11/12/2012 - By Matthew Shultz

This month’s nutrient of the month is appropriately potassium, the third major or macro nutrient, and naturally the third number on the fertilizer bag. Potassium is a mysterious plant nutrient that often does not get enough “cred”, but should. Many plant physiologists admit to not knowing exactly what potassium does, but they can tell you what happens when a plant is low or deficient in potassium.

Potassium actually plays many roles, but is not a part of the chemical “structure” of plants. A lot of its worth lies in its strong ionic charge, which aids in enzyme activation, improved stomatal activity (better turgor), sugar transportation, water absorption, increased crop yield and disease resistance. So when you’re running low in potassium, all of the above plant functions are less effective.

Potassium leaches readily in sandy soils or soils with a low cation exchange capacity. Because of this, controlled release fertilizers with coated potassium are especially effective. The potassium cation, or as it is often called, “potash”, is usually hooked up with a nitrate (as in many premium fertilizers), a sulfate (sulfate of potash) or a chloride (as in a muriate of potash). Sulfate of potash or sulfate of potash magnesia have the added bonus of being attached to the secondary nutrient sulfur (SOP) and magnesium (SOPM), while potassium nitrate is attached to a premium form of nitrogen.

The downside of muriate of potash is its chloride companion. But when put in a slow release form (coated), less sensitive plants are fine with it. All plants obviously need potassium, but some varieties need it in higher amounts. In the ornamental world, aroids like anthuriums, philodendrons or spathyphyllum love potassium. Palms and cycads are way up on the potash list as well. Agronomically tomatoes and peppers have a high potassium requirement.

But the one plant with the highest demand is.....? You guessed it, bananas! A palm field nursery requires 400 lbs elemental potassium per acre per year. Tomatoes- 400 lbs. per acre per crop. Bananas- a whopping 600 lbs. potassium per acre per year.

As potassium leaches easily, again a controlled release fertilizer (CRF) is recommended. But even these can often seem not to provide enough. Harrells CRF or CRF Plus mixes historically have better potash soil readings than one of our main competitors' slow release products. A good soil reading for a CRF in a soil sample is 60-80 ppm while quicker release readings would be good around 80-150 ppm. Finally, a great component of foliar sprays on most crops is potassium nitrate at 1.5 lbs per 100 gallons water. It is good to have a balance of nitrogen and potash in leaf tissue. A lopsided nitrogen level over potassium will encourage vegetative growth. On the contrary, raising potassium will encourage flowers and fruit.

By Matt Shultz, Horticultural Tech Support Specialist and Sales Representative, Harrell’s LLC and Marshall Horsman, Technical Services Manager, Harrell’s LLC.