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.