What do the NPK values on lawn fertilisers mean?

By | 15/04/2015
Spreading lawn fertiliser

Whenever you purchase fertiliser or soil improver for your garden, you will see that they are marked with an NPK value. But what does this mean?

N, P and K are the chemical symbols for the three main elements that plants require for healthy growth. These are nitrogen (N), phosphorous (P) and potassium (K). (Potassium is represent by a K, as that is taken from its Latin name “kalium”).

The three NPK values that appear on a fertiliser label represent the relative composition of each of the elements within the given product. For example, a good quality spring lawn feed may have an NPK rating of 12-4-8, which means it contains 12% nitrogen, 4% phosphorus and 8% potassium, therefore you can easily tell that the main component of that fertiliser is nitrogen.

Sometimes the NPK values can be a little misleading as the values of P and K represent the values of phosphates and potash (the element oxides), rather than the true phosphorus and potassium values. In this case the true value will be represented in brackets on the label i.e. 7% (5.8% K), but I won’t get into the technicalities of that issue just yet.

But what do nitrogen, phosphorus and potassium actually do for a plant?

Each element helps a plant grow in a specific way, just as calcium in our diets is great for teeth and bones, nitrogen in the diet of a plant is great for growing leaves. Each element has it’s own role, and each are very important for a healthy lawn. In summary:

  • Nitrogen helps the grass plants grow green leaves, making it look lusher and healthier.
  • Phosphorus helps the grass plants to maintain healthy roots and side-shoots, giving a more stable, thicker lawn.
  • Potassium boosts the grass plants hardiness and disease resistance, helping it to fight infection and cope during times of stress.

The seasons determine the growth cycle of grass. To maintain health it requires different amounts of each of the NPK elements at different times of the year.

In spring, a lawn needs a fertiliser that contains a higher amount of nitrogen, so that it can grow lots of new leaves as quickly as possible, making the best use of the increasing sunlight and warmer conditions.

Conversely, in the autumn, a lawn fertiliser needs to be lower in nitrogen, as leaf growth is slowing down. It need a good dose of potassium, as grass will be dormant through the winter and will be susceptible to disease and fungal infections.

Different NPK for different seasons

Many lawn treatment companies apply the same high-nitrogen fertiliser throughout the year, but this causes problems with excessive leaf growth. Consequently resulting in large amounts of thatch and an unhealthy lawn that can be difficult to bring back to its best.

This is why my lawn treatment service varies the ratio’s of NPK in it’s fertilisation treatments during the year. Spring is different to summer which is different to autumn. It is the best method to keep your grass green and healthy.

If you would like more information about fertilising your lawn, please don’t hesitate to get in touch and I will be happy to advise.

Kris Lord

Further reading about the fertiliser NPK Rating:

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