Soil Blocking vs. Seed Trays
A Cost-Effective, Eco-Friendly Choice for Starting Seeds
Starting seeds is one of the most exciting parts of gardening. There is nothing quite like planting a seed and watching it wake up and grow. It’s a promise of what’s to come, hope for the growing season and brighter days ahead… it’s truly a little bit of magic that’s being infiltrated into your daily life. When I plant seeds, I repeat little mantras or affirmations to myself, and I think about the symbolism of what I’m doing as a way to hopefully channel more of that growing energy into the world.
Sometimes I find it ironic that I started a flower farm having never even grown a zinnia prior, but sometimes I think it actually worked to my benefit. When it came to starting seeds, I knew immediately I wanted to use the soil blocking method instead of traditional seed trays for a number of reasons. It’s a practice that is gaining quite a bit of traction in recent years across farmers of all types, and as someone who was intent on following more environmentally friendly practices, who had limited space to work with and who was on a budget, it was the perfect option to get started.
So, let’s dive into what soil blocking is, how it compares to seed trays, and why it might be the perfect method for your garden, too!
Photo ©Arlyn Flower Farm 2025
What Is Soil Blocking?
Soil blocking is a seed-starting method that uses compacted blocks of soil to grow seedlings, which eliminates the need for plastic seed trays. These blocks are formed using a tool called a soil blocker, which compresses a custom soil mix into sturdy, freestanding blocks. Each block has a small indentation on top to hold a seed and you can get the tool in different sizes to accommodate varying seed types. Once your blocks are prepared, they’re placed on a flat, watertight surface like a tray or shallow pan until you’re ready to either “bump them up” or plant them outside. Full disclosure: I don’t bump mine up. I plant them all directly outside and they thrive.
Photo ©Arlyn Flower Farm 2025
Benefits of Soil Blocking
1. Environmentally Friendly
One of the biggest advantages of soil blocking is its low environmental impact. Traditional seed trays are often made of flimsy plastic, which can break down over time, contributing to waste. There are some that are more sturdy, but many are so flimsy that they break in the first season, which contributes to the “single use plastic” issues facing our world. In contrast, soil blocking uses no plastic containers. You can skip buying (and later discarding) seed trays altogether, which is a win for the planet. Now, you do need some sort of tray for the blocks, but you have more freedom in determining what type of material you want to use for this.
2. Healthier Root Systems
Soil blocks encourage air pruning, a natural process where roots stop growing once they reach the edge of the block. This prevents the plants from becoming root-bound, a common issue with traditional trays, which means you minimize the impact of transplant shock. Healthier roots mean stronger plants, better equipped to thrive once transplanted.
3. Cost Savings
While the initial investment in a soil blocker tool may seem significant (I think I paid $20-ish for mine), it pays for itself over time. You’ll no longer need to purchase replacement seed trays, and the ability to use homemade or bulk soil mixes can further cut costs. You will be shocked at how much less soil you use versus traditional seed trays.
4. Saves Space
Because soil blocks eliminate the need for rigid (often larger) seed trays, you can fit more seedlings into a smaller space. I’m able to fit 120 seedlings on a 10x12” cafeteria tray and 4-5 trays per growing shelf. The number of plants I can fit on my growing rack is wild. Needless to say, if you have a smaller space for starting seeds, soil blocking can be incredibly useful.
Photo ©Arlyn Flower Farm 2025
How to Get Started with Soil Blocking
Ready to give soil blocking a try? Here’s what you’ll need:
Soil Blocker Tool: Available in various sizes, depending on the seeds you’re starting. I started with a small ¾” one which is able to handle most seeds, although I did add a larger 2” blocker my second year for larger seeds.
Custom Soil Mix: Now, most people say that soil blocking works best with a mix designed for this purpose, which retains moisture and holds its shape. The first year I was meticulous about creating one of these mixes, but last year I opted to use what was available at my local greenhouse and it worked just fine.
Note: Because the soil blocks are small and you need the soil to hold it’s shape, you want to make sure your soil is made up of fine particles that will stick together. If you have soil that has larger chunks of things (as some soils do), you’d want to strain the soil first to make sure you don’t end up with large chunks.
Tray or Flat Surface: Use a shallow pan or non-plastic tray to hold your soil blocks. I use basic cafeteria trays, but people also use bakery cabinet trays (the long fiberglass ones), old meat trays (reusing the styrofoam ones… cleaned of course).
Seeds: Choose your favorite varieties!
A few extra tools that are handy to have:
A watering can with a small nozzle (so you can pour water without disrupting the blocks).
A drywall trowel/scraper to scrape off extra dirt from the bottom of your blocker as you’re making them.
A mister or spray bottle (that has a fine mist option) to keep the top of the blocks moist.
To make the blocks, moisten your soil mix until it’s the consistency of wet sand. Press the soil blocker into the mix, ensuring it’s fully packed, then eject the blocks onto your tray. Place one seed in each block, mist lightly, and keep the blocks moist until germination (this is where a humidity dome can be helpful).
Photo via Unsplash
Keeping Your Seedlings Alive
One potential downfall to soil blocks (although it’s never been a nuisance to me) is that they tend to dry out a bit quicker than normal seed trays. You’ll want to check them daily (maybe even twice a day as they get going). When you water them, lightly water the edges of the tray so as not to disrupt the blocks (they’re fragile in the beginning because the roots haven’t formed to hold them together).
You’ll notice the blocks soak up the water. You want to make sure they’re damp but not overly wet. If there is water left on the tray after the blocks look fully moist, either pour the water back into a container (I keep a bucket where I can pour excess) or tab it up with a towel. You don’t want the blocks sitting in standing water.
With a little TLC, before you know it, you’ll have little seedlings growing!
Is Soil Blocking Right for You?
If you’re looking for a sustainable, cost-effective seed-starting method, soil blocking is worth exploring. It’s especially appealing for those aiming to reduce plastic use and embrace environmentally friendly gardening practices. While it requires a bit of a learning curve, the benefits, like healthier seedlings and long-term savings, definitely make it a worthwhile addition to your gardening toolkit.
You can be a seasoned grower or starting seeds for the first time and make soil blocking work for you. It’s a wonderful way to nurture your plants while staying kind to the planet.
If you give it a try, let me know!
Happy soil blocking, friends!