If you are like me and scroll through all the flower farmer and gardener Instagrams, you might notice the term soil blocking pop up frequently. What is soil blocking, you ask? 

Soil blocking is a way to propagate seeds without using those large plastic seed trays. Some farmers say soil blocking comes from Europe, others say some farmers here in the states came up with it (I bet both are true.) In any case, I wanted to break down the pros and cons of soil blocking to really see if the hype is real and if seed propagating really is as fantastic as they say.

First, what is soil blocking?

Benefits of soil blocking - PROS

• Soil-blocked plants have healthier roots

  • The roots of plants grown inside a container circle the inside of the container whereas plants grown in soil blocks self-prune when they hit the air. This allows the soil-blocked roots to extend immediately into their surroundings when transplanted, unlike the plastic tray plants which have to unwind themselves first. This also makes for much less root disturbance.

• Soil blocking maximizes seed numbers in a small space.

  • Soil blockers come in different sizes: from ¾” up to 2”. Most seeds can grow in the ¾” soil block. Which means that I can fit 60 soil blocks onto a 6” x 10.5” styrofoam tray. For context: a typical plastic seed tray has 72 blocks in it and is about 2’ long by 1’ wide. With seed blocks, in that same 1’x 2’ space, I can have 3 of those styrofoam trays for a grand total of 180 seeds under one light. This also helps save resources, which makes a difference for beginning gardeners who might only have one light and one heat mat.  

• Soil blocking saves money.

  • You will have the initial purchase of the metal seed blockers, the plastic trays, and the soil blocking mix, but that’s all you need to be off to the races. You don’t need multiple plastic seed trays. And my experience with the cheaper seed trays is that they only last one season before breaking and cracking all over the place. As of this writing, I’ve reused the styrofoam trays for the last three years and they should last several years more. I only need to replenish the soil mix.

• No storage issues with soil blocking. 

  • Have a corner filled with broken seed trays? Me neither since I’ve switched to soil blocking.

• Healthier Soil Mix
  

  • The soil mix has compost and nutrients inside and is not sterile. This makes for healthier and more robust seedlings.
  • You don’t need very much of the soil blocking mix to get through the season of seeds. 

Downsides of soil blocking - CONS

• Soil blocking takes practice.

  • You need to make sure that the soil is packed down really well in the soil blocker. This is a technique that needs to be perfected because you want the densest soil blocks you can make.
  • Your first sets of soil blocks will most likely crumble. If this happens, they need to be packed-in more. I used my first four sets of soil blocks as practice, dumped them back into the mix and re-did them.

• Soil blocking takes a special soil mix. 

  • You can’t just use any soil for soil blocking. The consistency of the soil mix needs to be like fudge and needs to hold together over a period of time. I’ve seen certain farms and seed companies sell their own soil blocking mixes. I’ve also found recipes online to make my own. Most recipes tend to include compost, green sand, and rock of phosphate. 

• Making your own soil blocking mix can be tricky.

  • The first several years of my soil-blocking journey, I bought soil blocking mix from Johnnyseeds.com, but this year I decided to make my own. I noticed that if I deviated from the exact proportions of the ingredients in any way, my soil blocks would start to fall apart even before the seeds were ready to be put into the blocks.

• Easy to overwater.

  • You must let the soil blocks dry out completely between waterings. They need water only every 24 hours. If you overwater, your chances of dampening off are greater. Dampening off is a term used for a fungus that attacks the roots of the seedlings where they touch the soil. 

CONCLUSION

Due to the space limitations I have in my tiny garden, I found the switch to soil blocking to be full of benefits. I wholeheartedly recommend soil blocking as a fantastic way of propagating seeds that saves time, money, and space. It’s amazing how little soil seedlings actually need to grow strong. The air-pruning really makes the seedlings resilient and I have had a greater number of seedlings survive the transplanting. The price is right when it comes to the large number of seeds I propagate every year and, frankly, I wouldn’t be able to grow the volume and varieties of  the plants I do if it wasn’t for soil blocking. I’m so happy to see that soil blocking is becoming more mainstream and is being used by smaller gardeners around the world.