Potassium, the K in NPK, is essential for healthy plant growth and is deemed a
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.