Guaranteed Analysis 101

The detailed label on fertilizer bottles or soil bags is its “guaranteed analysis”. This is the elemental breakdown of what it is composed of. It is described in weight, so 1 L of liquid fertilizer (roughly 1 kG) that is 10% Nitrogen, would have 100 G’s of Nitrogen in it.

It’s common for products to list their macro-nutrients on the front of packages, these three numbers are representative of the Nitrogen (N), Phosphorus (P), and Potassium (K) in that order. The guaranteed analysis describes what the product is at least composed of.  A 10% Nitrogen product could have >10%, but not <10%.  Since fertilizers are prepared in batches, and the quality/ purity of source materials is variable, fertilizer products can slightly vary from batch to batch. 

Before looking at the K2O molecule deeper, it is beneficial to understand a little about what a mole is, not the animal, but the unit.  A mole is a measurement unit used to describe quantities of atoms.  It is very large and is only practical to use in the context of atoms.  To put it into perspective, 1 mol of sand grains would be enough sand to cover the earth several times over.  You can find the molar mass of elements on the periodic table listed as their atomic weight

K2O has a molar mass of 94.2 G/mol, so at 94.2 G of K2O, we know we have 1 mole of that molecule.  K has a molar mass of 39.10 G/ mol, and O has 16 G/ mol.

To figure out how much K we have in our 10% K2O product, we can multiply that 10% by the ratio of the weight of potassium in the K2O molecule and the weight of the molecule. 

So, there is 8.3% K in our product listed as 10%, because it is listing the molecule, not the element.  Many other compounds are listed this way on the product. 

Knowing how to read your fertilizer products is important for managing the health of your crops.  Hopefully after reading this, you will feel much more confident when picking out your next fertilizers.   

While the label depicts many of the element as singular atoms, this is not how this exist in the real world.  All elements either compose parts of molecules or ions.  There are a few different tests for determining fertilizer compositions.  One common test to determine the concentration of elements in a fertilizer product is by mixing them with a reagent.  Reagents are compounds specifically chosen to react with choice elements to form new compounds that can be readily measured.  Potassium is described as Potassium Oxide (K2O) or “Potash” in the analysis.  So, while a fertilizer product may list 10% K2O, this is not 10% potassium.  Remembering that the analysis is described in weight, we can find the percentage or amount of Potassium that is in our product by taking a deeper look at K2O.