Best Super Soil Recipe for Autoflowers: 4 Options Compared

The pick

Half-strength Subcool's Super Soil, layered into a 5-gallon pot. The bottom 40 percent of the pot holds the amended mix. The top 60 percent holds plain potting soil. Autoflowers start flower at week 3 or 4 and finish in 70 to 90 days from seed. A full-strength batch burns the roots before stretch ends.

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Why it wins

• Amendment weights are cut in half, so the mix acts as a mild feed instead of a hot one.
• Seedling roots start in plain soil and reach the amended layer at week 3, when demand climbs.
• One mix covers the whole run. Water only, no bottled nutrients.

Subcool's base recipe

The original calls for 8 bags of base soil at 1.5 cubic feet each, plus:

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• 25 lb worm castings
• 5 lb blood meal
• 5 lb bone meal
• 3 lb bat guano
• 3 lb rock phosphate
• 1/2 cup dolomite lime
• 1/2 cup azomite
• 1/4 cup Epsom salt
• 2 tbsp humic acid

That batch fills about 12 cubic feet. Cook time is 30 days at minimum. 60 days is better.

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Option 1: Half-strength Subcool's

Keep 8 bags of base soil. Cut every amendment weight in half. Add 1 cup kelp meal and 1 cup alfalfa meal for a slow nitrogen release. Mix, moisten, and cook 30 to 60 days in a bin at 65 to 80F. Turn the pile each week.

Option 2: Full-strength Subcool's, bottom third

Same recipe, no cuts. Fill the bottom 30 percent of a 7-gallon pot with the amended mix. Cap with a plain seedling mix. Roots hit the hot layer at week 4. This suits long autoflowers that run 90 days. It is a risk on fast 65-day strains.

Option 3: Clackamas Coot's mix

Equal parts sphagnum peat, aeration, and worm castings by volume. Per cubic foot, add 1/2 cup kelp meal, 1/2 cup neem meal, 1/2 cup crab meal, 1 cup gypsum, 1 cup oyster shell flour, and 4 cups rock dust. This mix runs milder than Subcool's. A plant on it needs a compost top dress at week 5. Cost per cubic foot is lower than the Subcool's batch.

Option 4: Bagged super soil

Kind Soil, Nature's Living Soil autoflower concentrate, and Coast of Maine Stonington Blend ship ready to use. Kind Soil and Nature's Living Soil go in the bottom third of the pot, same as the layer method. Stonington Blend is a base mix, not a concentrate layer, so add amendments or top dress. Prices differ by region and by bag size.

Mixing and cooking steps

1. Lay a tarp. Dump the base soil.
2. Add amendments. Break up clumps with a hoe.
3. Moisten to a squeeze test: one drop of water from a fistful.
4. Bin it. Cover the top. Keep it out of rain.
5. Turn the pile each week for 4 to 8 weeks.

Water and pH

Water at 6.2 to 6.8 pH. Lime and rock dust buffer the mix on their own. Skip nutrient bottles. A plant in a 5-gallon pot on this mix takes about 1 gallon of water every 3 days in flower.

Pot size and timing

5 gallons is the floor. 7 gallons gives more amended volume and less fade at week 6. Autoflowers veg for 21 to 28 days, so the cook happens before you plant, not after. Label the bin with the mix date. A batch that sits 90 days still works.