Oh man screw all that expensive shit. Ill show you a recipe that will save you hundreds and grow some killer pot.
50% Miracle Grow Organic Raised Bed Soil
This stuff here is super cheap and its just as good as their expensive organic potting soil and I use it indoors!
20% Mushroom Compost
20% Cow Manure Compost
10% Perlite
After that now is the amendments
This is how I do it. Ill do the math and mix around the exact amount of soil by converting the gallons total to cubic feet etc. I lay out a large heavy duty tarp I find it easier to mix if its more spread out when adding things rather than in a large heaping pile. So you said you have 16 x 15 gallon pots. Thats around 200 Gallons give or take of actual soil mix you'll be using since you're not filling up the pots to the very top. Thats roughly 25-30 cubic feet. So to make it easier lets say you need total 30 cubic feet.
You'll need:
10 bags of MG Raised Bed soil which is 15 CF $100
8 bags of mushroom compost $40
6 bags cow manure $40
3 to 4 Cubic feet of Perlite $50
After you've dumped all that onto the tarp now start putting in the amendments
16lbs worm castings 4 bags $30
Powdered Dolomite Lime ( dont use hydrated or fast acting you'll burn them) Ill use about 3 cups per 15 gallon pot one bag is enough for a few grows $5
Azomite 3 cups per 15 gallon pot one bag will be good for multiple grows. $50
Epsom salt about a cup per pot for now and more later in the grow if needed so 16 cups total. $10
I dont have a pic up there but you can also add gypsum for sulfur/calcium and soil conditioning. 1/2 cup per pot should be plenty 1 bag $6
3 to 4 bags of garden tone fertilize $60
Dont Add this to the mix yet
6 Bags of sand $30
Dont add to the mix
Now comes the important part. After you've dumped all that out onto a tarp) mix the shit out of it. I mix for ten minutes. Get in there up to your shoulder and mix it very well. Every now and then take the sides of the tarp and pick it up and recenter the pile.
Lay out your pots. Now fill the pots about 1/2 way, take 2 cups of garden tone and mix it into that half of the soil in each pot. Then add the other half amount of soil and put 1 cup of garden tone and mix it only into that half of soil. Try not to mix the first half and the second half of soil together. I do it this way so when the plant get bigger the roots will fill the bottom section and require more food. Its worked well for me this way. I like to water mine a little after all that and let them cook or just sit in the garage for a week or two. Ive also planted directly into it. I wouldn't plant seedlings into this but a transplant thats 3-4 weeks or so old will love it.
Don't forget the sand on top. I like to use DE under the sand so if anything tries to get out also gets shredded. 1.5-2" inches of sand on top will prevent lots of potential problems. I swear by it. Simple play sand is perfect!
Depending on how long you veg or how long your flowering is you probably will have to add towards the end. Using teas throughout or even fish emulsion is awesome. Not too much, but you'll be able to tell if they need more or less. You could just water this and they would make it just fine but if you want to push them a little then teas and fish is awesome.
Hope that helps man, this is what I use and have been using for a while. No need to spend ridiculous money on soil. If you dont use synthetics you can just re-amend the soil and use it again.
Thats a total of $420 for all of this. You will then have a base that you can reamend that will cost you less than $100 bucks a run after that. Also when you re-amend the dead roots from the plants add nice time released nutrients to the soil as the break down.
