Potassium in plant physiology, its effect on yield and quality formation

Source: IPI Bulletin, Berne, Switzerland

Potassium (K) is an essential element for all living organisms. In the plant tissue the content of K is higher than that of other cautions. In fact, potassium is the most important caution in many physiological and biochemical processes. A salient feature of K is the high rate at which it is absorbed by the plant from the soil. Though K is not a constituent of organic compounds, it is universally present in the plant and is very mobile. Some major physiological functions of K can be summarized as follows:

The main function of K is the activation of numerous enzyme systems involved in the formation of organic substances and in the build-up of compounds such as starch or protein.

What K does in plant?

The physiological functions of potassium are of great practical significance in plant growth and horticulture:

K is essential for the development of the root system

K increases the sugar content of crops such as fruits, carrots and sweet potatoes.

K increases the size of fruits

K improves the colour of fruits and flowers

K improves the keeping quality of fruits and vegetables

K is essential for the efficient nitrogen fixation by leguminous crops (e.g. peas and beans)

K makes plant more resistant to drought.

K makes plants more resistant to frost

K makes plants more resistant to a number of diseases and pests.

Of course, potassium does not carry out these functions in isolation. It will display its full effect when all the other growth factors are at an optimum, including the plant nutrients nitrogen, phosphors, calcium, magnesium, sulphur, as well as the micronutrients, such as zinc, boron and others.

Symptoms of potassium deficiency

General description

Potassium deficiency is likely to occur under a wide range of conditions on light textured soils, on soils with very low or very high pH (very acid or alkaline), on peaty soils, under high rainfall, in case of imbalanced use of fertilizers (e.g. relative excess of N, Ca, Mg) and under conditions where crop residues are removed from the field.

In general, symptoms of K deficiency develop as follows:

Reduction in growth rates (giving a stumpy appearance, for example, celery and carrots develop a rosette-like growth).

Leaves are smaller with narrow blades, often coloured dark to blue-green.

As K is mobile, it is transported in the plant from older leaves to actively growing parts. Therefore, the typical K deficiency symptoms start to appear on older leaves.

White, yellow or orange chlorotic sports or stripes, usually starting from the leaf tips and margins.

The chlorotic areas become necrotic (leaf scorch). The symptoms spread to younger leaves, and eventually the whole plant can die.

Roots are poorly developed. Stems remain thin.

K deficient plants are often affected by diseases.

Crop quality of fruits and vegetables is severely reduced; fruits remain small and are dull-coloured.

Symptoms, which look similar to K deficiency can be caused by salt injury fungus attack, spray damage, etc. To avoid a wrong diagnosis, the history of the field should be studied first.

Hidden hunger

It is well to remember that crop yield and quality decline long before visual symptoms develop on the plants. In other cases, the symptoms may disappear after some time, so that hunger signs are visible only at a certain stage of growth. This does not mean that potassium is deficient only during that particular stage. The general appearance of the plants and, finally, yield and quality show clearly that the nutrient supply has been imbalanced throughout the growth. The state of deficiency without an exhibition of visual symptoms has also been called "hidden hunger". There is no doubt that hidden hunger is limiting the full performance of many crops.

It is comparatively easy to convince the grower of his advisor of the necessity to correct visible deficiency by additional application of the respective fertilizer. However, he will have difficulties to understand why he should apply more of the fertilizer, if the plant has hidden hunger, and he cannot see any symptoms. Experiments have shown that fertilizer application, delayed until deficiency symptoms appear, may come too late to avoid low yields and poor quality. The strategy should not be to correct hunger but to prevent it.

Potassium nutrition of horticultural crops

It has been mentioned earlier that nutrient uptake by horticultural crops is equal or higher than that of cereal crops. Table 1 shows the uptake of nitrogen (N), phosphorus (P₂O₅), potassium (K₂O), magnesium (MgO) and sulphur (S)) by a number of fruits and vegetables in comparison to rice.

Generally the uptake of K₂O is higher than the uptake of N or of other nutrients. To ensure an adequate supply of potassium, the grower is advised to develop his fertilizer practices, keeping in mind the following:

The potassium removal (uptake at the expected yield level minus K in residues left on the field)

The soil K status as determined by soil analysis (and the characteristics of the soil to release or fix potassium)

Leaf or petiole K contents as determined by plant analysis (where applicable)

And a number of other factors such as rainfall or irrigation water availability etc.

If possible, part of the nutrient requirement should be met by organic manures (farm-yard manure or compost), the balance by mineral fertilizer. The two major potash fertilizers are:

Potassium Chloride (muriate of potash or MOP) containing 60% K2O and

Potassium Sulphate (sulphate of potash of SOP) containing 50% K2O plus 18% S (Sulphur)

Another potash fertilizer used in horticulture is potassium nitrate, 44% K₂O plus 13% N. All these materials are water soluble. Potassium may also be applied in the form of NPK compound or complex fertilizers.

While chloride is needed only in traces, except for some special plants such as palm trees, sugar or tables beets, sulphur is an essential macro nutrient for all plants. Thus, it is not surprising that SOP has been found superior to MOP in the production of high quality fruits and vegetables in many experiments. As a chloride-free fertilizer with low salt index, SOP is particularly recommended under conditions of intensive cultivation, high rates of fertilizer application and in semi-arid regions with salinity problems.

As rates below 100 kg K₂O/ha, all the potash can be applied as basal dressing at the time of planting. Higher rates should be split into several applications. Also perennial crops like fruits trees may require two or more applications per year, depending on the rainy season(s) or the irrigation schedule. Usually solid fertilizers are applied at some distance from the plant and incorporated into the soil. If the K status of the solid is high, broadcast application of potash fertilizer is the adequate method. At low soil K level, which can be expected when deficiency symptoms are visible, or in the case of clay soils with high K fixation capacity, placement of the potash fertilizer is recommended to increase efficiency. As an ameliorative measure to improve the potassium status of low K soils, deep plowing combined with a very high dressing of K before the establishment of an orchard for deep-rooting crops (e.g. grapes) has been found beneficial in Europe. The rates used range between 800 and 1150 kg K₂O/ha.

Foliar spray of potash is possible. Potash fertilizers are water soluble. However, the rates of K, which can be applied in this way, are too small for a substantial contribution to the nutrition of high yielding crops. Moreover, if MOP is used, leaves may be damaged by scorching.

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