Lack of potassium produces a greater range of symptoms
among different species than does lack of nitrogen. Plants like
the cereal grains, which produce central stalks or shoots, die
prematurely unless provided with this element from the very
beginning. In crops such as lettuce, beets, and carrots, how-
ever, the effects of potassium deficiency are less severe, and are quickly remedied by addition of small amounts of the element
to the nutrient solution.
Unlike nitrogen, potassium does not enter into fixed chemical compounds inside the plant. However, crops like potatoes,
which are predominantly starch, absorb large amounts of it
and produce better-quality crops if an ample supply is avail-
able. It is assumed that the element plays an important role
in photosynthesis of crops rich in starch and sugar.
The deficiency symptoms of potassium can be described as a
premature breakdown of the younger foliage. The leaves
droop and, as deficiency continues, may change in color (be-
coming either lighter or darker according to species) and die.
The first symptoms to be noticed are brown spots on the older
portion of the leaves.
An excess of potassium makes straw and leaves of plants stiffer
than normal. Sometimes the tips of the youngest leaves wilt
and those of the older leaves turn brown. There is no charac-
teristic off-color of the foliage unless growth is slowed down
Very few species can reproduce themselves from the small
amount of potassium contained in their seed. The tuber crops,
such as potato, are a notable exception. They contain fairly
large amounts of this element, can make considerable growth
from that contained in the tuber, and reproduce small tubers
without additional supplies from outside sources.