Monotub Substrate Calculator

A monotub is a large plastic tub used for bulk mushroom cultivation. The substrate fills the tub to a depth of 3–5 inches and consists of colonized grain spawn mixed with a bulk substrate (typically pasteurized coir, straw, or manure-based mixes).

Core Formula: Total Substrate Volume = Tub Length × Tub Width × Substrate Depth

Spawn-to-Substrate Ratio: The ratio of colonized grain spawn to bulk substrate significantly affects colonization speed and contamination resistance.

The recommended ratio for beginners is 1:2 (one part spawn to two parts bulk substrate by volume). Experienced growers can stretch to 1:3 or 1:4.

Bulk Substrate — CVG Recipe (Coir-Vermiculite-Gypsum): Per 1 quart of substrate:

• Coco coir: 1 brick (650 g) makes ~8–9 quarts hydrated
• Vermiculite: 2 quarts per brick of coir
• Gypsum: 1 cup per brick of coir
• Water: 3–4 quarts per brick (field capacity)

Worked Example — 66-quart monotub, 4-inch depth, 1:2 ratio:

• Tub interior: ~24" × 16" = 384 in²
• Substrate depth: 4 inches
• Volume: 384 × 4 = 1,536 in³ ÷ 57.75 = 26.6 quarts total
• Spawn (1:2): 26.6 / 3 = 8.9 quarts grain spawn
• Bulk sub: 26.6 × 2/3 = 17.7 quarts CVG substrate
• Coir needed: ~2 bricks
• Vermiculite: ~4 quarts
• Gypsum: ~2 cups

Yield Expectations: A well-colonized monotub typically produces 1–3 flushes. First flush is the largest. Expected yield for oyster mushrooms: 1–2 lb per flush. For cubensis-type: 1–3 oz dry per flush from a 66-qt tub.

Substrate Depth Guide:

• 3 inches: faster colonization, lower total yield
• 4 inches: balanced (recommended)
• 5 inches: higher yield potential but slower colonization and increased contamination risk