All lawn fertilisers indicate the quantities of elemental nutrients found within the product. The standard convention for designating these quantities is by a percentage ratio. The three main numbers in fertiliser labeling represent the percentage of nitrogen (N), phosphorus (P), and potassium (K), respectively. For example, if a 20 litre bag of Amgrow’s Shirleys fertiliser is labeled (9.5 – 4 – 5) it means there are 9.5% of  nitrogen, 4% phosphorus, and 5% potassium.

Lets have a look at Potassium

Potassium (chemical symbol K) is one of the three major elements most necessary for plant nutrition. 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 fertiliser 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 high clay content tend to have more available potassium than sandy soils. Potassium also occurs naturally in organic fertiliser and compost sources, such as seaweed products, wood ash, and animal feeds and bedding materials.

How Grass Uses 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 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 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.

Fertiliser blends which are high in K (potassium) are good to apply in the lead up to winter as potassium helps the cold hardiness of grass.

Excess potassium

Excess potassium is relatively harmless to the lawn and the environment, but too much potassium likely also means an excess of nitrogen and/or phosphorus, both of which can be harmful. And over-applying nitrogen fertiliser can be detrimental to the lawn itself—either through creating too much top growth or possibly burning the grass plants.