The reason for this post is that I want an inexpensive basic 10-10-10 slow release organic fertilizer, and I can't find any. Maybe for good reason, I don't really know. I do have the components, courtesy of a clearance sale at a local garden store, to make my own blend.

*******5.22.10 - I've since found out that the ideal ratio for NPK is 3:1:2.

3:1:2 ratio fertilizers don't supply 'excessive' N. Plants use about 6 times more N than P, so 1:1:1 ratio fertilizers actually supply far more (excessive) P than plants can use in relation to N. The 15-30-15 you suggest actually supplies 12x as much P as plants can use in relation to N, unnecessarily raises the electrical conductivity and level of total dissolved solids of/in the soil, unnecessarily raises pH, and makes it more difficult for plants to absorb water and the nutrients dissolved in water - particularly Fe and Mn.This post is still useful. I'll use the same methodology to make a 9-3-6 fertilizer.*******

Here's how I do it.

Home gardeners and their suppliers are trained to think of volume, not weight, when it comes to fertilizer, i.e. 3 cups of 10-10-10 fertilizer per Earthbox™/18 gallon SIP. Farmers and Growers measure fertilizer by the weight of the available (to the plants) N-P-K, expressed as a percentage of the total weight of the fertilizer. Those are the 3 numbers you'll find on the back of all fertilizer bags. So to come up with a blend, you'll need to use weight to make a batch with the proper proportions and then measure out the amount to use from your blend by volume (in cups).

My goal is a 10-10-10 fertilizer. Meaning 1 pound* of fertilizer with .10 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.

* Chemistry can be very precise. The way it plays with the meaning of words is confusing as hell. Remember that 1 lb of delivered NPK rated 10-10-10--expressed as a percentage of the total weight and available to be used by the plants--will weigh more than 1 pound. We're determining the weight of the available NPK, not the total weight of the blend.

I have Miracle-Gro Organic 7-1-2, Espoma Triple Phosphorus 0-46-0, and Espoma Epsom Plus 0-0-22. Following the basic formulas laid out at this site, it's possible to combine these into a 10-10-10.

You can skip down to the end if you want the answer. In the meantime, I'm going to show my work.

The numbers behind the making of 10-10-10.

To simplify(!) the math, I'm basing this all on 1 pound of fertilizer.

Start with the most complete element, the 7-1-2. It provides .07 lb of N per 1 pound of fertilizer. To get .10 lbs of N, I'll need .10/.07= 1.43 lbs of 7-1-2.

In that 1.43 lbs of 7-1-2 there is 1% of P or .01 x 1.43=.0143 lbs available. We need to credit ourselves for the P supplied in the 7-1-2, 1%, so we subtract .0143 lbs from the .10 lbs required, leaving a deficit of .0857 lbs to be made up using 0-46-0. We divide the .0857 lbs needed by 46%, or .46, to get .186 lbs. That is the amount of 0-46-0 we need to add to our blend to get .10 lb of P available.

Finally we need to account for the K supplied by the 7-1-2. In the 1.43 lb of 7-1-2 there is 2% of K or 1.43 x .02= .0286 lbs. Subtracting that from the .10 lb of K needed leaves a deficit of .0714 lbs to be made up by the 0-0-22. So divide .0714 by .22 to get .323 lbs of 0-0-22 necessary to make .10 lbs of K available to the plants.

Now we have the formula for 10-10-10, delivering 1 lb of N-P-K:

1.43 lbs of 7-1-2

.186 lbs of 0-46-0

.323 lbs of 0-0-22

To use this formula I converted lbs to grams by multiplying each of the amounts by 454. (454 grams = 1 lb). This let me easily weigh them on my kitchen scale, which I enclosed in a zip-lock bag to keep it clean.

I weighed each component separately, adding to a bucket as I went along. As I was measuring them, I made a point of weighing 1 cup of each. This way I can recreate the recipe with a cup instead of a scale.

One cup of 7-1-2 weighs 165 grams

One cup of 0-46-0 weighs 320 grams

One cup of 0-0-22 weighs 390 grams

The Simple Answer

To make a little more than 4.5 cups of 10-10-10

4 cups of 7-1-2

.25 cup of 0-46-0

.375 (3/8ths) cup of 0-0-22

To make 22 cups of 10-10-10.

19 cups of 7-1-2

1.25 cups of 0-46-0

1.80 cups of 0-0-22

Because of rounding errors, i.e. I turned 3.93 cups into 4, the amounts don't scale exactly. You can vary the quantity. Just keep the ratios between them the same.

Each of my thirty SIPs needs 3 cups of 10-10-10, so I'll be making this a few more times this spring.

## 14 comments:

That's much easier. Thanks.

some information about roof garden is also here. But only drainage subject :)

http://www.geosyntheticsworld.com/search/label/Roof%20garden

Epsoma no longer offers Epsom Plus. Any ready substitutes?

Thanks, LH

Not really L,

Assuming it's a brand you trust--based on a reading of the label if nothing else--I'd say any fertilizer that has a high P--potassium--relative to it's other numbers. If the numbers were much different than the Espoma 0-0-22, you'd have to play with the formula a bit.

I meant to say "... any fertilizer that has a high K--potassium--relative to it's other numbers."

I actually found both of these products and ended up mixing up the large batch. I take it you have already planted your lettuces and wonder how this formula seems to be working thus far. Have you done this in the past? Beginners jitters. Thank you.

One addition to the post above-just read your planting notes and see you are putting your 10-10-10 into the strip as well as the Espoma tone at 5-3-3. What is your thinking on using both? Thanks, L

Hi L,

The first year I grew on my roof, 2007, I followed the official Earthbox planting instructions. I think that's a good idea for anyone just starting out. Otherwise you don't have a baseline to compare any adjustments you might make in succeeding years.

I wasn't sure about any of this; as you say, beginners jitters (?). Last year we changed our blend after noticing that the larger plants weren't making it all the way through the growing season.

There's a couple, at least!, pieces of conflicting information around how long the slow release fertilizer continues to feed plants: The Earthbox site claims that you fertilize once in the spring and that's it. The Miracle Gro organic, that I used the last 2 years claims "lasts up to 2 months". That takes me all the way to July 15th, two months short of my goal. The larger plants showed all the signs of not enough fert. Google is a great diagnostic tool, btw. Two years ago I added water soluble (non-organic) Miracle gro to the boxes in mid July. Last year I ended up cutting open the plastic bag I have on top of my boxes to put more organic slow release fert on top of the soil. A real pain. Plus all the organic fertilizers aren't balanced--the reason for this post.

My hope is that with more n-p-k delivered to the plants I won't have to add more fert in the middle of the growing season. It's really trial and error.

I can understand why the official EB company site says one size fits all. It makes things much easier, and it generally works. I think that contrary to what they claim, pre-packaged organic fertilizers--even using 3 cups versus 2 cups of petro-chem--might not have enough n-p-k to make it all the way to fall.

I mix a cup of Espoma 5-3-3 Tone in the box after reading some of the posts on the Earthbox forum claiming that it was a way to improve yields. I can't find that post on the forum, but look here to start. So far everything is going great; nice healthy robust plants.

FYI, I have seen a number of posts that echoed your sentiment that the fertilizer seemed to peter out midsummer. Given that we are both in the Chicago area, my impulse is to build on what you and others nearby have learned from experience. If I caught you right, you seemingly had the same shortfall two summers in a row. I have the very ingredients you mentioned in your recipe, EB says any balanced fertilizer will do, and so unless there is something I am not seeing here, it would seem to be that there's no real harm in beginning by building on that. Is there something I'm missing?

In any event, you have been very generous and gracious with your advice and I really thank you.

Sincerely, L

L,

It sounds like you have the right idea. I think building on other people's experiences is a good place to start.

Are you sure you're really ending up with a 10-10-10? The only nitrogen source in this particular mix is the 7 in the 7-1-2 and it seems like by adding more mass (the P and K rich fertilizers) without adding more N, you can only ever reduce the proportion (down from 7), not increase it. Or are you really just trying to end up with an even proportion, the amount of which you apply can be scaled up to be equivalent to 10-10-10 in nutrients available to the plants? Or maybe I'm misunderstanding something about this wacky standard.

I see I'm not the only one who's confused by the jargon.

We're after a certain amount of each N-P-K component, we don't care about their weight relative to the total amount.

That's the confusing part, because the formula is all about the weight of the 3 components, never about the percentage of, for example, N relative to the total weight of the fertilizer.

From the only relevant link I could find, and the one I relied on to help me sort this out:

"What do the numbers on this soil test report mean? It is a question I hear often, so don’t feel alone if you have asked it. I have previously written an article on what the individual nutrient results mean (Soil Test Results: What Do They Mean?, December 2000 issue of Ag News and Views). However, the nutrient values mean nothing if you don’t know how to apply the fertilizer. I hope this article will answer the application portion of this question. Figure 1 shows a sample soil test report. In the box in the lower left hand corner are the fertilizer recommendations in pounds per acre of actual nutrient. You will notice it says N, P2O5 and K2O. This is the same way nutrients are expressed on a fertilizer tag.

So if the number under the N in the fertilizer recommendation box is 60, it means 60 pounds per acre of actual nitrogen, not 60 pounds of nitrogen fertilizer. It is the same for P and K.A 40 under P2O5 in the fertilizer recommendation box is 40 pounds of actual phosphate not 40 pounds of phosphorus fertilizer."The formula is based on the weight of the 3 compounds and not the percentage.I think this is where people confuse the principle behind the calculation.By the way i don't know of any slow release "Organic Fertilizer".I only know of the Dry fertilizers that comes in two forms,slow release and fast release.It's what most people use since it last the longest resulting in being cost convenient.Also visit my profile to see what my site says about fertilizers.You might like it,or you might not.

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