[Newer post with an easier to use formula, here.]Fertilizer has been a sticking point for a while. I could never shake the suspicion that all the talk online, and from Official Garden people, never really explained what those numbers meant. More to the point, how do I combine them to get what I want--which method do I use? Do I just add the numbers together? I managed to pass a chemistry class in college--a while ago--why can't I figure this out!?

I get the basics. N-P-K + vital nutrients; 3 cups of organic or 2 cups of petro-chem slow release fertilizer rated between 5-5-5 and 15-15-15. Blah, blah, blah. And it's not just N-P-K that's important. Ideally, all the nutrients would come from soil that we "grew" via compost.

The catch is that we're using closed containers that are watered from below and use peat/coir as their growing medium--in order to wick moisture properly--and as a result need outside nutrients. The loop is not closed. Because we don't want to spend a lot of money to reinforce our roof structures, we can't use heavy composted soil; light weight medium like peat and coir are our only options.

So.

We use organic--with a few exceptions--which limits our choices, has lower nutrient levels, and is more expensive. Unless there's a sale.

A

local gardening center was changing their product line and in the process getting rid of all their

Espoma products at 50% off. I bought plenty. If you're in Chicago, you might want to see if they've any left.

With all these different nutrients, I have a chance to make my own blend. Let me back up a bit and say that in the past we've used some

Miracle-Gro Organic 7-1-2. Plenty of negatives: Big company that makes most of it's money polluting; Not balanced- too much N, not enough P and K. I think it showed in our results. Add to that our

local supplier of

Bradfield Organics stopped selling large bags. So I thought, this year I'm going to try mixing my own. How hard could it be? A few clicks with google and it'd be sorted out.

Not true. And so the reason for this post, which is basically a copy of an email I sent to H2, my co-blogger.

To begin, I decided what blend I wanted to end up with, and how much I needed.

For my 2

Mammoth Melting Pea SIPs, I wanted 6 cups of slow release 5-10-10. (

Peas/beans fix nitrogen, so the N should be lower. And don't forget the

pea inoculant.)

My base, and all the nitrogen (N), came from Espoma 5-3-3. This means 5% (by weight) of N, 3% P, and 3% K are available to the plant. I also have a bag of Phosphorus (P) from Espoma 0-46-0, as well as Potassium (K) from Espoma 0-0-22

The

5-3-3 Plant-tone (pdf) has all the trace elements.

Now the math.

This link is the first place that I have seen a detailed explanation. Fortunately it's not that complicated. It does make it clear to me that all the people I've talked to/read about how to combine fertilizers really had no idea how to do it and were blowing smoke. Home gardeners just don't do this kind of thing.

A recipe to make 1 pound, roughly 3.5 cups, of 5-10-10.Meaning 1 pound of fertilizer with .05 lb of N available, .10 lb of P, and .10 lb of K. The basic math can be used to create any mix you'd like.

All the fertilizer numbers are by weight, not volume.

The trick is to remember that 1 lb of delivered NPK rated 5-10-10--available to be used by the plants--will weigh more than 1 pound. We're after the weight of the NPK, not the total weight.

• Start with the most complete component, one with a little of all three.

By definition 1 lb of 5-3-3 has .05 lb of N available.

• The 1 pound of 5-3-3 also has .03 lb of P available. I subtract this from the .10 lb that I require, leaving a deficit of .07 lb. A pound of 0-46-0 has .46 of a pound of P available. I don't need that much, only .07 lb. The total amount of 0-46-0 to add to the mix is .07 divided by .46 or 0.15 lb.

• The 5-3-3 has .03 lb of K available. I subtract this from the .10 lb required, leaving a similar deficit of .07 lb. A pound of 0-0-22 has .22 of a pound of K available. I only need .07 lb. So the total 0-0-22 added to the mix is .07 divided by .22 or 0.32 lb.

Result: 1 lb of 5-3-3 plus 0.15 lb of 0-46-0 plus 0.32 lb of 0-0-22 gives me 1 pound of 5-10-10.

Like I said it's confusing. If you put the blend on a scale, you'll have 1.47 lbs of mix to each pound of the actual nutrients delivered. But that's how fertilizer is "measured".

Because SIPs/earthboxes are fertilized by volume not weight, I'll have to estimate how much to use. My SIPs need 3 cups per box. I'll be mixing my fertilizer by weight and applying it by the cupful.

[Newer post with an easier to use formula, here.]

Edited on 6.5.09

To use the above formula, I measured the density of the three components.

• 3 cups of Espoma 5-3-3 weighs 1 pound (454 grams)

• One cup of 0-46-0 weighs 320 grams• One cup of 0-0-22 weighs 390 gramsTo make approximately 3.5 cups of 5-10-10 fertilizer, I need:.15 x 320=.20 or 1/5 th of a cup of 0-46-0

.32 x 390 = .375 or 3/8ths of a cup of 0-0-22

3 cups of 5-3-3