Understanding NPK

As growers, we can feed plants in different ways to get the results we want.

NPK relates to the amount of Nitrogen(N), Phosphorus(P) and Potassium(K) which a fertilizer contains as various compounds.

• Nitrogen(N) supports (and also seems to stimulate) leaf growth

• Phosphorus(p) supports (and also seems to stimulate) root growth

• Potassium(K) supports (and also seems to stimulate) flower growth

You might have noticed that up till now, we have been interested in developing roots in our lawn. I did not recommend a traditional high nitrogen feed (which promotes leaf growth) so early in the year, but recommended an autumn feed containing a lower nitrogen level. The idea here is to hold back leaf growth to develop the roots as much as possible.

Soon we will stop feeding the roots and concentrate on growing green turf (i.e. leaf growth). For this we need a higher nitrogen feed.

How do we know what is in the feed?

Fertilizer manufacturers, by law, have to print an analysis on the container. Usually this is on the side, but in some cases it is underneath.

This autumn lawn feed contains 4%N, 4%P and 12%K. The NPK ratio is 1:1:3 (divide through by the lowest number). This feed contains equal amounts of nitrogen(N) and phosphorus(P). If you are wondering why the autumn feed contains so much potassium(K), it is to strengthen the grass against frost.


This spring lawn feed from the same manufacturer contains 15%N, 3%P and 3%K. The NPK ratio is 5:1:1. This feed contains five times more nitrogen than any other nutrient. It is ideal to promote leaf growth.


I said that we could use 'Phostrogen' in place of an autumn lawn feed. The analysis of 'Phostrogen' is 14-10-27. The NPK ratio is therefore 1.4:1:2.7. Notice that although 'Phostrogen' contains 14% nitrogen, versus 4% for our autumn lawn feed, the ratios are very close: i.e 1.4:1:2.7 versus 1:1:3. This is why it is an effective substitute. The percentages appear so different because they are applied at different rates.

I don't know if you are now more or less confused. But the point is that we want to try to give nutrients in the amounts needed by the plants. Too much of the wrong one is wasteful; it can also cause a build up in the soil, which can lead to other problems.

As far as we as novice growers are concerned, it is only important that we appreciate that there are different fertilizers for different purposes, and to try to apply the correct one at the correct time.

It is the amount of nitrogen in a fertilizer which can cause the most problems. Nitrogen is also used in far larger quantities than the other nutrients, so is most likely to become deficient. We shall see later how to watch for this.

Thank you for being so patient. Next time you are in your garden centre, look at the different fertilizer analyses and try to understand how they relate to their uses. I had better shut up now.