Substrate Question

The best thing to remember when adding substrate to a tank that is "full" is to remove some water before hand so you don't have a big spill. otherwise, be sure to wash it beforehand so it doesn't make a mess, and then put it in.

For plants, most substrates I believe are capable of growing plants. If you buy something inert and not specifically for plants (no nutrients), you'll want to use root tabs and fertilizers (which you will want to use eventually anyways. You don't want the grains of substrate to be too big or too small. Some sands are almost too small and basic aquarium gravel too big. Although, all should work fairly well either way, the best size is in between, at least from my research. If you're keeping harder water fish like guppies I'd suggest Eco complete. The granule sizes vary which is nice. I have also heard ADA aqua soil and fluorite. Many people also put organic potting soil underneath their substrate. There's really a lot of choices! If you have the right lights and some ferts you can grow many plants either way. If you want cories I believe you're stuck with sand (or that's the best).

For depth I tend to think 2 inches is good but if you have big rooting plants you may want more.

what i remember for substrates (notes are home, i'm not)
for minimum recommended depths

1/4" for a shrimp tank
1" for stem plants
2" for other plants
3" for heavily rooting plants

as for what to use, ... i'd recommend sand, play sand for cheap
pool filter sand is cheap too, but i'm queezy about it's sharp edges

after that if you want colors or anything specific, the price starts to go up.
you can get stuff as simple as play sand, ... or as expensive as (i'm intrigued by this one) sand-blasting glass beads, or ceramic beads (foundry casting media - cerabeads)... if i have the cash i am thinking cerabeads in the future (light weight, round, small/fine - sadly i'm expecting an expensive bill

if your looking for more exotic stuff, chromite, sub-round shape so it's not too rough (relatively speaking as it can be fine grade as well)

most sand-blasting media i'd stay away from due to being very angular (sharp edges - even on sand-sized particles)

then there is basic cheap sand, play sand, ... one of those KISS concepts (keep it simple s___ )

Edit:
as austin said, i'd recommonend organic soil underneith any cap of sand your adding

in a tank full of water, i'd recommend removing all ornaments & fish, just dump in some soil, let it settle, place in some paper towel (not bleached) cover with your substrate cap, ... put your oraments back, plant any plants you have, give the tank time for things to settle down (i honestly have no idea how long) add fish & whatever critters your after

easier to lay down your substrate in an empty tank (no water)

organic soil at the bottom of substrate, ... i agree with the concern about it rotting, that's what we are familiar with for composting.

yet, it's like something else is going on and what comes out is healthy. ... i think.

walstad method, is the biggest name for this (or started this, i dono) concerns are satisfied with very fast growing plants, i don't get the details, but apparently safe for fish in a week or so i think... so i may not get the details about what's going on, but the results say good things.

so worries about rotting, ... i think there's more to it. we are familiar with rotting under certain circumstances, then what is so different about a pile of leaves or grass clippings or other composting projects, ... i haven't explored any details, i am encouraged by the results though