A Closer Look at Potassium for Grass

The last number on a fertilizer label, K is also an essential plant nutrient

Close-Up Of Green Grass Growing On Field
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Lawn fertility is one of the most important aspects of lawn care, so it is important to know even a little about what is in the bag of fertilizer and how it affects your lawn. All lawn fertilizer should be labeled so as to indicate the amount of elemental nutrients in a percentage clearly. The three main numbers represent the percentage of nitrogen, phosphorus, and potassium respectively. For example, if a 50-pound bag of fertilizer contains 20-20-20, there will be 10 pounds each of nitrogen, phosphorus, and potassium. Choosing the appropriate blend should be based on soil type, soil test results, and other factors including personal preference (organic) or legislation (fertilizer bans).

What is Potassium?

Potassium (chemical symbol K) is one of the three major elements most necessary for plant nutrition, along with nitrogen (chemical symbol N) and phosphorus (chemical symbol P). Potassium is mined and manufactured in the form of potash which refers to salts that contain potassium in water-soluble form. It is most commonly used for fertilizer in its inorganic versions muriate of potash (potassium chloride), and sulfate of potash (potassium sulfate).

Potash is abundant in many different soils, but not all of it is available for uptake by the plant. Soils with a high clay content tend to have more available potassium than sandy soils. Potassium also occurs naturally in organic fertilizer and compost sources like seaweed products, wood ash, and animal feeds and bedding materials.

Why Does Grass Need Potassium?

Along with nitrogen and phosphorus, potassium is one of the essential macro-nutrients required in the largest quantities by plants for growth and vigor.Potassium is important in the synthesis of some plant components and the regulation of processes including the more efficient use of nitrogen by the plant. Adding soluble potash (K2O) to the soil helps grass withstand stress, drought, and disease. Specifically, potassium helps maintain turgor pressure in the cells of the plant resulting in a positive influence on drought tolerance, cold hardiness, and disease resistance. As a result, potassium deficiencies in turf may cause and increased susceptibility to drought, winter injury, and disease.

Potassium is mobile in plants and can be taken up in quantities greater than needed for optimal growth. It can be difficult to identify if overconsumption is a problem because little is known about the optimal concentration of potassium in the turf. Although soil tests are the best way to determine the nutrient requirements of the lawn, in some cases it can be difficult to determine anything more than a potassium deficiency. Plant available potassium is constantly changing in the soil and is dependent on many factors which are interconnected. An overall healthy soil should be the goal with potassium levels falling in line naturally - and with the addition of fertilizers.

Fertilizer blends which are high in K (potassium) are often sold as a winterizing fertilizer due to the potassiums effect on the cold hardiness of grass. Consumers need to be aware that terms like winterizer or summer fertilizer are more marketing terms than actual claims of fertilizer benefits.

Is Potassium Run-off Dangerous to the Environment?​​

Because potassium salts are water soluble, they are readily leached into groundwater and can be present in the run-off if over-used. However, potash is not a known pollutant though and is rarely present in concentrations toxic to humans or wildlife. Potassium does not deplete the water of available oxygen like other elements contained in fertilizers.

An excess of potassium would be relatively harmless to the lawn and the environment, but it would probably also mean an excess of nitrogen and/or phosphorus, both of which can be harmful to the environment and over applying nitrogen fertilizer can be detrimental to the lawn - either through too much top growth or even burning the grass.