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Potassium is one of the major elements for crops. It is naturally present in most rocks and soils but must be soluble in water to be absorbed by plants. Potassium is necessary for the storage of nutrients by crops, such as fruits or root crops. Potassium fertilizer supplements potassium from soil organic matter and from bedrock. As a result, a mineral pool is settled in clays and supplies the soil solution progressively according to requirements.
IMPORTANCE FOR PLANT LIFE
Potassium is one of the three major elements essential for crops.
It is involved in the stomata’s functioning and therefore contributes to reduce the plants
transpiration, thus increasing resistance to drought.
It regulates intracellular exchanges and supports the formation of carbohydrates in the leaf. It fosters
their migration to storage organs (tubers, roots and fruits).
Potassium strengthens the cell walls, provides plants with better lodging resistance and resistance
against diseases or parasites aggression.
Potassium is absorbed quite easily by the roots as ions in the soil solution. It is taken up passively with water in proportion with its concentration in the soil solution. Potassium is also very mobile in the plant and between the cells.
Since plants consume potassium without much regulation, an excess of potassium is often released by the root exudates.
Soluble potassium in the soil solution that is available for crops is replenished from the clay-humic complex. This replenishment depends significantly on the humidity conditions in the soil (wet and dry phases) and the seasonal climate. Since potassium is soluble in water, its availability is quickly affected by dry conditions. Conversely, an excess of water can cause potassium leaching.
1. Livestock effluent, crop residue and other organic by-products from human activities are an important resource for fertilization.
2. Potassium is extracted from mines mainly as a mixture of sodium, potassium and sometimes magnesium salts. It undergoes purification to be transformed into potassium fertilizer usable in agriculture.
3. Potassium evolves in the soil in its mineral form as cation K+ . It may be bound in the crystal structure of the soil, adsorbed at the surface of clay minerals and also dissolved in soil water.
4. The leaching of dissolved potassium is a more pronounced in sandy soils with low cation exchange capacity (CEC).
5. Runoff and erosion (potassium bound to solid particles) are other mechanisms that remove potassium from agricultural plots.
6. Root absorption of plants is exclusively from potassium K+ dissolved in the soil solution.
Soil analyses measure exchangeable potassium from fairly similar extraction methods in all laboratories. The interpretation is made by evaluating the analysed potassium with respect to an optimum that would be, in ionic form, 4% of the CEC. It is therefore necessary to know the CEC, in order to correctly estimate the supply of soil potassium.
Sensibility table & Symptomes
Potassium deficiency primarily concerns the older leaves; it is characterised by a yellowing, then browning and desiccation of the limb and finally the edges of the leaf.
Excess & Needs
Too much potassium can affect the quality of production (less extractable sugar in sugar beets or less dry matter in potatoes). Excess potassium can also reduce magnesium absorption. Excess potassium can also interfere with the absorption of iron and manganese if these elements are not readily available.
The total potassium present in the soil is often derived from magma (mica, potassium feldspar), but is blocked in core particles from the original bedrock. Such geological potassium is altered and released, but that takes place over a long period of time and is insufficient in view of crop needs.
Sources of water-soluble potassium that can be used for fertilisation are rarer and are mainly located in in Eastern Europe and North America. These deposits come from old salt and sea beds where the water has evaporated, and which were then covered by other sediments and protected from erosion. These mines mainly contain sylvanite, which is a mixture of different water-soluble salts such as potassium chloride, sodium chloride and magnesium salts, and which are sorted and purified using physical separation processes.
The required fertilization is calculated according to the amount extracted by crops, and how much is needed to maintain an appropriate level. The absolute content of extractable potassium is less relevant but the percentage of K+ cations with respect to the CEC. A rate of 4% is considered satisfactory. The higher the CEC, the higher the K level.
Alternating drying and wetting cycles are favourable. On the other hand, a long wet period or a long dry period prevent the liberation of potassium from clays and the replenishment of in the soil solution.
pH has a lower, albeit indirect effect on the quality of the clay-humic complex and the level of microbial activity – the more microbial activity, the more potassium will be mineralized and transferred into soil solution.
Potassium interferes with magnesium. The order of priority for binding the ions to the clay complex is Ca>Mg>K>Na.