aem wrote:

Kathy wrote:

Aha! So the numbers must refer to something. So in a 2-3-4, that's two
somethings of nitrogen per some standard measure. Do you know what the
units are?


N is nitrogen, P is phosphorus, K is potassium. The numbers are
percentages. So if you see 30-10-10 on a bag of fertilizer you know
that it will deliver 30 percent nitrogen, etc., with the remaining 50
percent being filler/carrier and maybe trace elements. -aem


For those interested in a little advanced information (trivia?) there is
a little more to what the numbers on the bag mean.

The first number is Nitrogen and it is expressed as the amount
(percentage) of actual N in the container. Now, you can't have a bucket
of pure N and just put an amount in the fertilizer bag. Nitrogen comes
in different forms. Different fertilizers use Nitrate nitrogen,
ammoniacal nitrogen, urea, or (often) a combination. No matter which
form is used, the first number listed is just the N part of the molecule.

You might think it would make sense if the other 2 numbers used the same
rule. It would make sense but that's not how they do it:

The 2nd number is P phosphorous. Not just the weight of the P but the
weight of the entire P2O5 molecule. The weird part is, if the
manufacturer uses something other than P2O5 to provide the P, the number
on the bag refers to what the weight (percentage) would be if they HAD
used P2O5 to provide the P.

The same for the 3rd number. K, potassium is weighed in as the entire
K2O molecule, even if they actually use no K2O in the bag at all. The
number is how much K2O it would take to provide that much K IF that is
what they had used.

Weird, huh?

Steve