b'In the soil, more than 90% of the potassium is very strongly bound to soil particles. Very little of this will become available to the plant.The rest is bound to, for example, (clay) minerals and is slowly becoming available. A small portion of the total potassium stock in the soil (up to 2%) is readily available and is dissolved in the soil solution or bound to minerals and organic matter. Exchange can then take place with hydrogen, calcium and magnesium.MagnesiumThe fourth main element, magnesium, is the most abundant in the plant after potassium and plays a central role in the chlorophyl, literally and figuratively. Magnesium regulates the activity of important enzymes involved in photosynthesis and is important in the production of proteins by enzymes. More than 300 different enzymes in the plant depend on magnesium.Magnesium easily leaches into the soil.Deficiency symptomsDefect symptoms can be recognized by the yellowing of the old leaves. In very serious deficiency, the leaves turn brown. The leaf yellowing is a result of processes that do not go well in the plant; starch and then sucrose accumulation takes place and photosynthesis is reduced because the photosynthetic molecule is damaged. This damage is irreversible, even after magnesium fertilization, there is no recovery. The root system is smaller and the fruits are softer. Magnesium, like potassium, is stored in the plant cell.113'