Discovering Soil-Block Making, by Eliot Coleman

Soil blocking is a clever way to start strong seedlings whose roots take off as soon as they are transplanted, and it does away with the cost, waste and storage hassle of plastic pots. Here is how market gardener and author Eliot Coleman came to develop and refine the system for small commercial growers.

Artichoke seedlings growing in wooden flats

When Coleman began growing vegetables commercially in 1969, he raised seedlings in wooden flats of potting soil. He kept a set of marker boards with dowels at the right spacing for each crop and pressed them into the soil to mark where each plant should go. Some crops were sown straight into the flats; others were germinated in seed flats and then pricked out into growing flats at the correct spacing.

Watering newly emerged seedlings

When the seedlings were big enough for the field, he cut the flat into cubes with a sharp knife, one plant per cube, like slicing a pan of brownies. Compared with bare-root transplants, each seedling kept its own nearly undisturbed root ball and settled into the field quickly.

He tried early Speedling and plug tray systems when they appeared but liked the results less, because plants seemed less vigorous and slower to grow. Simple geometry explained it: an upside-down pyramid holds only a third of the volume of a cube with the same top width. So he returned to wooden flats while still looking for something better.

Onion seedlings air-pruned in soil blocks

He found it while touring European farms in 1976. One small farm was raising seedlings in soil blocks. The blocks had the larger soil volume of a cube, and because they were separated before sowing, the roots were hardly disturbed at all.

European research also showed that roots of seedlings grown in containers circle the inside wall, while roots in soil blocks air-prune at the edges because of the gap between blocks. After transplanting, soil-block roots grow straight out into the surrounding soil and the plants establish about three days sooner than container seedlings, whose roots first have to stop circling.

Coleman was sold, but he wanted more. Early hand blockers made nothing smaller than a 1½" block, and he wanted tiny germination blocks for crops started on bottom heat, such as tomatoes, peppers, eggplants, and celery, so that more seedlings would fit on each heating mat.

He persuaded a British maker to produce a mini blocker for germination (¾" x ¾" blocks) and matching pins, or dibbles, that press a hole in the top of 2" blocks so sprouted seedlings can be potted on quickly: a square block into a square hole. That made it a complete system for starting seedlings.

Setting out seedlings grown in soil blocks

A few blocks in each flat always end up empty where seeds didn't sprout, but even those had a use. When Coleman's children were young and helped transplant in the field, the empty blocks became summer's version of snowballs, "dirtballs" they gleefully threw at each other.

Further Resources

• Soil blocking: a better way to start seedlings • Article
• Starting seeds indoors: tips and troubleshooting for healthy seedlings • Article
• Hand-held 12-block soil blocker • instructions • PDF tech sheet
• Seed-Starting Date • Calculator
• Three systems for starting seeds indoors • Article
• Soil block makers • instructions • PDF tech sheet
• Shop soil block makers • Products
• Shop seed-starting supplies • Products
• Crops to start early as transplants • Products
• Books by Eliot Coleman • Products