How to DIY Soil Blocks for Vegetable Seedlings -- And Save Money on Gardening (Photos)

Black pin, bag of seed starting mix, and a square mold

Treehugger / John Laumer

Serious gardeners who've kept track of their spending know how expensive it can be to purchase peat pots, potting soil, & soil blocks each year. With any sort of plant root bundle packaging, you are forced to buy more polypropylene and labels and trays. There is better way.

Note: Garden and home centers seldom offer seedlings or transplant-ready vegetables in soil blocks for practical reasons: root bundles dry out too fast if not surrounded by plastic; plant root bundles can't be shipped without crumbling unless they are either potted or shrink wrapped (the latest awful design to deal with when transplanting store bought plants is the plastic wrapped root bundle -- it sucks to get off without exposing the roots); and, distributors like to have something to print a label and a bar code on.

Skip all that.

Finished seed blocks sitting in trays

Treehugger / John Laumer

The pictured 2" X 2" blocks have a dollop of wet seeding soil on their tops, covering the seeds I had put in the little concave depressions left on the block tops by the press-plate nipples shown in the third photo of this post.

1. Get the Right Tools

soil blocker mold

Treehugger / John Laumer

Buy a small, mechanical soil blocker molding device -- I got my stamped steel, four block molding device from Johnny's for 30 bucks. Other outlets carry them and there are several sizes available. You'll also need a wide bottomed mixing pan or similar.

2. Use a Sterile Seed Starting Mix

Be sure to use sterile seed starting mix intended for soil blocking. If it is not sterile or close to it you may have issues with 'damping off' when sprouting tomatoes and such that need several weeks before they are big enough for transplanting. Especially if it is a cold damp cloudy spring and the seedlings get little real sun.

Tip: Don't buy a soil blocker mail order until you know where you can get sterile seed sprouting mix meant for that use. Regular seed-starting mixes don't have enough binder in the formulation and will crumble.

Adding clayey soil to an ordinary, non-sterile peat and vermiculite mixture, though it may hold formed blocks together, can introduce fungus that could cause young seedlings to wilt and die. I'll show how you can do it for non-organic, larger seeds that sprout and grow quickly in early spring, though.
Here we go.

3. Wet the Seed Blend

Shovel in bin with soil

Treehugger / John Laumer

You'll want to wet the seeding blend to the consistency of fresh concrete.

4. Add Soil for Binding

Soil reading to be mixed into seed starting mix

Treehugger / John Laumer

Add some clayey garden soil to bind the peat and vermiculite -- enabling the block to hold its shape.

5. Form the Cubes

Person using the soil blocker

Treehugger / John Laumer

Form cubes with the soil blocker. Push down firmly; wiggle and twist while pressing downward; slide the blocker out into an open area on the pan; and then...

6. Remove the Excess Water

Soil blocker sitting in bin

Treehugger / John Laumer

Squeeze free water out by pulling on the hand lever (see the muddy water oozing out on the top of the blocker forms?).

7. Remove the Soil Blocker

Seeds being added to the soil blocks

Treehugger / John Laumer

After squeezing out excess water, lift the soil blocker up, set it down on the sprouting tray, pull the trigger and out they row after all you have left to do is drop in some seeds and put on a dollop of the soil/water mixture.

I used pea seeds in this case, which are easily seen and not likely to have issues with fungal growth as they are non-organic. The only substantive difference between organic and non-organic seeds, by the way, is use of anti-fungal treatment on the non-organic seeds -- a common practice to enhance fertility.

Seeds in seed blocks

Treehugger / John Laumer

All the soil blocks with seeds...ready to cap with a glop of wet soil on the top of each.

Mister sitting next to tray with seed blocks

Treehugger / John Laumer

All done with seeding. Every day you must water one or more times with a gentle mist - small drops only. Do not use a hose attachment and don't leave it out in the rain or the blocks will erode.

You may read that it is advisable to cover newly-seeded flats with plastic wrap or plastic sheeting until sprouts are up. I want as little petrochemical input in my gardening, as possible so I'm going with frequent watering and no covers. Plus, ultraviolet rays, unimpeded by the plastic cover, will help reduce the fungal growth on blocks that otherwise might hinder my new plants. We'll see how that works.

Time to wash off all the wet soil and put it away 'til next spring. It should last a couple of generations if you keep 'er clean. I want my kids to grow their own.

Incidentally, kids can mess with this all they want and the result can be mixed back into the wet seeding mix until they either get it right or give up.