# Calcium Nitrate - Why only 18.8% Calcium for fertiliser?

I brought a kilo of Calcium Nitrate for my hydroponic setup.

The chemical formula for Calium Nitrate is Ca(NO3)2. I did a quick calculation of its weight:

Ca = 1*40.078

N = 2*14.006 = 28.012

O = 6*15.999 = 95.999

Total = 164.085

Hence the % in weight of calcium is ~24.42%.

BUT I see on the fertilizer pack that Calcium is only 18.8% and Nitrogen is 15.5%

Why is this?

• Your math seems fine. The manufacturer, on the other hand, doesn't know how to market the stuff properly. The URL suggests "The content of Ca in our fertilizer is 24% that exceed our foreign analogues on 5%, their calcium nitrate contains only 19% of Ca.", whereas label on the package says "Ca–18.80%". If it's really 98% Ca(No3)2 as they claim, then it's about 24% Ca, as you suggested. – andselisk Apr 21 at 8:05
• Also note that the element content in fertilizers is sometimes given for the corresponding oxide – this is a common practice for potassium (K2O). – andselisk Apr 21 at 8:07
• I understand your point. But even other suppliers, I think this is generally the case. ex: amazon.com/Calcium-Nitrate-15-5-0-0-Fertilizer-POUNDS/dp/… – Nujufas Apr 21 at 8:10
• That's due to impurities. They list a bunch of extra stuff even on the label (chlorine, ammonia salts etc.) and it's easier to claim slightly lower content of the main product to avoid a lawsuit if the product doesn't meet the standards (for instance, gets moisturized due to improper storing conditions). If you really want to know the exact content, don't rely on what manufacturer tells you and do quantitative analysis for calcium yourself. – andselisk Apr 21 at 8:14
• Also note the the calcium nitrate is hygroscopic and is usually provided as tetrahydrate. en.wikipedia.org/wiki/Calcium_nitrate – Poutnik Apr 21 at 8:15