Plastic-free Seed Starting with Soil Blocks
Several years ago I set out to reduce the amount of plastic that enters our lives. From food packaging to farm materials, I am always on the look-out for alternative ways to make, use, or buy without adding to the plastic legacy we are leaving behind.
I've always been mindful of our plastic consumption but I am became particularly intent on weaning ourselves off it after learning that half of all plastics ever manufactured were made in the last 15 or so years! Only 9% has been recycled and over 40% is used in packaging. Most of this plastic is not made from recycled material, hence the staggering amount of new plastic we collectively produce and dispose of each year.
One area on the farm that plastic shows up every year is when we start seeds in our reused plastic pots. As we head into the 2021 growing season – our first time planting a market garden – we needed to expand our seed starting operation, yet we didn’t want to buy new plastic pots or seedling trays.
Therefore, this spring we finally bought soil blockers that are handy little gadgets that turn potting soil into sturdy little soil blocks. These little blocks firm up as the seedling roots grow and hold the soil together. There are a handful of sizes available, but we went with the 1.5 inch and 2 inch sizes.
I was initially hesitant because I imagined the process of making soil blocks to be time-consuming and tricky with finding the right potting soil that would hold together initially while the roots developed to firm up the block. None of those initial fears were founded. We were able to use the same potting soil we have been using (it just needs to be much wetter) and it took far less time to make dozens of soil blocks than it does to fill and seed a comparable number of plastic pots!
Soil Blocker Benefits – People, Planet, Profit
Aside from the plastic-free aspect of soil blocks, they have other benefits.
Less Stuff to Wash and Store
Unlike plastic pots, the only thing to store is the soil blocker itself. Phasing out pots also reduces the clean up after the seedlings are transplanted because unlike cleaning out dozens of trays or pots, you only need to rinse off the soil blocker and the container used to make the blocks in.
Less Stuff to Landfill
Despite my best efforts to endlessly reuse plastic pots, each year several inevitably break or wear out. Getting more pots from gardening friends isn’t a problem, but unfortunately the damaged pots head to the landfill since many are not recyclable.
Even pots with the recycling symbol shown can be made of many different plastics, not all of which is accepted in all areas. Furthermore, the dies used to color pots, the purity of the plastic, and residual dirt can reduce the likelihood that pots are recycled (though I’m sure many are wish-cycled by well-meaning gardeners). It is best to check your local recycling guidelines before throwing old pots in.
Less Space for Seedlings
One of the benefits I had not given much thought to but really appreciate now is how much more efficient space-wise soil blocks are. I can fit several dozen little soil blocks in the same area that 8-10 pots fit in. As we continue to grow our garden and start more seedlings indoors, this is a game changer for our little farm.
The roughly $75 I invested in our two soil blockers is approximately $75 more than I have ever spent on seed starting trays or pots. However, since we are starting our market garden this year, we would have paid a comparable amount on plastic seed starting supplies to start the same number of seedlings. We could reuse the plastic trays and liners for many years (hopefully) but as discussed earlier, they would have eventually worn out requiring us to buy replacements.
As with many other high demand items during this pandemic era, pots and seed starting trays can be difficult to find in some places. With a soil blocker we never need to worry about sourcing new pots or shipping delays! Also, with a well built and cared for soil blocker, we wouldn’t need to invest in new seed starting containers for years if not decades.
Better for Seedlings
Aside from the benefits for the gardener and the environment, using soil blocks has many benefits for the plants themselves – healthier root system and reduced transplant shock.
Soil blocks produce more robust seedlings because of how the roots behave when they hit the air “wall” of the soil block vs. the plastic wall of a pot. In a pot the roots can become root-bound as they continue to grow and circle around the pot. In a soil block they will pause and wait to grow until they are planted, which helps the roots retain their vigor.
Avoiding root-bound conditions will help seedlings to acclimate to the garden soil after transplanting. The roots from seedling in a soil block that paused when they reached the soil edge will resume growing quickly. Transplanting at the proper time is key to avoiding the trauma of transplanting (this is true for both pots and soil blocks).
Reduce and Reuse
Although I do love my new soil blockers, we are still reusing existing plastic pots and plan to do so in the future. Perhaps one day I will invest in larger soil blockers, but until then I plan to reuse 4 inch and 6 inch pots for our tomatoes, peppers, eggplants, zucchini, cucumbers, and other plants that require longer lead indoors in northern climates. We will also use up our plastic pots until they wear out, since it is best to reduce and reuse before recycling (or landfilling depending on the pot).
Soil Block Planting Guide
Here is a handy little guide for sorting out what size soil blocker to use for certain plants:
1.5” Soil Blocks: Onion, Leeks, Scallion, Kale, Swiss chard, Spinach, Kohlrabi, Radicchio
2” Soil Blocks: Basil, Beans, Beets, Broccoli, Cabbage, Cauliflower, Celeriac, Celery, Lettuce, Parsley, Peas
Cucumbers, melon, winter squash, zucchini can be started in 2” soil blocks and then potted into a 4 inch pot. The same goes for artichoke, eggplant, pepper, and tomato, except they should go into a 6 inch pot – usually when they develop their first true leaves. There is also a tiny ¾ inch soil blocker used for germinating seeds that will be potted into larger soil blocks or pots, but at this point we are sticking with our two sizes.
We are also experimenting with the type of trays to set the soil blocks in. Over the years we have been given plastic trays that we use for holding and carrying pots and now soil blocks, though we certainly don’t have enough of these and I haven’t been able to bring myself to buy new ones. In the short-term, however, we have been reusing cardboard flats from our local Co-op.
There are some downsides to using cardboard. For one thing, I suspect that the roots from the soil blocks will grow into the cardboard for the plants with the longest indoor growing time. For another, the repeated misting/watering softens the cardboard that weakens the structural integrity over time. This makes turning and transporting the trays holding 30-100 soil blocks more difficult to work with.