Substitute fertilizers optimally based on N-P-K numbers

Goal or Problem

In a garden center, you can find fertilizers for almost any sort of plant in your garden. The problem is that you may end up having too many boxes/bags with fertilizers at home, each of them for a plant or two. Wouldn’t it be nice to know whether some of the fertilizers are interchangeable? Perhaps buying two or three types would be enough for the whole garden.

Idea

You can easily determine whether fertilizers are interchangeable by the N-P-K numbers. You can find them on all fertilizer packagings. The numbers represent the percentage (by weight) of the three major elements required for healthy plant growth: nitrogen (N), phosphorus (P) and potassium (K). Nitrogen contributes to faster growth of plants and to their green color (chlorophyll is largely composed of nitrogen). Phosphorus aids in root system development and flowering ability. Potassium has many functions, such as disease resistance, increase of cold tolerance and drought protection.

Next time you buy fertilizers, look at the N-P-K numbers. If you have two (or more) packagings with the same (or very similar) N-P-K and you need each of them for several plants only, you may consider using just one fertilizer for all of those plants. You will be probably surprised how many “very specialized” fertilizers have the same N-P-K. A typical example is an azalea & rhododendron fertilizer that can be used also for hortensias even though “specialized” hortensia fertilizers are available, too. To double-check, read the small print on the packaging. It may contain more complete list of plant sorts, for which the fertilizer is suitable.

If you already have various fertilizers at home, make the inventory of their N-P-K numbers. You may find out that they are often very similar. Use them where appropriate before buying new fertilizers.

Here is the inventory of the N-P-K numbers of the organic fertilizers I currently have.

Lawn: 9-3-5
Conifer: 9-3-6
Buxus: 9-3-6
Rose: 7-6-12
Clematis: 6-6-8
Rhododendron and azalea: 5-6-8

The fertilizers for lawn, buxuses and conifers contain more nitrogen that makes the plants greener. The fertilizers for plants, which we grow for their beautiful flowers, are richer on potassium. Note that fertilizers for conifers and buxuses have the same N-P-K numbers. Fertilizers for rhododendrons/azaleas and for clematises differ just in the percentage of nitrogen.

Limitations and Side Effects

If in doubt about interchanging fertilizers with the same or similar N-P-K, ask the personnel in the garden center.

Sometimes it makes sense to go for a very specialized fertilizer. It may contain other nutrients, minerals or microbes which aid the growth or bloom of a specific plant. This should be mentioned on the packaging.

References

Understanding Fertilizer Numbers: N-P-K