The N-P-K numbers that are printed on the package of nutrients or fertilizer indicate the percentage of "those" elements (by wieght) that the product contains "As Is". By "As Is" I mean before being mixed, diluted, or spread out in any way, shape, or form - just as it sits, in the package!
Somewhere on that package you will find a section that reads "How To Use" (or something like that). Basically, these are the directions that tell you how much of this product to "mix in", with water or "soil", for diferent applications. It is this "Mixing" of the product that determines the actual, final, strength or potency of the product that the plants are exposed to.
Here is a perfect example! I have two different, 4 lb., packages of "Organic Fertilizer". Both are made of "similar" ingredients (a combination of various organic meals and composted manures or guanos). The main, appearant, difference is that one of them has N-P-K numbers about twice as "Strong" as the other:
Whitney Farms "All purpose" Fertilizer (5-5-5).
FoxFarm "Marine Cuisine" Fertilizer (10-7-7).
The "real" difference is in the directions on the packages. Whitney Farms says to use twice as much product per gallon of soil (2 Tblspns.) as opposed to FoxFarm (1 Tblspn.) per gallon of soil. Following this example, the Whitney Farms product actually delivers about 30% more P&K (but the FoxFarm product "goes farther").
To make a long story short, "It's all about how much you mix in - not just what the N-P-K numbers say!".
