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Soil mix calculator

Pick a living soil recipe that THCFarmer growers actually run, enter your pots, and get a shopping list in gallons, cups and tablespoons. Then come back for the top dress.

Checked Oct 20, 2026 Source: THCFarmer member recipes (threads 90967, 146147)
What goes in the pot Schematic cross-section of a pot of living soil. The base mix is compost or castings, peat and aeration in roughly equal parts by volume. Dry amendments, measured in cups, are blended through the base. A top dress is added to the surface later and not mixed in. Ratios vary by recipe. FIG. 02 · ANATOMY OF A LIVING SOIL SCHEMATIC, RATIOS VARY BY RECIPE What goes in the pot TOP DRESS Added to the surface later. Not mixed in. BASE MIX Roughly equal parts by volume. CASTINGS PEAT AERATION AMENDMENTS Measured in cups, blended through the base.
01

Mix your soil

Or work out a top dress

Ingredient Amount Buy
Turn on JavaScript to use the calculator, or read the recipes below.

Feeding with Gaia Green instead? Use the Gaia Green schedule (Shaded's method) for amend and top-dress amounts through the whole grow.

02

The recipes

Straight from the members who run them

Clackamas Coot's mix

The classic living soil: equal parts worm castings, peat and pumice, amended with neem, kelp, malted barley, rock dust and gypsum. Organikz posted Coot's current version on THCFarmer.

Shared by Organikz Read the post →

Base mix (by volume)

  • 1 part Premium worm castings
  • 1 part Canadian sphagnum peat moss
  • 1 part Pumice, 1/8 to 3/8 inch

Amendments per cubic foot

  • 1/2 to 1 cup Neem or karanja cake
  • 1 cup Kelp meal
  • 1 cup Malted barley powder, freshly ground
  • 2 to 3 cups Rock dust (basalt)
  • 1/2 cup Gypsum

How they make it

  • Organikz lines the bottom 1.5 to 2 inches of each container with porous lava rock.
  • He uses uncharged biochar as half of the aeration share, and lets the soil cold cook for 2 to 4 weeks, watering and feeding it as if plants were already in it.
  • Add red wigglers and nightcrawlers from the start, about a handful per 2 x 2 feet.
  • Use big containers. He says 15 gallons or larger gives a smooth grow; 10 gallons works but is a struggle.

Coot's top dress (per 10 gallons of soil)

  • 1 cup Malted barley powder
  • 1 1/2 cups Kelp meal
  • 1/2 cup Neem or karanja cake

Organikz tops the soil with 2 inches of good compost, then this top dress. After that, top dress kelp meal and neem cake monthly, and add half an inch of compost or castings between cycles.

7munkee's reusable living soil

A Coot's-style mix 7munkee has reused for three cycles, adding only seed meal and castings between grows. Water only from seed to harvest.

Shared by 7munkee Read the post →

Base mix (by volume)

  • 1 part Worm castings or compost
  • 1 part Pumice, perlite or lava rock
  • 1 part Peat moss

Amendments per cubic foot

  • 3 cups Rock dust
  • 1 cup Gypsum
  • 1/2 cup Neem seed meal
  • 1/2 cup Kelp meal
  • 1/2 cup Crustacean meal
  • 1/2 cup Malted barley

How they make it

  • Add water equal to about 10 percent of the soil volume, plus some worms.
  • Wait a week, then plant.
  • Water only from seed to harvest. Between grows, 7munkee adds only seed meal (and some castings) before the next run.

Happyreefing's Gaia Green mix

A simple amended mix for autos and photos: castings, peat and perlite with Gaia Green 4-4-4, Azomite and seabird guano, then a flower top dress. Waters when dry and adds a weekly compost tea.

Shared by Happyreefing Read the post →

Base mix (by volume)

  • 40% Worm castings or compost
  • 40% Peat moss
  • 20% Perlite

Amendments per gallon

  • 3 tbsp Gaia Green All Purpose 4-4-4
  • 2 tbsp Azomite
  • 1 tbsp Seabird guano

How they make it

  • Water when dry, and give a compost tea once a week for the microbes.

Happyreefing's flower top dress (per gallon of soil)

  • 1 tbsp Gaia Green Power Bloom 2-8-4
  • 1 tbsp Seabird guano

Posted as "top dress in flower with Gaia Green Power Bloom 2-8-4 and seabird guano, 1 tablespoon per gallon". Read as 1 tablespoon of each.

CookiesLikeWhoa's simple ProMix mix

The quickest mix here: ProMix, perlite and castings with Gaia Green 4-4-4 and mycorrhizae. Built for 5-gallon pots.

Shared by CookiesLikeWhoa Read the post →

Base mix (by volume)

  • 50% ProMix
  • 25% Perlite
  • 25% Worm castings

Amendments per gallon

  • 3 tbsp Gaia Green All Purpose 4-4-4

How they make it

  • Add some mycorrhizae and you're done.
03

What growers here actually run

From the THCFarmer archive

  • Trying ProMix BX with perlite and Gaia Green: 20 gallons of BX, 1.5 cups of 4-4-4 and 6 to 7 gallons of perlite, rested 3 weeks before planting.

    Goodshit97 Read the post →

  • Lets the soil sit at least 90 days before use. Adds about 100 worms and lake shell and leaf mold collected locally.

    Novaracer69 Read the post →
04

Threads worth reading

Discussions this page draws on

05

Go deeper

Learn guides on THCFarmer

06

Common questions

What does cold cooking mean?

Mixing the soil and letting it sit moist for a few weeks before planting, so the microbes start breaking down the amendments. Organikz cooks Coot's mix for 2 to 4 weeks. 7munkee waits a week.

Are the parts measured by volume?

Yes. One part is the same volume of each ingredient: a bucket, a gallon or a cubic foot. The calculator splits your total volume between them.

Can I reuse living soil?

That's the point of these mixes. Between grows, top dress or re-amend instead of throwing the soil out. 7munkee is on a third cycle adding only seed meal and castings.

Where do I find malted barley powder?

Home-brewing shops sell malted barley. Grind it fresh just before use, as Organikz recommends.

How much soil do I need for my pots?

A 5-gallon pot holds roughly 4 to 5 gallons of soil, about two-thirds of a cubic foot. Enter your pot size and count and the calculator does the rest. Buy a little extra, since loose mixes settle after watering.

Still not sure? Ask the growers.

Post your recipe, pot size and what you're growing. The living soil crowd here will tell you what they'd add or leave out.

07

More grow tools

These numbers are starting points, not rules. Strains, water, medium and light all change what a plant wants, so watch your plants and adjust.

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