Cannabis Soil Mix for Autoflowers: Ratios, pH, Pot Size

Short answer

Autoflowers grow best in a light mix that drains fast. A base of 60 to 70 percent peat or coco coir, 25 to 30 percent perlite, and 10 to 15 percent compost works in most rooms. Target pH is 6.0 to 6.8. Plant the seed in the final pot, 3 to 5 gallons, because autoflowers start to flower at week 3 to week 5 and do not recover from root stress.

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Base ingredients

• Sphagnum peat or coco coir: holds water and holds nutrient ions. Coco needs a calcium and magnesium buffer before use.
• Perlite: adds air space. Use coarse grade, not fine.
• Compost or earthworm castings: slow nitrogen and microbes. Keep the share at 10 to 15 percent or the mix holds too much water.
• Dolomite lime: 1 to 2 tablespoons per gallon of peat mix to lift pH.
• Rice hulls or pumice: swap for perlite where perlite floats or blows away.

A 5 gallon recipe

1. 3 gallons peat or coco coir
2. 1 gallon coarse perlite
3. 0.75 gallon compost or castings
4. 5 tablespoons dolomite lime if the base is peat
5. Mix, wet the batch, and let it sit 24 hours before planting

pH and feeding

Test the mix pH after it sits wet. Peat starts near pH 4.0 and needs lime to reach 6.0 to 6.8. Water at 6.2 to 6.8. Autoflowers in a charged mix need no added nutrients for 2 to 3 weeks. Start at one quarter strength after that and raise the dose each week. Flowering runs 5 to 7 weeks. Overfeeding shows as clawed leaves and dark green color. Flush with plain water at pH 6.5 if that happens.

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Pot size and watering

Use 3 gallons for small strains under 24 inches, 5 gallons for most plants, and 7 gallons for large plants. Water when the top inch of mix dries. Young plants in a large pot drink little, so water in a ring around the stem and wait. Soggy mix causes root rot in 3 to 5 days. A pot with one drainage hole of 1/2 inch or more is enough.

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Common mistakes

• Hot super soil in the top layer. Roots burn in week 1.
• Transplanting. Taproots break and flowering does not wait.
• Fine perlite that packs into the peat and closes air space.
• No drainage hole. Standing water kills roots.
• Feeding at full strength in week 2.