The nutrients in fertilizer determine how much to use

The nutrients in fertilizer need to be worked out before we can see how much and what type of fertilizer to use. Also, there are so many fertilizer suppliers with so many products, you may be wondering "How can I use a SWEP soil test to compare them?"

Fertilizers – whether organic or ‘synthetic’ – are compounds that contain the nutrient elements we want. The trick is to apply the amount of fertilizer that contains the quantity of nutrient your plants need. So grab a pocket calculator and let’s see how it’s done!

First, let’s look at what is meant by NPK. These numbers represent the percentage of major nutrients in fertilizer - Nitrogen (N), Phosphorus (P) and Potassium (K). Because they are percentages, you can simply multiply by 10 to get the number of grams per kilogram. For example, Single Superphosphate is 9% Phosphorus, so you will get 90g of actual P with every kg of fertilizer. Other elements in this fertilizer are Calcium and Sulphur, but they are not included on the NPK.

One word of caution, however, North America and some other countries actually use N : P₂O₅ : K₂O, which confuses things a bit (especially when the label only says "NPK). To convert these percentages into true NPK, simply multiply P₂O₅ by 0.44 and K₂O by 0.83.

Many fertilizers contain more than one nutrient and this tends to complicate things. For example, a typical NPK for Poultry Manure is 2:1:1. That is 2%N, 1%P & 1%K. When looking at nutrients in fertilizer, many people make the mistake of looking for an NPK that matches the numbers on their soil test. If you find one, then OK, but chances are you won't.

Also, remember that the g/kg is 10 times the number in the NPK so even if your requirement was for 20g N/m², 10g P/m² and 10g K/m², you would need one kilogram per square metre. Unfortunately things are almost never that simple.

Suppose your requirement was actually 15g N/m², 5g P/m² and no potash, let’s look at some fertilizers to compare:

First, choose the nutrient that is more important to you. Let’s say this is phosphorus. To work out an application of a blood and bone that would supply 5 g/m² of P, use this calculation:

Of course, each of these fertilizers will supply nitrogen as well. So once you have worked out the amount of each fertilizer that will provide the required 5 g/m² of P, you need to work backwards to see how much Nitrogen you get as well. The table below shows the results for each of the four fertilizers above.

You can now pick the best match and decide how to make up the remaining Nitrogen.

Blends

Often, you will not be able to find a suitable fertilizer, so you may opt for a blend. The easiest way to do this is ask your supplier to prepare one for you, but you can also do it yourself.

The process is the same - choose your preferred fertilizers (assuming they are compatible in a blend – for instance, you cannot blend Superphosphate and Urea). You work out how much of each is required and the resulting mixture is the blend you need.

Lets suppose the previous example also included 15 g/m² of Potassium (K), so the requirement is now 15g N, 5g P and 15g K. Lets also assume the list of fertilizers to choose from (together with the percentage of nutrient they contain and required application rate) is:

The total of the three application rates is 157 g/m², but to make a blend you need to know how much of each to include in 1 kilogram. In other words, the amounts need to total 1,000g, not 157g.

To do this, first work out the proportions you need. Simply divide the total amount by 1,000:

Now divide each of the required amounts by this to get the amount needed to make up 1kg:

So the final blend is:

Now you have the blend, simply treat this as if you had found the ideal product for your needs - simply divide any of the nutrient requirements by the amount of that nutrient in the blend to get the amount to apply per square metre:

NB. There is no need to multiply by 10, because you are not starting with percentages this time.

For instance, we used single superphosphate as our phosphorus source in this blend, so the blend contains 354g Superphosphate/kg x 9% P in superhposphate = 31.86g P/kg of the fertilizer blend.

Then:

This gives an application rate of 157g of the fertilizer blend per square metre.