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.So.

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.

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 grams

To 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

## 7 comments:

My brain hurts.

Mine too.

I spent 3 hours trying to figure this out. It's almost as bad as doing taxes.

What I took away from all of this is that the NPK numbers get thrown around by people who don't have a clue as to what they mean. Why even try to understand it, as long as you follow the simple formulas, a la Earthbox.

I don't think it's that hard to do, to be honest with you. After baking so much bread by weight, I'm really more comfortable with the process.

You could easily convert the weights to volumes. Measure out various amounts by weight and put them in a cup. Make a mark on the side of the cup and use it as your basic measuring tool. You'd need to do it for each of the three components, and that's all.

I think you could boost the 7-1-2 with some P and K using this formula.

I was wondering about combining the Espoma 5-3-3 with that Black Gold tomato and veg fertilizer I got at Grand Street.

The latter has 4-5-3.

It gets me closer...but too much nitrogen for peas?

Or...just use the tomato/veg fert for the peas?

Fertilizer by the Numbers has it's problems. The numbers imply a certain amount of , well, certainty.

I went through the math to show (myself) where the numbers come from and how to combine them in a useful way.

I think that most normal fertilizers would work fine for just about all the configurations we're using. But since I was trying to sort out what works best for Peas--and the internet consensus is lower N--playing with the numbers gives me a way to test various scenarios. Another way is to try a specific fertilizer, unadulterated, and see what it produces.

I get the feeling that many (most?) of our readers don't want to mess around with the fertilizer mix. "It's too complicated! That's not why I garden. I just want to relax." Ugh.

Anyway, on to your question. If you want less N, and more P and K, don't combine the two. You're just adding more N. You'll need a fertilizer with higher P and K to combine with your 5-3-3. Don't know if you have that. I've got extra; come on over and we'll mix up a batch.

There's a similar divide between weight and volume in baking. Home bakers are taught that some version of "It's too complicated to

weighall those ingredients. Don't worry your pretty little head. Here's a measuring cup." Weight is king in any commercial recipe. And for good reason.I put my scale inside a zip lock bag protect it from the fertilizer and I'm in business.

In my case, it's not complicated, just time-consuming. I'd rather be planting.

So the peas are in--thanks much for the nitrogen dust. I have to say it's a trip planting an Earthbox after 14 bucket SIPs.

Didn't intend for my comments on formulas to read as a personal swipe. I see this divide between science and gardening, with science having the numbers to make it more "real", always looking down it's nose at mere gardening. I was trying to demystify the formulas, to say they're something used by blowhards to hide behind. If you (and I) understand them, that power fades away.

I'm putting together a 10-10-10 mix made of leftover Miracle-Gro Organic (7-1-2), Espoma 0-46-0, and Espoma 0-0-22. I'll put up the measurements, in cups, once I get it weighed out.

My thought was to use that as my general fertilizer, dropping down to 5-10-10 for the Peas and Beans.

No swipe taken, Bruce. And power to the people indeed.

Will you sell us some of your 10-10-10?

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