I think it’s imperative to actually know the npk values (lab tested) of your soil before you could add anything to change the values.
Yes, it’s ratiometric. However, that is done by weight, not by volume. You’re completely correct with this math conceptually, but not the right units. The N-P-K values are based on percent by WEIGHT. You can either work in weight directly, or use density measurements to derive weights from the volumes you mix.
If I have that 15-15-15 by weight, and I want to ammend a media that weighs 70 lb/cuft (equal to 9lb/gallon). To obtain 0.15 (100x) I add the soil weight DIVIDED by the ratio (100) or 0.09lb. So you’d add about 40g of fertilizer. Densities vary, so it goes by weight rather than volume. For soil and things like that we talk about dry weight.
I like using ppm because you preserve the ratio when you do calculations, makes it easy to sort out individual element contributions - mg per kg.
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You might want to call the manufacturer for help as they are best suited to answer your questions and give you the proper equations for you to plug the numbers into. Once I work things out I double check with them for clarification when in doubt. They are a very reliable source for these types of questions to help you dial in the dosing. Hope this helps you out.
time to hire someone to tell you how to do it. hypothetically. then send it to a lab for the guaranteed part. sorry, i just couldn’t resist myself.
it has already been mentioned that mixing uses weight not volume. back to the math, it confused the hell out of me just trying to figure it out, but i did find the reason that 15-15-15 is not 15 in all three categories here. more good info here. this subject is not easy to look up for sure. i see the need for the thread and am curious to see the results.
Say you want to mix up 1000kg of soil, at 0.15-0.15-0.15.
That means you need 0.15% of 1000Kg in each element. So that would be 1.5kg of each N-P-K.
Now if the fertilizer you are using is 15-15-15 that means in every kg of fertilizer you have 0.15kg of each element.
Since in this example you are looking for 1.5Kg of each element N-P-K and each Kg of fertilizer is 0.15kg per element you will need to add 10Kg of fertilizer.
Might not be right here so if anyone sees a mistake plz speak up 
that’s what i thought too until i followed a few links to see if i was right. this shit isn’t that easy to figure out. all sorts of other variables like “how much urea is needed to make a 15% solution of nitrogen?”, then you have to figure out how much n is in urea (46%) and figure it from there, so if urea is one ingredient you would need just over 300kg of urea in a 1000kg mix to get 15%, then you need to figure how that affects the other parts. i guess if it was easy everyone would do it. hell, i guess i did learn something this morning.
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They should put the actual elemental amount on those NPK ratios, rediculous that you have to look at the molecular compound then figure out the actual N component etc. I’m struggling to understand the reason behind doing it this way hmmm
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I think you looking for the perfect values that salts can give you.
Organics are not precise, nor does it matter, to a degree.
I simply follow the back of the DTE products, and use what I want to.
NO bagged fertilized organic product is as perfect as the printing on the label, NONE of them.
That is the why to my just mixing up my own, once I realized they are just guessing also.
Plus, there are for me, certain N values, I do not want in my mix.
There are certain NPK values, I want, and the only way around that is to do it, adjust, retry, adjust.
I love salts and organics equally, just build your mix and tweak it.
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Maybe calculate molarity and approach it that way, purely quantitative.
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3 tbsp would actually be .01175
When u figure 256 tbsp in a gallon then 3 tablespoon is only a lil over 1% of that gallon
Times that .01175 by the 15% or .15 of say nitrogen and you have .00175, not even 1% will be nitrogen
You will be 0.175% of n
What is listed NPK wise on the label is measured in the following
N = N 1:1
P2O5 = P which is 43.6% by weight P
K2O = K which is 83% by weight K
so your NPK ratio on a label is actually measuring N,P2O5, and K2O not direct NPK
Also looks like i accidental swapped the p and k percentages last night so i corrected them in that first post.
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That’s not really how soil works. What you need is a soil analysis, then you can calculate what amendments need to be added. If you want to go down that rabbit hole, I suggest you read The Intelligent Gardener by Steve Solomon.
The Intelligent Gardener: Growing Nutrient-Dense Food https://a.co/d/aPxM5oB
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This is exactly why I start on a very low dose and bump a little each time I feed. 
@Motaco I haven’t had a chance to engage with the specifics in this post much yet but I’ll do my normal thing and point towards resources we can use to figure this out together:
This thread basically walks someone through your exact question and the math for it, along with considerations of elemental vs oxide forms and solubility/availability. I love finding these threads on ResearchGate forums or Quora where a bunch of PhDs from around the world all pitch in, the Southeast Asian and MENA agronomists always have the most patient and detailed answers while being super technical about it (shoutout to Drs Antoun and Rao). I find a lot of good tech stuff on these sites when I’m researching something. Hopefully this first link answers your question and the others are useful in doing the calculations for your particular soil.
https://www.researchgate.net/post/Calculation-of-fertiliser-for-potted-experiment
https://aesl.ces.uga.edu/soil/fertcalc/
https://www.haifa-group.com/fertilizer-conversion-calculator
@iceman thanks for the tag
@ReikoX is definitely someone you want to listen to on this one IMO
@FieldEffect is deep in this question right now too so maybe this is just the spring thing for us, what’s up with our dirt, mang?
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Out of likes bro…I’m very happy u dropped by. 
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Thanks for everyone’s help. This is much more complicated than I had imagined.
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Call the manufacturer for the best answer.
@ReikoX I think this is the best soil literature I’ve read, just finishing it now. Writing style is straightforward with enough technical accuracy and engagement to be VERY productive. Some humor sprinkled in too. A+ Recommendation (it’s led me deeper into, and back out the other side of the VERY deep soil rabbit hole)
@Dirt_Wizard nice resources as always. Appreciate you jumping in to help us with our soil questions 
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This topic is underrated…should be utilized more…lol…so I’ll go ahead…lol…
See post linked…