Soil Blocking for Transplants: Mix Ratios and Block Sizes That Work

Soil blocks replace plastic cells with compressed mix. The roots air-prune at the face of the block, so they do not spiral. The system only works if the mix holds together when wet and breaks apart when the plant is set. Most failed blocks are a peat-heavy slop or a sandy mix that crumbles on the way to the bed.

A mix that stands up

Measure by volume, not weight. A 5-gallon bucket is one unit.

Base mix (makes ~4.5–5 cu ft, enough for 800–900 of the 1.5-inch blocks):

• 2 buckets brown peat moss or coconut coir (if coir, buy low-salt, buffered)
• 1 bucket finished screened compost
• 1 bucket coarse perlite or horticultural vermiculite
• 1 cup lime if the peat tests below pH 6.0 (skip or cut if compost is already alkaline)
• 1/2 cup balanced organic fertilizer (roughly 4-4-4) per bucket of peat, or 2 cups total for this batch

Optional: 1/2 bucket screened soil from a clean bed if you want microbial life and extra weight. Do not exceed one part soil in four. More soil makes a brick.

Wet the mix the day before. Target a squeezed handful that holds a ball and releases a few drops — like a wrung sponge. Too dry and the blocker will not fill. Too wet and the blocks slump into each other in 48 hours.

Block sizes by crop

Seed one seed per block for coated lettuce and most brassicas. Two seeds for uncoated onions, then snip. Cucurbits: one seed, point down, 1/2 inch deep.

A 20-block 1.5-inch tool turns out 20 blocks per press. At a steady pace that is 400–500 blocks an hour including filling the tray. Plan 40–50 blocks per sowing if you sell 40 heads of lettuce a week, plus 10% extras.

Making the block

1. Fill a tub with mix. The mix should glisten but not puddle.
2. Press the blocker straight down until every chamber is packed. Twist 10 degrees, lift.
3. Set on a solid 1020 tray, not a webbed tray. Blocks need a floor for the first week.
4. Seed. Cover lettuce lightly; cover brassicas 1/4 inch; leave celery almost surface.

Do not tamp with your fingers after the press. That smears the face and glazes it.

Water and climate

First 48 hours: mist or bottom-water so the surface never crusts. After germination, bottom-water. Set the tray in 1/2 inch of water until the tops darken, then drain. Top-watering collapses young 1.5-inch blocks.

Greenhouse target: 65–75°F days for lettuce and brassicas, 70–80°F for solanums. Night 10°F cooler. Humidity 50–70%. A dome for the first 2–3 days, then off.

Blocks shrink when they dry. A 1.5-inch block that looks 1.25 inches is thirsty. Water before it gets there or the edges tear.

Planting blocks into the bed

Plant the whole block. Do not shake mix off. Seat it so the top of the block is 1/4–1/2 inch below the bed surface, then pull soil to the stem. Water in with the drip for 30–45 minutes the same day.

A block that sits proud dries out in one afternoon wind. A block buried an inch too deep rots the crown on lettuce.

When blocks fail

• Collapse in the tray: mix too wet or too much compost. Cut compost to 1 part in 4 and dry the batch.
• Will not come out of the tool: mix too dry, or peat not pre-wetted.
• Algae on the face: constant top-watering and low light. Bottom-water and add airflow.
• Spindly stems: 16–18 hours of light too far away. Keep fluorescents or LEDs 2–4 inches off the canopy.

Potting on without wrecking the cube

Tomatoes started in 1.5-inch blocks move to 3-inch blocks at 14–18 days, when roots show on two faces. Set the small block in a pre-pressed hole in the larger one. Do not tease roots. Water from the bottom. Field set 10–14 days later, after a 4-day outdoor harden: 50% shade day one, full sun by day four, still under 40°F nights they stay inside.

Discard mix older than three weeks if it smells sour or grows fungus gnats. Make smaller batches. A 2-bucket batch every Sunday beats a 10-bucket pile that sits and sours by the greenhouse door.