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
plants showing marginal leaf scorch characteristic
of potassium deficiency.