Potassium (K) or Potash (K2O) - Potassium rivals nitrogen as the nutrient element absorbed in greatest amounts by plants. Like nitrogen, a relatively large proportion of plant-available potassium is taken up by crops each growing season. Plants deficient in potassium are unable to utilize nitrogen and water efficiently, and are more susceptible to disease. Most available potassium exists as an exchangeable cation (see below). The slow release of potassium from native soil minerals can replenish some of the potassium lost by crop removal and leaching. This ability, however, is limited and variable. Fertilization is often necessary to maintain optimum yields. Mineral potash is the main source of potassium in fertilizers. Sometimes ashes from burned wood are used to supply this nutrient and to substitute for needed lime. (see Using Wood Ash )
The ratings of the potassium soil test levels are similar to those made for phosphorus. "Below optimum" soil K levels strongly indicate that crops will respond to the application of potassium. Band application (applying fertilizer along the row instead of broadcasting it over the whole soil surface) of a portion of the total requirement may also be advantageous. Crop response means a noticeable difference in growth, appearance, health and/or productivity.
"Optimum" soil K levels indicate that crops may respond to potassium if climatic conditions are favorable.
Soils with just "above optimum" K levels are not likely to respond to application, but they should be maintained at this level. Very high above optimum soil K tests show that you can allow crops to deplete potassium until the soil test drops into the high range.
Potassium is an important plant nutrient for several reasons. Potassium:
 | Increases drought resistance of plants by regulating the opening of stomata in the leaves, which in turn regulates gas exchange between the plant and the atmosphere (carbon dioxide, water vapor). |
| Is associated with stronger stalks and stems, thus reduces lodging. |
| Assists in more than 60 enzyme systems affecting metabolism. |
| Assists in photosynthesis and conversion of sugars to starches and cellulose. |
| Aids in translocation of food within a plant. |
| Increases protein, starch, and oil content of plants. |
| Has been associated with resistance to diseases. |
With all this activity, potassium has not been shown to be a part of the plant's organic structure. Instead, it acts as a policeman in the plant, keeping traffic ways operating efficiently. More specifically, washing dried plant leaves will remove most of the potassium, while leaving essentially all of the nitrogen and phosphorus behind in the leaf's structure.
Plant leaves, when functioning properly, will contain about 2 percent or more potassium. Thus, when grass clippings are removed, or entire plants are removed from the garden, large amounts of potassium are also removed. However, when leaves, stalks and straw are tilled into the soil after harvest, much of the potassium in the plant is returned to the soil.
Potassium does not leach down into most soils as Nitrogen does from surface applications, but it will leach into and through very sandy soils. On sandy soils, K fertilizer should be surface applied at no more than 1 lb. K2O per 1000 square feet.
Cations have charges The clay and organic components of the soil have a negative charge. As a result of these charges, positively charged ions (cations) such as hydrogen H+, potassium K+, ammonium NH+4, calcium Ca2+, magnesium Mg2+, aluminum Al3+, etc. are attracted to the surfaces of clay and organic soil particles. They are held there and exchanged with other ions in the solution or with ions at the plant root's surface.
Clay particles have layers. The layered structure of clay particles provides lots of surface area for holding cations. Most available potassium exists as exchangeable cations.
This site was last updated 11/22/06