What's the Difference Between a Fertilizer Ratio and a Guaranteed Analysis on the Bag?

Confused About Fertilizer Ratios?

Ok folks, we need to knuckle down and have a horticultural heart-to-heart about fertilizer ratios and guaranteed analysis…because they are not synonymous!

Pretty much everyone online gets this wrong, therefore most of AI responses that are getting spit out at you through aggregation are also wrong. Just about any time you plug in a question about what type of fertilizer ratio is best for a particular plant, you’re instead provided with a specific guaranteed analysis of a fertilizer. This is misleading and could cause you to use the wrong fertilizer or over apply fertilizer. Whether you're fertilizing your indoor houseplants, garden shrubs and trees, or your vegetable garden, it's key to understand both ratios and guaranteed analysis!

Fertilizer Ratios Explained

Let me explain. (I’ll give you the short form explanation on this topic here, but if you want to dig deeper you can always join me in either my Indoor Plants or Botany courses!)

A fertilizer guaranteed analysis provides the percentage of the macronutrients (N-P-K) (N)= nitrogen (P) = phophorus, and (K) = potassium of the total product weight in pounds, or in pounds per gallon volume, of the bag or container. For example, a 20-20-20 N-P-K GA fertilizer (that’s the number you’ll see printed on the bag) means the weight of the product is made up of 20% nitrogen, 20% phosphorus (as phosphate P2O5), and 20% potassium (as potash K2O). This can help you determine exactly how many pounds of each are actually being applied to your land or containers. The remaining 40% of product weight in that bag or container is composed of other elements from the N-P-K compounds and micronutrients, inert ingredients, amendments, binders, etc. If there are available micronutrients, those will also be listed by percentages in the full guaranteed analysis breakdown on the label.

It’s also possible to see a fourth number, which usually would represent iron (Fe) or sulfur (S), but usually these will be listed in the detailed analysis.

Here’s where things are often misrepresented online: When applying a 20-20-20 guaranteed analysis fertilizer (the numbers you see printed on the bag), you’re not applying a 20-20-20 ratio fertilizer: You’re applying a 1-1-1 ratio fertilizer.

The fertilizer ratio is just that: the ratio of each nutrient against the others. For example a 5-5-5 N-P-K fertilizer is also a 1-1-1 ratio fertilizer. Examples of 3-1-2 fertilizers are a 15-5-10 GA fertilizer and a 30-10-20 GA fertilizer. An example of a 2-3-1 fertilizer is a 10-15-5 GA fertilizer.

Now, pound for pound, while both 5-5-5 GA and 10-10-10 GA fertilizers are 1-1-1 ratio fertilizers, the 10-10-10 provides twice the nitrogen by weight than a 5-5-5 fertilizer. A 20-20-20 fertilizer (also a 1-1-1 ratio) would provide four times the nitrogen by weight than a 5-5-5.

As you can see, a specific ratio fertilizer can manifest as very significant differences in the amounts of each element you are applying. By understanding this concept, you could have a 20-20-20 N-P-K fertilizer sitting on your shelf, but plants you are growing can’t handle or don’t need that high a percentage of N in one dose. By cutting the product application dose of a 20-20-20 in half, you could deliver the same balance of elements and get similar results, using less overall quantity by weight.

For example, fruit trees are reliant on nitrogen stores from a late-winter feeding of a slow release balanced 1-1-1 fertilizer, such as a 10-10-10 N-P-K, usually delivered in a slow-release form, for their spring bud break growth and development. But a higher dose of nitrogen, say in a 20-20-20, either in a quick release granular or liquid, could overdose the nitrogen, encouraging more shoot and foliage development, versus flower and fruit set. If all you have is a 20-20-20, you could cut the application dose in half to apply the recommended balance and percentage of nitrogen for the desired results.

How to Calculate the Ratio of any Fertilizer

Using a simple calculation, you can determine the ratio of any given fertilizer by dividing each number in the GA by the smallest number.

For example for a 15-5-10:

15/5 = 3

5/5 = 1

10/5 =2

So a 15-5-10 GA fertilizer = a 3-1-2 ratio fertilizer.

When someone references using a “balanced” fertilizer, that means one with a 1-1-1 ratio. The same balance and percentages of each macronutrient. An “imbalanced” fertilizer could come in the form of a 3-1-2, 2-0-3, 1-3-2, and so on.

Why are fertilizer ratios useful?

The fertilizer ratio is important because it shows you the relationship between N - P2O5 - and K2O. Different species of plants, as well as plants in different stages of growth (vegetative versus flowering) and age will benefit from different balances of these elements. Why? Because different species of plants use elements differently, and different biological processes use more or less of them during different stages of growth or development.

Guaranteed Analysis - Not So Fast!

Wait a minute, like you said my fertilizer GA shows compounds containing N, P, and K. Am I getting 20% of that compound, or 20% of elemental nitrogen, phosphorus, and potassium? Good question. This is where things get a little complicated.

Only nitrogen is represented in the guaranteed analysis as a total percentage of the free elemental form of N. Phosphorus and Potassium, on the other hand, are represented as a percentage of their respective compounds P2O5, and K2O. That means in a 20-20-20 guarantee analysis N-P-K fertilizer you are getting 20% by weight of elemental nitrogen, and 20% P205 (phosphate), and 20% K20 (potash). So, how much elemental P and K are you actually getting in a 20-20-20?

Let’s break this down how to do this math, starting with nitrogen:

Even though your fertilizer GA % will tell you how much elemental N you’re getting, you won’t see elemental nitrogen listed as an ingredient on your fertilizer. Instead, it needs to be delivered as part of a compound, usually more than one. Common compounds containing nitrogen you’ll find in fertilizers include:

Urea (CH₄N₂O)
Ammonium Nitrate (NH₄NO₃)
Nitrate (NO₃)
Ammonia (NH₃)
Ammonium (NH₄)

There are other forms of nitrogen compounds as well, but these are the most common. To figure out exactly how much nitrogen, phosphorus, or potassium you’re actually getting in a fertilizer, you must use the atomic weight of the element and calculate the percentage of it within the atomic weight of the molecule.

For example, in Urea CH₄N₂O, elemental nitrogen represents about 46% of the molecular compound.

First. We take the atomic weight of each element. You can get these numbers from the periodic table:

Carbon12.01 g/mol

Hydrogen 1.01 g/mol

Nitrogen 14.01 g/mol

Oxygen 16.00 g/mol

Now, we calculate the molar mass

Carbon = 12.01 x 1 = 12.01 g/mol

Hydrogen = 1.01 x 4 + 4.04 g/mol

Nitrogen = 14.01 x 2 = 28.02 g/mol

Oxygen = 16.00 x 1 = 16 g/mol

Total = 60.07 g/mol

To calculate the percentage of nitrogen in urea we divide the total mass of N by the total molar mass of a urea compound, then multiply by 100 to get the percentage.

28.02 g/mol N / 60.07 g/mol Urea = 0.466 x 100 = 46.6 % N

We call this percentage the “conversion factor” when calculating the content of an element within a compound.

Depending on the amount of urea or other compound used in the fertilizer by weight, manufacturers will calculate this conversion and provide the percentage by weight of elemental N on the bag. Therefore you don’t need to run the numbers on N. But you do for P and K if you want to know the exact amount of each in the elemental form.

For phosphorus (P) and potassium (K):

(You can go to the periodic table to find the atomic weights for each element in phosphate and potash and calculate the total molecular weight for each compound and do the same math as for Urea, but I’ve done those conversion factors for you already here).

• Based on molecular weights the conversion factor for phosphorus within phosphate (P205) is 44%.

If you have a 20-20-20 N-P-K fertilizer, you multiply .44% x 20% = 8.8% elemental phosphorus

• Based on molecular weight the conversion factor for potassium within potash (K2O) is .83%

In the same 20-20-20 fertilizer you multiply .83% x 20% = 16.6% elemental potassium.

If your fertilizer bag weighs 20 pounds then by simple arithmetic

20 lbs x 20% N = 4 lbs elemental N

20 lbs x 8.8% P = 1.76 lbs elemental P

20 lbs x 16.6 x 20% = 3.32 lbs elemental K

The remaining 10.2 lbs of material in the bag are inert materials, other elements/micronutrients, or amendments (such as compost).

Understanding how to calculate this is very helpful when fertilizer application instructions for specific plants or crops are offered up in pounds of macronutrients (and micronutrients) per area, size of plant, or diameter of trunk. As they often are.

Soluble or Insoluble N: What’s the difference?

You’ll also find N listed as soluble or insoluble forms. Soluble N (WSN) will be the percentage included that is water-soluble and therefore can be taken up by your plants right away. This can be helpful if a plant is suffering from a nitrogen or other macronutrient deficiency that needs to be quickly corrected. Insoluble N (WIN) is in compounds that need to be broken down to release the elemental N and thus provide a “slow-release” component. In fertilizers labeled “slow release”, at least 33% of the elemental N needs to come from an insoluble form.

There is a lot more we could get into here, but I for a basic understanding of ratios versus guaranteed analysis, I figure this is plenty to take in! If you'd like to dig into more horticultural learning, you can join some of my courses through UCLA Extension, Indoor Plants and Botany