Fertilizer Numbers: How To Make Sense Of The Numbers On Fertilizer Bags

So you’re trying to work out which fertilizer is best for your garden, but all you see is a bunch of numbers that don’t make sense. You’re starting to feel like you need to have a chemistry degree just to feed your lovely plants. Welcome to the intriguing world of numbers on fertilizer bags!

Let’s break it down. Fertilizer labels have three main numbers that are usually bolded. These three numbers tell you what the main macronutrient make up of the fertilizer is – that is, the primary nutrients that plants require being nitrogen(N), phosphorus(P), and potassium(K).

The first number is the amount of nitrogen (N). The second number is the amount of phosphate (P2O5). The third number is the amount of potash (K2O) it contains.

Thus the label you read is known as the fertilizer grade and it is a national standard. It is colloquially know as NPK.

So a bag of fertilizer (or a bottle of liquid fertilizer) with an NPK of 10-10-10 contains 10 percent nitrogen, 10 percent phosphate and 10 percent potash.

These different grades of fertilizer are manufactured by mixing two or more sources of nutrient together to form a fertilizer blend, thus they are commonly called ‘mixed fertilizers’.

There are many different blends made depending on the type of plants it is for. This is because different types of plants require different percentages of the 3 main nutrients in NPK fertilizer.

It’s also possible to get fertilizers with only a single one of the nutrients. A couple of the common Nitrogen sources are ammonium nitrate (33.5-0-0), urea nitrogen (46-0-0), sodium nitrate (16-0-0) and liquid nitrogen (30-0-0). Notice how the last two numbers of the NPK are ‘0’. That is because it only contains Nitrogen as a single source.

Phosphorus is usually provided as 0-46-0 and potash (the common name for Pottasium) as 0-0-60 or 0-0-50.

Calculating Nutrient Content

An example using kilograms:

If you want to get fancy like the farmers do, you can calculate the pounds or kilos of nitrogen in a bag of 10-10-10 fertilizer by multiplying the weight of the bag by 0.10. You can do the same process for calculating the amounts of phosphate and potash. So as an example, a 25kg bag of 10-10-10 contains a total of 7.5 kgs of nutrients: 2.5 kgs nitrogen, 2.5 kgs phosphate and 2.5 kgs of potash. The remaining 17.5 kg weight is just filler, usually sand or limestone.

Another example using pounds:

To calculate the pounds of nitrogen in a 50-lb. bag of 8-0-24 fertilizer, you would multiply 50 by .08, which equals 4. To calculate the pounds of phosphate is very easy because the ‘0’ tells us there is none. To work out the pounds of potash simply multiply 50 by .24, giving you 12.
So a 50-pound bag of 8-0-24 fertilizer has a total of 16 lbs of nutrients with 4 lbs nitrogen, absolutely 0 lbs of phosphate, and 12 lbs potash. The filler amount is this case is 34 lbs.

Selecting a Fertilizer Grade

If you want to really drill down into the detail on what fertilizer is best for your garden, you can have your soil tested. The results of such a test usually comes back with a recommend fertilizer grade you should use. However, another approach is just to get one of the standard home gardener fertilizer packs at a garden or hardware store which usually labels what it is best used for. In general, lawn requires higher amounts of nitrogen than typical vegetable crops, although leafy greens also require a significant amount of nitrogen as well. Some of the typical grades you’ll find labeled for lawns and gardens include:

  • 6-6-18
  • 5-10-5
  • 5-10-10
  • 10-10-10
  • 8-0-24
  • 20-20-20

Spreading Fertilizer

It is very important to spread fertilizer evenly, otherwise, plant growth will be uneven. You may have noticed lawns that look somewhat striped. This can occur when fertilizers are spread unevenly.

To make sure that you get even coverage when spreading fertilizer, it is best to use a spreader.

The two most popular types of fertilizer spreaders are cyclone spreaders and drop spreaders. Cyclone spreaders generally better at spreading evenly, therefore giving you the best results. When doing the job, make sure you overlap your pattern by proceeding in a cross-hatch manner. Apply half the fertilizer in one direction and the rest in the opposite direction. Also, make sure you break up any fertilizer clumps before proceeding to prevent clogging. Clumps tend to form if any moisture gets into the stored fertilizer so make sure you store it in a dry place.

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