It doesn't contain K2O, simply that the amount of potassium expressed as K2O is 46%. It's a standard.
Since the molecular weights of varying ions are different, they standardized the reading to K2O. If the potassium was in the form of K2O, it would equate to 46% of the product weight. But it's not, it's just elemental K attached to NO3. Lets look at N first.
Here, they've listed the product as containing 13% by weight as N, and then described that N as being in the form of NO3.
The molecular weight
of NO3 = 62.0049
The molecular weight of N = 14.0067
62.0049 / 14.0067 = 4.42
So the conversion factor between N and NO3 is 4.42. Applying that to the description on the label gives 57.5% NO3. The easiest way to look at it is to say, the product contains 57.5% NO3, of which 13% is N.
The molecular weight of K2O = 94.196
The molecular weight of K = 39.0983
We need to see that K2O is a measure of a substance with two K for each O. So we must double the weight of K before finding the factor. 39.0983 * 2 = 78.1786
94.196 / 78.1786 = 1.2
So the conversion factor between K and K2O is 1.2. Applying that to the description on the label gives 38.33% K.
Adding these together nets 95.83%. The rest of the weight is made up by impurities.
It looks like they repackage this one
One of the special grades
would be better.