A card-carrying member of the elite 17 elements deemed essential for the growth, development and reproduction of plants, potassium (K) is not an element to grow without. Categorized as a macronutrient, along with nitrogen (N) and phosphorus (P), potassium is required by plants in relatively large amounts. Although it is not an active component of plant organic matter and compounds, potassium is closely involved in the activation and regulation of numerous plant processes. The importance of potassium in these processes is the reason it has earned the nickname “The Regulator”.

What are the Different forms of Potassium?

On fertilizer labels, potassium is represented as its oxide, K2O, and is referred to as soluble potash because of the test method used to determine its concentration levels. In soil and hydroponic solutions, potassium exists as the positively charged ion K+. In the ion form, potassium is readily taken up by plant roots. Soil often contains high levels of potassium as part of other soil mineral compounds, but it is generally in a form unavailable for plant consumption.

There are a few forms of potassium found in soil that are accessible to plants, including readily available, slowly available or fixed and exchangeable forms. Slowly available and exchangeable forms of potassium are found attached or stuck between negatively charged soil particles or colloids. Available-soil potassium, as well as the fertilizer, moves along with water through the root zone and the potassium ions are taken into plant roots when direct contact is made.

Potassium in the Garden

Once potassium has entered a plant, it behaves differently than nitrogen and phosphorus in that it does not become a part of the plant’s organic matter structures. Instead, it enters a vast array of plant cells and is crucial in the activation and successful execution of numerous plant processes, both chemical and physical.

One of the most notable jobs potassium has within plants is its role in the activation and manipulation of plant enzymes. Enzymes act as a catalyst and help induce and speed up the rate of chemical reactions within plants. Potassium activates more than 60 enzymes by helping to manipulate the shapes of enzyme molecules so their active sites are accessible and ready to work.

Potassium can also neutralize negatively charged ions and other compounds within the plant to stabilize its internal pH between 7 and 8—the desired level for optimal enzyme activity. Adequate levels of potassium within a plant’s cells help ensure a higher level of enzyme activity at rates that support vigorous growth.

Potassium also plays a vital role in water and elemental nutrient uptake, and the regulation of the stomata—tiny, pore-like structures on the leaves and stems that open and close to allow oxygen and water vapor to exit plants, and carbon dioxide to enter. Stomata will also stay closed during times of water scarcity to help maintain proper water levels inside plants. The opening and closing of these structures is controlled by specialized cells on their outer edges called guard cells.

Potassium is involved in regulating the function of these cells. When potassium ions and water flood into guard cells, the cells swell and the stomata open. As potassium leaves the guard cells, the stomata will close tightly to conserve water. When potassium levels are low, the stomata will be slower to open or close, and will have a harder time staying closed tight. Stomata that stay closed tight help reduce water loss from plants, and stomata that are more quick to open will allow more carbon dioxide to enter the leaves, resulting in increased rates of photosynthesis. Potassium also assists in the translocation of sugars and starches.

Potassium Deficiencies in the Garden

When potassium levels are too low, it can lead to less disease-resistant plants and also have a negative effect on the structural strength of fruits and vegetables, resulting in a shorter shelf life. Periods of potassium deficiency, even short ones, can cause plants to produce lower yields. This is especially true when deficiencies occur during the critical stages of development, such as during the reproductive phase when flowers and fruits are formed. Potassium requirements and uptake rates increase during the fruiting stage.

Visual symptoms of potassium deficiency will first appear on the older leaves since this element is highly mobile and is moved by the plant to newer areas of growth. The older leaves will begin to develop marginal chlorosis, which will spread to the edges of the leaves and result in a burnt or scorched appearance. Potassium-deficient plants may also wilt more readily as moisture levels are not properly regulated. If a deficiency progresses into the reproductive growth phases, harvestable yields will likely be reduced.

Excess Potassium in Your Plants

It is important to pay close attention to the amount of potassium being supplied to plants. Most plants can absorb more potassium than they need to function properly. This is referred to as luxury consumption and the excess potassium can accumulate within plants.

Excessive amounts of potassium can result in a nitrogen deficiency, and interact unfavorably with calcium and magnesium, causing a deficiency of one or both nutrients. Excessive levels of calcium in the root zone can result in reduced uptake of potassium. In general, plants prefer higher levels of nitrogen and lower levels of potassium during the vegetative growth stage, and the opposite scenario for the flowering and fruiting stages.

Potassium & Hydroponics

Hydroponic nutrients usually contain one of the following forms of potassium: potassium nitrate, mono-potassium phosphate (MKP) or potassium thiosulfate (KTS), but they are definitely not limited to just these three forms. If you are looking for a potassium source void of extra nitrogen or phosphorus, try potassium sulfate. Some potassium sulfates have been approved for organic crop production, and some are even OMRI-certified. Other organic fertilizers containing potassium include composted organic materials such as poultry manure and vermicompost (worm castings).

Before making any changes to your current fertilizer regimen, you need to adequately research and understand the nutrient requirements of each particular plant type being grown at each stage of development. Supplying your garden with the proper amounts of all elemental nutrients will help ensure plants are healthy and productive. Once potassium requirements are carefully dialed in, this essential element will work to regulate the numerous internal functions of plants, allowing them to grow to their fullest potential.