Plant Doctor Archive

 FOR a long time I have been puzzled when comparing different packaged fertilisers. Whilst I understand that the 8.3.6 (for example) shown on a packet is the manufacturers’ claimed percentage by weight of N.P.K., how do I compare this with another brand that states 14.44.22? I have two questions:

1. Even the second example only adds up to 40.9% — is the rest of the bag just a filler, apart from minute quantities of trace elements?

2. The first example adds up to 17% — does this mean it has less than half actual fertiliser compared with the other brand. This is important when comparing prices.

  THE debate over fertiliser make-up is always a tricky one. The first consideration is that the percentage of fertiliser in the bag is made up by a weight count of European origins.

The N.P.K content listed on the bag stands for Nitrogen, Phosphorus and Potassium. Therefore, when comparing a fertiliser of N.P.K. 8.3.6 against one of 14.44.22, you would say that there is a lot more nutrients in the higher one, and therefore this may feed for a longer time, or give out a heavier dose of fertiliser at over time.

Each of the three main ingredients looks after different parts of the plants and their function. The easiest way to judge their use is to think in terms of Leaf (N), Root (P) and Fruit (K). A fertiliser of 8.3.6 will be sufficient for the household gardener and will give a good balanced feed to plants. The other fertiliser you mention may be used for commercial purposes and will give good root growth to the plant (P), and flowering/fruiting (K), with less emphasis on the foliage growth.

As said before, fertiliser is made up of mineral rocks and filler. The filler includes other minerals that are important to how the fertiliser will work, how the fertiliser will be taken up, and trace elements are usually included. They are usually don’t appear as a notation of percentage on the bag as it is NPK make-up that you want to know.

Weekend Gardener, Issue 105, 2002, Page 22

• The views expressed here are not necessarily those of the RNZIH
• Content or advice may be out-of-date.