Potassium deficiency of woody ornamentals in the landscape

Published: Thursday, February 7, 2013 at 12:23 PM.

As with nitrogen, Florida’s sandy soils are naturally low in potassium.  But unlike nitrogen, potassium deficiency can be kind of tricky to diagnose and treat.  One reason for this is that plants seem to be able to tolerate a wide range of potassium levels without showing signs of deficiency.  Also, high levels of certain elements in the soil will prevent plants from using potassium.  But applying potassium where a deficiency doesn’t exist may lead to a deficiency in some other elements.

In this article we’ll talk about potassium deficiency and describe in some detail its symptoms and treatments.  My information was provided by Professor Emeritus Dr. Robert Black of the University of Florida, Institute of Food and Agricultural Sciences.

Potassium must be carefully balanced with other nutrients, so you have to be very sure that you’re diagnosing any deficiencies correctly.  One general symptom of potassium deficiency is interveinal chlorosis of the leaves.  Interveinal chlorosis is a yellowing of leaf tissue between the veins of the leaf.  You may notice leaf drop occurring later in the season.  If potassium deficiency persists, growth is slowed down, and limbs and branches of the plants may die.

These symptoms, however, don’t always mean potassium deficiency.  Magnesium deficiency, for example, shows almost exactly the same symptoms.  Yet, treating one deficiency, when the problem is really the other, will only make matters worse. If you suspect potassium deficiency, take a sample of soil to your local County Extension Office in which they can send it to the IFAS Soil Testing Laboratory and have analyzed, to determine exactly what the problem is.

After you have determined that your soil needs more potassium, you can correct the problem with fertilizer treatments.  You may use commercial garden fertilizer, such as 8-8-8.  High analysis fertilizers, such as 16-4-8, or you can supply just the potassium by using a compound such as potassium sulfate.

If you decide to use a basic gardening fertilizer, apply it four times a year at the rate of two or four pounds for every 100 square feet of soil.  You may decide to use a high analysis fertilizer.  Theoretically, the high nitrogen content in the high analysis fertilizer could aggravate potassium deficiency symptoms.  But in the landscape, this is rarely, if ever, a problem.

Most soils could benefit from the increase in nitrogen, so it’s okay to use a high analysis fertilizer to treat potassium deficiency.  Apply it four times a year but only use one to two pounds for a 100 square foot are.



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