Potassium, the K in NPK, is essential for healthy plant growth and is deemed a
. It plays roles in photosynthesis and plant food formation as well as transport and storage of plant food. As a result of this, it is also important for nitrogen fixation in legumes because nitrogen fixing bacteria rely on food from their associated plant in order to convert nitrogen from the atmosphere into nitrogen that plants can use. In conjunction with calcium and boron, it is also very important for the development of plant cell walls so among other things, it helps plants resist frost and cold damage. It also controls a plant’s ability to cope with drought and helps plants combat disease and insect damage. Many gardeners know potassium as potash but this term more accurately describes potassium chloride. This is an important distinction as gardeners who add potash to their soil to boost potassium levels may end up increasing soil chloride levels enough to cause toxicity. Potassium is a component of wood ash and this is where the term potash comes from.
A wide variety of factors affect potassium availability including:
- Cation exchange capacity – high levels of clay and organic matter in the soil prevent leaching of potassium though if soil contains little potassium, it also prevents plants from taking it up.
- Other cations – a large excess of other cations in the soil prevents plants from taking up potassium. This is especially true of sodium.
- Moisture – a lack of water prevents potassium uptake.
- pH – low soil pH reduces the availability of potassium.
- Temperature – potassium is less available in cold soils.
- Soil drainage, compaction and aeration – potassium becomes less available as the soil becomes compacted, water logged and poorly aerated.
Ideally, for healthy and productive soil you should aim for a potassium concentration of at least 0.5 meq/100g (milliequivalents – this is a special term used to describe the amount of some elements in soil).
Plants are most likely to develop a potassium deficiency if they are grown in soil that is cold, too acidic, too dry, compacted, waterlogged or otherwise poorly aerated (or any combination of these). Other nutrient imbalances can increase the risk of potassium deficiency.
Symptoms of Potassium Deficiency
The most common symptom of plant potassium deficiency is yellowing of older plant leaves along the edges and/or between the veins. Leaf tissue may also die along the edges and at the tips and in extreme cases, necrosis may spread between the veins. Having said that, plant yields will likely be reduced before either of these symptoms appear. Plants also wilt earlier in hot weather and succumb to diseases and pest attacks more quickly and more often. Legumes will often display symptoms of nitrogen deficiency as well.
Treating Potassium Deficiency
If you notice signs of potassium deficiency, first improve the availability of potassium by ensuring your soil is well drained and aerated and that the pH is appropriate. Potassium is most available to plants when the soil has a pH of 6.5-7.5 but provided it’s between 6 and 8, most plants should be able to obtain sufficient amounts of potassium. If it’s early in the season, a cloche may help to warm the soil and improve potassium uptake. Also ensure that plants are receiving adequate water. The soil should be well drained but that doesn’t mean it should be allowed to dry right out on a regular basis. They key is water as frequently as is required to keep the soil moist but not water logged. If you are adding fertiliser that is high in other cations, change to a different fertiliser and ensure you add plenty of organic matter to the soil.
If your plants are still exhibiting symptoms of potassium deficiency after these actions and a soil test reveals the soil contains insufficient potassium, then a potassium containing fertiliser may be applied. Organic sources of potassium include kelp, wood ash and many plant residues (banana peel is particularly high in potassium). Rock powder contains a variety of nutrients including potassium. Sulfate of potash and potassium nitrate are examples of inorganic fertilisers that contain potassium. Your choice of inorganic fertiliser in particular will be influenced by the levels of other nutrients in the soil (for example sulfur and nitrogen).
Excess potassium does not appear to have a toxic effect on plants. It can induce deficiencies of other nutrients however (particularly nitrogen, calcium and magnesium) so care should be taken to avoid an excess by only ever applying potassium containing fertilisers when required and according to the directions on the packaging.
Potassium in plants and soil
Ways to Treat High Potassium in Soil
Of the three nutrients that all plants need in the largest amounts, potassium is the least understood. Unlike nitrogen and phosphorous, it does not form any part of the plant, but exists in the soil, where it acts as a catalyst to enzyme reactions necessary for plant growth But as with all beneficial soil components, too much potassium can be detrimental to plant growth, as it interferes with the uptake of other substances. There are ways to combat this, however, to assure that your plants get all the nutrients they need and in the right amounts.
The primary purpose of fertilizer is not to feed the plant but to enable it to more readily absorb the nutrients present in the soil. A soil test is the only accurate and definitive way to determine how much of any substance is present in your soil. You can send samples to your local county extension or to a testing lab, or you can use a test kit purchased from a nursery or garden center. If the test shows a high concentration of potassium, it could indicate dense clay soil, which traps the mineral and allows it to build to highly concentrated levels. The results could also mean that the fertilizer you are using contains too much of it.
Establishing the right balance between potassium and other soil components is more about controlling how much goes into the soil than trying to reduce it once it’s there. If a soil test indicates a high level of potassium, literally start from the ground up by not adding more to it in the form of a multipurpose fertilizer. Typical fertilizer blends are generally composed of the three most important substances — nitrogen, phosphorus and potassium — which are indicated on the packaging by the letters N, P and K. Selecting a blend that is low in potassium, or K, or contains none at all, is a first step in assuring that it doesn’t build up to unsuitable levels in the soil.
Plant Distress Signals
Too much potassium disrupts the uptake of other important nutrients, such as calcium, nitrogen and magnesium, creating deficiencies that usually produce visible effects. A calcium deficiency produces irregularly shaped new leaves and blossom end rot on plants, such as tomatoes (Lycopersicon esculentum ), that produce fruit. As frost-tender plants, tomatoes can be grown in U.S. Department of Agriculture plant hardiness zones 2 to 10 if set out into the garden once the soil has warmed to at least 60 degrees Fahrenheit. A nitrogen deficiency is suspected when older lower leaves on plants turn yellow while the rest remain a light green. Plants lacking magnesium will exhibit yellowing of the edges of older leaves that may also develop an arrowhead shape in their centers. While adding more of these substances to correct the imbalance may help, the excess potassium will most likely impact their long-term effectiveness.
A Healthy Balance
When present in the soil in proper amounts, potassium helps with photosynthesis, the process by which plants manufacture their own food using the sun’s energy; helps plants absorb other nutrients more efficiently; creates a favorable environment for microbacterial action; and provides turgor, or the ability of plants to stay upright. Distribute excess potassium more evenly by thoroughly working dense soil until it is loose and friable. Dilute and flush out large amounts of potassium by watering the soil any time it appears dry to a depth of one inch. Schedule any fertilizing within several weeks before planting, so that the potassium doesn’t have time to accumulate during the off-season. To minimize long-term potassium buildup, consider using aged or composted animal manure as a substitute for commercial fertilizers, as its components break down more slowly to keep up with plant demand. If using manure, apply it at a rate of 40 pounds for every 100 feet, and work it into the soil to a depth of 6 to 9 inches.
Ways to Treat High Potassium in Soil. Of the three nutrients that all plants need in the largest amounts, potassium is the least understood. Unlike nitrogen and phosphorous, it does not form any part of the plant, but exists in the soil, where it acts as a catalyst to enzyme reactions necessary for plant growth But …