A major plant food is potassium, more commonly referred to as potash when talking about fertilizer. Potassium is certainly the most elusive plant food. In one acre of ground there may be 40,000 pounds of potassium in the top six inches of soil, but only 1 percent of it might be immediately available. Potassium seems to be needed for all the functions of the plant, but the plant does not build it into the structure of its parts. The element may move back into the soil as the plant matures.
It also moves around in the plant, usually going from older to younger tissue. Once the plant dies, the potassium is very easily leached out of it. Scientists don’t know yet exactly why potassium is so important to plants. It just is. The most commonly held theory is that potassium helps the plant resist disease, protects it from cold, and protects it during dry weather by preventing excessive losses of water. It also helps in the formation of plant sugars.
A plant lacking potassium grows slower. The leaves may get yellow streaks in them. Edges and tips of leaves become dry and scorched. In corn, the ears that do develop arc often just nubbins and on the stalks, the space between leaf nodes is abnormally foreshortened. The dwarfing makes the whole stalk shorter and the leaves appear too long for the plant. Stalks of corn or other grains are generally weak where potassium is in short supply and will break or blow over more easily.
In tomatoes, potassium shortage stunts plants. The young leaves become wrinkled, older leaves grayish and yellow along the edges. Light-colored spots between veins turn eventually to a bright orange color before the leaves die. Fruits, if any, ripen unevenly and are abnormally soft. Cabbage along leaf borders turns bronze-colored, then a scorched brown. In carrots, the leaves curl. Beets grow tapered roots instead of fat bulbs. Radishes first show unusually deep green in the center of the leaf and scorching on the edges later.
How To Supply Potassium
Potash is the most difficult fertilizer for organic growers to obtain in quantity, and therefore presents the biggest problem in any large-scale organic venture. Muriate of potash is the main source of potash fertilizer in the United States, but is not recommended nor certified by organic gardeners even though it is mined from natural deposits laid down by ancient oceans. The potash in these deposits is potassium chloride salts, and organicists claim that both the salts and the chlorine in them leave residues that are harmful to the soil.
Instead, organic growers use greensand, also an oceanic deposit. Sometimes called glauconite or glauconite potash, greensand contains approximately 7 percent potash – all in a form available to plants. Greensand is actually more of a granulated clay than a sand, which helps explain why it absorbs and holds water to further aid plant growth. Greensand also contains silica, iron, lime, and phosphorus, plus traces of many other elements. Traditional recommendations call for an application rate of 1 pound per square foot. That’s fine for small gardens, but you may want to spread it thinner on larger plots or fields.