21 december 2017
Potassium (K) is one of the main nutritional elements in a growing medium. A plant needs it for proper functioning of the stomata in the leaves, for cell elongation and for the functioning of the enzymes in the plant. This month in ‘Focus on nutritional elements’ everything about potassium.
Potassium is absorbed by the roots of the plant as K+ ion. The uptake of potassium strongly increases during flowering and fructification. Flowers and fruits therefore contain relatively a lot of potassium. The nutritional element plays an important role in the functioning of the stomata in the leaves, because potassium is an important part of the guard cells of those stomata. Apart from that, potassium takes care of the cell elongation while also maintaining sturdiness of the crop by osmotic functioning. Potassium is also necessary to make sure that the enzymes in the plant function well. The dry matter of a plant contains approximately 1% potassium.
There is potassium in mineral and organic fertilizers. Potassium can be bound to an adsorption complex, like those that appear in heavily decomposed peat (frozen black peat), clay (in potting soil with clay) and coir pith. In potting soil mixtures with frozen black peat there is always a little bit more adsorption of potassium compared to potting soil mixtures with white peat. This is because the size of the adsorption complex differs per type of peat. Potassium in a potting soil with clay can be adsorbed more strongly. The amount depends on
the quantity of clay in the potting soil and the type of clay (the size of the adsorption complex and its occupation). It is known that the adsorption complex of coir pith is naturally approximately 75% occupied with potassium. Whenever coir pith is not treated before use, a significant amount of potassium (up to 20 mmol/l) will be released in the culture leading to a disbalance in fertilization. Compost contains a high level of potassium, so mostly no potassium needs to be added in a potting soil mixture that contains compost.
A potassium deficiency is firstly noticeable on the edges of the older leaves. Here chlorosis starts, which may be followed by necrosis. The elongation growth is also slowed down, causing the plants to become more thick-set. The potassium deficiency may also cause the leaves to ripple. In conifers the needles may even turn yellow and eventually fall. Plants with a potassium deficiency may become slack sooner and are prone to diseases. A surplus of potassium causes a dark green colour of the crop combined with slowed growth. Also the uptake of calcium and magnesium strongly decreases (antagonistic effect), that may cause blossom end rot in tomatoes and tip burn in lettuce.
Potassium deficiency in pepper (left) and in Cornus alba (right).
Fertilizers used in RHP substrates are checked for the specified levels under the RHP quality mark. Consumers that choose for potting soil with the RHP Consumer quality mark can count on a proper growth of their plants with a minimal level of potassium. Extra fertilizing is generally always necessary after 4 to 6 weeks. For substrates with the RHP Horticulture quality mark, the substrate producer and the grower decide together what the potassium level needs to be, suitable to the culture.
