Potassium (K) is positively charged but only slightly, so it sticks to clay particles but loosely. It is generally available to the plant but is leached out of the soil albeit slower than nitrates. 

Some potassium moves into the centre of clay particles and become unavailable. Potassium has many important roles in the plant. 

Enzymes require K to carry out their catalytic role. K is used in transpiration; it controls the guard cells known as stomata which in turn allow CO2 to enter the leaf and water to leave. Without this phenomenon photosynthesis would not take place and the energy for growth would diminish. Magnesium deficiency can be a consequence of high levels of potassium. Mild deficiency causes margins of lower leaves to become yellow and quickly turn brown. 

Right: Two plants showing marginal leaf scorch characteristic of potassium deficiency.

Above: Chlorosis and browning of lower leaves

Severe deficiency results in shortened plants and small dark green leaves. Marginal browning spreads up the plant and lower leaves die prematurely. Flowering is delayed, bloom size reduced and sprays carry fewer blooms than normal. 

Mild excess causes leaves to be very dark green

Severe excess results in checked growth and premature wilting. Symptoms of magnesium deficiency are likely to develop. Calcium deficiency may also develop if the rooting medium is too acidic.

For excess water heavily to leach away excess, omit potash from subsequent feeding programme. Be aware that calcium or magnesium deficiency may follow.