If it were me ( I like total control )
Buy a ppm meter
Fill a bottle up with salts 1/4 full , add water , shake
Then add this to Rez till ppm of feed is require and then feed
Use ppm as constant
1 Like
I appreciate the advice, but my point here is I am trying to learn how to calculate accurate nutrient ratios from scratch within a home mixed soil.
Sorry brother…a little confused here.
Are you using soil and plan to water down with nutrients, or are you planning on building your soil organically?
Below is a thread that may interest you if you plan on building your own soil.
1 Like
On the bag it mentions npk values but in reality it’s alctually taken as N:P205:K20 and this way the values will be different. Not sure on the values of what @Mr.Sparkle mentioned, but it will change when calculating with hydrobuddy or another similar nutrient calculation app.
Also most fertilizers would state P2O5 and K2O % at the back than just P or K.
1 Like
In bag soil , they usually put in some salts so food is avail instantly ( small amount ) and the rest is organic so like time release
If they put 4w worth of salt in it and watered , it would all be avail instantly and fry plants
Okay. A new Hypothetical situation. I have won the lottery. I have decided to go into business making high quality pre mixed soil. I want to compete with Miracle Grow, Fox Farm, etc.
I am convinced I have mixed the perfect soilless Mix known to man. Whatever you are picturing… there it is. One problem. The nutrient value is 0-0-0. I don’t want to sell a soil with zero nutrient value. I want it to be like my competitors. I have a fertilizer rated at 15-15-15 that I intend to add. I want to sell my soil with a guaranteed analysis at 0.15-0.15-0.15. How much fertilizer do I add to one gallon of soil to make that happen?
By my calculations it would be roughly a little under 3 tbsp. 3tbsp gives me 0.175
2 Likes
I think we may need a lab and a scientist now…
1 Like
Promix is 0-0-0 that’s why so many use it : )
2 Likes
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.
@Dirt_Wizard could you pls chime in on this issue…
1 Like
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.
2 Likes
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.
2 Likes
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
1 Like
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.
2 Likes
Maybe calculate molarity and approach it that way, purely quantitative.
1 Like
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.
4 Likes