What do Fertilizer numbers mean?

The fertilizer numbers you see when you pick up a bag from your local nurseryman – also known as the fertilizer analysis, or NPK – tell you what plant nutrients it will provide. They represent the percentage of the elemental nutrient in the fertilizer. This page give a brief description of this process, for more details read how to use fertilizer numbers to work out how much fertilizer to use. Fertilizers – whether organic or synthetic – are compounds that contain nutrient elements in forms that will be available to plants. The trick is to apply the amount of fertilizer that contains the quantity of nutrient you need. For example, if the NPK of a fertilizer was 3:2:1, it would contain 3% elemental Nitrogen, 2% Phosphorus and 1% Potassium. To provide, say, 5 grams of Phosphorus per square metre, you would need to apply 5 ÷ 2% = 250 g/m2.

Because the fertilizer numbers are percentages, if you multiply them by 10, you get the number of grams of each nutrient per kilogram of fertilizer. For example, Single Superphosphate is 9% Phosphorus (NPK = 0:9:0), so you will get 90g of actual Phosphorus in every kilo. Other nutrients in this fertilizer are Calcium and Sulfur, they are not included on the NPK, but their analysis would be dealt with in the same way.

A word of warning, however, not all countries use this system. Some fertilizers (and books) may give the fertilizer analysis as N:P2O5:K2O. This is interesting, because in most cases, these compounds do not actually exist in the fertilizer (for instance, there is no K2O at all in Sulphate of Potash, its actually K2SO4) – the numbers are only the calculated equivalent of what would be there if those two nutrients were present in that form. To convert these to NPK, simply multiply P2O5 by 0.44 and K2O by 0.83.

Many fertilizers contain more than one nutrient and can have a wide range of nutrient concentrations. For example, a typical NPK for Poultry Manure is 2:1:1. That is 2%N, 1%P & 1%K. Now compare this with a so-called “High-analysis” fertilizer like DAP (di-Ammonium Phosphate), which has an NPK of 18:20:0. The perceived advantage of high analysis fertilizer is that you need to apply much less to get the same amount of nutrient, but there is actually much more to it than this, especially when comparing organic vs chemical fertilizers.

When looking for a fertilizer, it is best to start by considering the recommendations on a soil test. These may be given in either kg/ha or g/m2, so they don’t compare them directly with the percentages of the fertilizer numbers. Instead, there are a couple of things you can do to help choose the best fertilizer to use:

1. You can work with single-nutrient fertilizers, dividing the amount of each nutrient by the fertilizer number percentage (as above) and then apply each nutrient as a separate fertilizer, or
2. You can also work with compound fertilizers (having more than one nutrient) by using the relative proportions of the fertilizer numbers. Divide all three fertilizer numbers by the Nitrogen (N) number (or one of the others if N = 0) to convert the fertilizer numbers into a ratio. To see how this is useful, let’s look at two different sets of fertilizer numbers: 12:11:5 and 20:18:8. When we divide the numbers in each set by their respective Nitrogen numbers we get 1.0 : 0.9 : 0.4 – they both have the same balance of nutrients.

Trace elements

The process is the same for trace elements, but you will need to look up the nutrient percentage of a separate fertilizer for each one and use this to calculate the various application rates.

An alternative is to use a so-called “shot-gun” product (one that has some of everything). Of course, it will be next to impossible to find one that even comes close to providing the balance of nutrients you need. However, if your requirements are small, you could use a weak trace element foliar spray two or three times during the season, just as a sort of ‘top-up’.

Finally, remember that if your soil pH is below 5.7 – do not apply the trace elements to the soil until Lime applications have had time to bring it above this level. In this case, a foliar spray will be the best option. Also, if the soil pH is above 8.0, do not apply trace elements to the soil at all – again, use foliar sprays instead.