Large Raised Bed Soil Mix: Master Gardener Recipe Review

Proposed Mix (9 cu yd for 21×12×1 ft bed)

• 31% Topsoil
• 31% Mushroom compost
• 20% Composted manure
• 5–10% Sand (bag weight confusion)
• 8% Peat moss

Assessment

• Composted manure at 20% is fine. Fully composted manure has low nutrient content (<0.5% N) and won't burn. "Pig manure" vs. "composted manure" distinction is moot if both are composted.
• Mix is reasonable for drainage and fertility.
• Critical flaw: Volume. 9 cu yd fills the excavated hole exactly. Bed must be elevated above grade for drainage. Add 3+ cu yd to crown the bed 6–12 inches above surrounding soil. Perennials especially need root zone above water table.

Revised Plan

1. Order ~12 cu yd total.
2. Build bed 6–12 inches above grade.
3. Mix thoroughly on-site or pre-blended.
4. Test pH after settling; adjust to 6.0–6.5.

Why Elevation Matters

A 21×12 ft excavated bowl in clay will hold water. Roots drown. Elevation + good mix = oxygen in root zone.