A purpose-built mix for soil blocking. Getting soil blocking right starts with getting the mix right, and most standard potting soils are too light and fluffy to compress into blocks that hold their shape through watering and handling.
Coco coir provides the backbone. It holds moisture evenly, resists compaction over time, and stays consistently hydrated through bottom watering better than peat moss. Sunshine #5 Germination Mix contributes fine sphagnum peat, perlite, and a long-lasting wetting agent that helps the mix wick water uniformly from the tray up through the block. Coarse sand adds weight and grit, so blocks hold their shape and don't tip during watering. Pumice adds porosity without floating to the surface the way perlite does. Worm castings provide gentle, evenly distributed fertility and microbial life with no hot spots that could burn seedling roots. Finished compost rounds out the biology and adds slow-release nutrition for the weeks between germination and transplant.
The result is a mix that compresses cleanly, holds a sharp edge, wicks water from the bottom of the tray, and releases seedlings at transplant time with an intact, dense root ball and no root disturbance.
Works with all blocker sizes:
- ¾" micro blocks for lettuce and fast-germinating crops
- 2" blocks for tomatoes, peppers, brassicas, herbs, and most vegetables
- 4" blocks for potting up — nest your 2" block directly into the 4" block with no root disturbance
Coverage:
The squeeze test: Grab a handful and squeeze hard. It should form a firm ball, hold its shape when you open your hand, and release a drop or two of water at the squeeze. If it crumbles, add water. If it's soupy, add a little more dry mix.
Soil blockers in all sizes are available to use onsite through the East Portland Plant Club Tool Library. Free to use by appointment. Come back when your 2" blocks are rooted out and use the 4" blocker here to pot up without disturbing a single root.
Potting Mix: Soil Blocking Mix
How to activate and use soil blocking mix
What you need:
- This bag of soil blocking mix
- A large bucket or tub (5 gallon minimum)
- Water
- Your soil blocker
- Wide-mesh propagation tray
Step 1 Add water gradually. Pour the mix into your tub. Add water slowly, about 1 part water to 3 parts mix, working it in as you go.
Step 2 The squeeze test: Grab a handful and squeeze hard. The mix should form a firm ball, hold its shape when you open your hand, and release a drop or two of water at full squeeze. If it crumbles, add a little more water. If it's soupy, add a small amount of dry mix.
Step 3 Let the mix sit for 5–10 minutes after the initial mixing, then mix once more. This gives the coir and peat time to fully absorb the water evenly throughout the batch.
Step 4 Fill and press. Load your soil blocker by pressing it firmly into the mix in your tub, wiggling slightly side to side to pack the chambers fully. Eject directly onto your wide-mesh tray. Blocks should hold a sharp edge and not crumble or slump.
Step 5 : Sow and label. Drop your seed into the dibble (the indentation on top of each block). Cover with a small pinch of dry mix or vermiculite. Label your tray immediately.
Step 6 Bottom water only. Never water soil blocks from the top once seeded. Pour water into the tray and let the blocks wick it up from below. Keep about ½" of water in the tray and let it absorb fully before refilling.
Storage: Unused dry mix stores indefinitely in a sealed bag or bucket in a cool, dry place.