Soil Blockers: My New Favorite Toy

By Marsha Goldberg, Fairfax Master Gardener
soil blocksIf you are a seed starter, like me — someone who appreciates the challenge of starting seeds, not to mention the economy and wide range of choices — you’ve probably tried many different containers for getting your seeds started. I tried many over the years — plastic trays with cells, cells in Styrofoam blocks, peat pots and pellets, containers made from folded newspapers — but none ever worked as well as I would have liked. So it was with great interest that I read about soil blockers in a number of books, including The New Organic Grower by Eliot Coleman and Cool Flowers by Lisa Ziegler.

A soil blocker is a metal mold with a plunger-like device that allows you to make cubes of soil. You push the mold into a soil mix, then push the blocks out onto a tray. You can make cubes in a variety of sizes from ¾ of an inch to 4 inches. You can sow seeds in the ¾-inch cube and when they germinate, put them into a ¾-inch indentation that you have made with the blocker in the 2- or 3-inch cube. Or, you can skip the ¾-inch cube and sow directly into the larger cubes, although this is not as space efficient if you’re sowing a large number of seeds. If you use the right soil mixture, it holds together so you don’t need pots or containers. When you pick up the cubes to transplant them, the soil does not fall away from the roots of the seedling, so you don’t disturb the roots at all; the plants do not suffer from transplant shock.

Since the seedlings are not in pots, the roots are stronger because they receive extra oxygen and they are air-pruned; that is, the roots reach the edge of the block but don’t go past it until you place the plant in the larger block or in your bed. Many companies such as Johnny’s Selected Seeds sell a soil mix specially formulated for soil blockers, but you can easily make your own. There are numerous recipes on the Internet and some differing thoughts as to whether you can use plain potting soil. The keys are to include fibrous material such as peat or coir so the cubes hold together and to use just the right amount of water. Too much or too little water, and the cubes turn to mush or fall apart. I was able to get it right in two or three attempts. I usually mix up a big batch of all-purpose potting soil at the beginning of the planting season to keep handy: equal parts garden soil, compost, perlite (or vermiculite), and coir (which I now use instead of peat). That worked perfectly once I achieved the right moisture level. Some websites recommend adding greensand, colloidal phosphate, and bone meal as well for the minerals they provide, but that is not strictly necessary. Eliot Coleman also recommends using lime to offset the acidity of peat, but if you are using coir that should not be needed.

There are a few downsides to using soil blockers. Most notably, the initial expense is somewhat high. I bought a set for $55 with a ¾-inch blocker, which makes 20 cubes at once; a 2-inch blocker, which makes 5 cubes at a time; and a set of inserts. You can attach the inserts to the blocker to make divots in the cubes in which to place seeds or, in the 2- or 3-inch block, to make the indentation in which to place the smaller cube. You can buy the blockers separately for about $30 each depending on the size. Since the 2-inch blocks worked well for me, I don’t have a need for the 3- or 4-inch blockers, although I may splurge at a later time. If you are hesitant about the cost but a bit handy, see the link below to instructions for making your own blocker. In considering the initial expense of buying the blockers, remember that you won’t have the recurring expense of buying pots or soilless seed-starting mix each year; in time, the blockers will pay for themselves. I also had to buy the materials to make the potting mix, but that seems more economical than buying seed-starting mix, and I always keep a big batch of potting mix on hand anyway.

Also, making the blocks takes time and can be tedious, and mixing the soil and water to make the blocks is a bit messy. I mixed mine up in a deep plastic bin and worked near a water source.

The advantages? I start a lot of seeds, so the system will pay for itself within a year or two. The cubes — both sizes — are easy to handle, and when it is time to transplant I merely pop the 2-inch cube into the ground. There’s no need to dislodge the seedling from a cell or a pot, so I don’t have to handle the plant excessively and this means a lot less damage to the seedlings. I am able to make a large batch of ¾-inch blocks that takes up a relatively small amount of space. I collect clear plastic clamshell containers from the salad bar at the market — I sneak a few out each time I shop — because the 7- by 6-inch size is perfect for holding 20 mini-cubes. With the top snapped on, this creates a mini-greenhouse that holds in the moisture. I stack up the containers in a neat and uniform process.

Space becomes more of an issue when I transplant hundreds of seedlings to 2-inch blocks, although these take up less room than if I were using 3- or 4-inch plastic pots. At that point I can also start moving some of the hardier plants outside to an inexpensive plastic greenhouse that I bought a few years ago. I am happy that I won’t have to buy peat pots each year or wash and store all the old plastic pots that I formerly used for transplants.

Soil blockers may not be for everyone, and some folks may shy away from the initial expense. But if you are still searching for a better way to start seeds, you may want to consider this method.

Cool Flowers: How to Grow and Enjoy Long-Blooming Hardy Annual Flowers, Lisa Mason Ziegler, 2014
DIY Soil Block Maker, The Prairie Homestead
Ladbrooke Soil Blockers Product Guide, Peaceful Garden Farm and Garden Supply
Making Soil Blocks, Penn State University Extension
The New Organic Grower: A Master’s Manual of Tools and Techniques for the Home and Market Gardener,
   Eliot Coleman, 1990
Soil-Block Making — A Better Way to Start Seedlings, Johnny’s Selected Seeds