Fertilizer mixes (N-P-K amounts)

[Graham]

Ray, I have carefully re-read your original post

The macronutrients - N, P, & K - are expressed as elemental nitrogen, but as oxides of the other two. Therefore, while a 30-10-10 does contain 30% nitrogen, it contains the equivalent of 10% P2O5, and 10% K2O, meaning that in terms of the elemental ingredients, it's 30% N, 4.4% P, and 8.3% K.

Okay, taking into account my beginner-level knowledge-base...

...what you're saying is, the Nitrogen (N) is the main nutritional element of the fertilizer.

You say the Phosphorous (P) and Potassium (K) elements are oxides... by that, I take it they're of a lesser concentration than their original elementary form... I've seen a number of different definitions for the term Oxide...What do you define an oxide to be?

So, N is the major element in fertilizer and both P & K are still considered Macronutrients but of a lower status than Nitrogen.

Now, Ray, given your 30-10-10 example, the N element is straight-forward; but, how do you reach the P value of 4.4% and the K value of 8.3%?