that you would see a deficiency of potassium. Heavy clay particles can also interfere with the uptake of potassium since they can displace other mineral ions such as calcium. When potassium is attached to the clay micelle, it becomes unavailable to the plant.


Excessive Potassium causes chlorosis, poor root growth, and wilted new shoots. Since potassium leaches quickly like nitrogen, excesses are rarely seen. pH also rarely affects potassium uptake since its tolerance range is roughly 4.5 to 8.5. Some mushroom composts and manures are very high in potassium and may cause a problem.


Potassium is normally supplied by manmade chemicals such as potassium sulfate (50% K2O), muriate of potash (potassium chloride) (60%), which is the most commonly used chemical, and potassium nitrate (44%). I would not consider buying large amounts of potassium nitrate since it is one of the main ingredients for the production of gunpowder and bombs. A few natural sources of potassium are wood ashes (6%, but very acid), bananas, and blood meal (2.5%). Both superphosphate and Hi Bloom contains 10% K2O. Teemed with phosphorous, potassium is necessary to promote large blooms with good color.