Tool for Making Soil Blocks (3D Printed)

You can help your garden plants grow more vigorously by starting your seeds in soil blocks. Soil blocks are lightly compressed cubes or cylinders of specially formulated soil. In a soil block, a seedling's roots do not coil around and around as they may when started in plastic. Instead, the roots grow until they reach air at the sides of the block and then wait until transplanted into the garden, where they smoothly transition into a full-blooded plant with extensive roots. sturdy stems and ample leaves. With soil blocks, you can avoid purchasing, storing, and disposing of plastic cell packs and pots.

In this Instructable, I will show you how to make a soil blocking tool using a 3D printer, and how to use that tool to make the actual soil blocks. I will do my best to answer any questions you might have. Over the years, I have made quite a few soil block makers, but this version is the easiest to make and use.

You can buy a commercially made soil blocking tool online or from gardening supply stores. A spring-loaded metal soil blocker that pops out four 2" cubes costs about $35. However, if you enjoy making and customizing your tools, this is a simple, ergonomic, and durable design you can 3D print for just a few dollars worth of plastic filament. With this device, you can produce several 2.75" diameter soil blocks per minute, and that’s good enough for an avid home gardener.  If desired, you can customize the design using a CAD program such as Fusion 360 or TinkerCad to change the size or gang several together for higher volume soil block production.  Unlike other DIY plans for this type of tool, this design does not require an assortment of tools and materials. Just find a public library, maker space, or friend with access to a 3D printer.

Here is a video of me demonstrating how to use this tool. That sweet pup in the background is our dog, Steinbeck.

For more information on starting your seedlings with soil blocks, visit the Johnny’s Selected Seeds website (www.johnnyseeds.com) or read “Chapter 13 Transplanting” in The New Organic Grower by Eliot Coleman. As for what soil mix to use, you can make your own following Eliot Coleman’s recipe or buy a readymade mixture such as Vermont Compost Company Organic “Fort Vee” Potting Mix.

For the Soil Blocking Tool

• Access to a 3D printer (I used a Prusa Printer with PLA filament, but any printer using PLA filament will work)
• 2 STL files to upload to slicer software for the 3D printer. These are below.
• Optional - basic knowledge of Fusion 360 if you want to modify the design

Here are the two STL files you need to create the soil block maker precisely as I've designed it. The "hollow cylinder v2.stl" will print a hollow cylinder that is about 3" in both height and diameter. The "innie and outie parts v1.stl" will print 2 separate but fairly similar pieces, one with a hole running through it and one with a nub that makes an impression in the soil. You use one of these pieces to compress the soil, and the two of them are stacked together to push the compressed block out of the cylinder. All three pieces stack together for compact storage. Watch my demonstration video if you have any questions about using this soil blocker.

Both of these files should be easy to print. If your printer works well, you should not need a brim or support structure. I used a 20% infill on a Prusa i3 MK3S and a Prusa MINI.

Here is the Fusion 360 file for these parts so you can modify them if you'd like. It's an easy part to model in Fusion 360 or recreate in TinkerCad and you can experiment with different sizes.