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Potassium is needed by plants

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By Tom Woods
Master Gardener
As we deal with nutrient management laws and high nitrogen prices, we sometimes lose track of just why we really use fertilizers to produce crops.
We know we need them because the soil sample results tell us to apply them. We know adequate fertility leads to improved yields, but why? We have spent so much time looking at the negative aspects of fertilizer that I thought it might be enjoyable to look at why a nutrient like potassium is needed by plants.
Potassium is an essential, major nutrient for plant growth. No other nutrient can do what potassium can do for plants. In many high yielding crops, the potassium content of the plant exceeds the nitrogen content. With an 8-ton yield of alfalfa, 480 pounds of potassium are removed from the soil, 160 bushels of corn removes more than 200 pounds of potassium.
In the plant, the exact function of potassium is not well understood but its primary function appears to be related to plant metabolism. Potassium is vital to photosynthesis and plant respiration (breathing). If potassium is deficient, photosynthesis decreases and respiration increases. This leads to a reduction of the plants carbohydrate supply.
Potassium is also essential to the synthesis of protein in the plant and helps the plant regulate adequate water in the cells. Research has shown proper potassium fertilization has improved corn yields 30-40 bushels per acre in years that have been either excessively wet years or excessively dry.
In soybeans, potassium helps to prevent lodging, reduces mold and mildew growth, and can help to reduce plant stress from cyst nematode, which can certainly translate into dollars at harvest time.
According to the USDA yearbook of agriculture, “More plant diseases have been retarded by the use of potassium fertilizer than any other substance.”
When potassium helps a plant resist disease, it does not do it in a direct way against a specific disease; it however strengthens plant parts that limit entry of invading diseases and improves overall standability of the crop.
If there is a deficiency of potassium, one of the first signs on the plant is leaf scorching or a burned appearance around the margin of the leaves. These will usually appear on the older foliage first but can also occur on young foliage under certain conditions.
Plants will tend to grow more slowly and will have a poorly developed root system. These plants tend to be more susceptible environmental stress and in a crop like alfalfa can directly lead to thinning of the stand. If the alfalfa is mixed with grass, the grass will prematurely crowd out the alfalfa because grasses are better at absorbing potassium than alfalfa.
There is no best way to apply potassium. A combination of pre-plant broadcasting and row application provide the potassium needed for an early start and an adequate reservoir for season long plant growth while limiting the potential of applying too much in the row, which can cause damage to the seedling from excessive salt. Potassium Chloride (KCL), which contains 0-0-60, accounts for more than 90 percent of the potassium sold in the U.S
Send your gardening questions or comments to: Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.