Soil Blocks: A Potless Approach to Seed Starting
For years we considered changing our seed starting system from self-watering 72-plug trays we had purchased at the big box stores to an innovative tool that extrudes stable cubes via a 4-block square form that marry pot and soil into one. The magic is in the mixture and its ideal moisture content, which then utilizes the air space around it as it’s ‘pot’.
Soil blocks work by providing a nutrient rich medium for the establishing plants. At first, the moisture is a key component to the strength of the potting mixture, yet as the plant emerges and establishes, the plant roots become a key binding agent as they permeate the block. Soil blocks are a very sturdy garden invention.
There are several sizes of blocks to choose from when investing in this new tool: from a mini-block at 3/4” to large transplant size of 4”. We hemmed and hawed last Winter when we were investing in this implement, and decided, in the end, to order just one size: a 2” soil blocker; the other finalist was an 1 1/2” blocker. We chose the larger size as we read it could be used as a single pot for many crops, going straight from this block into the field. And that is what we do for almost all of our brassicas, beets, lettuce, and many flowers. The smaller sizes will almost definitely need potting up, so we opted to give some veggies extra room for their first home.
For starts that need extra indoor time or that establish quickly and need to be potted up, we have developed a 4.25” newspaper pot method in lieu of the 4” soil blocker. We had the opportunity to try the 4” soil blocker last Winter, and despite it being the same recipe as the 2” soil block mixture, we could not get the thing to hold together. So, next best is a cubic pot made out of paper. It feels a close approximation to what we hope to achieve by using these plastic-free soil-full, pot-less pots.
This recipe is from The New Organic Grower by Eliot Coleman. I highly recommend this book for your garden shelves. It is a well-worn book on our homestead and one we reference throughout the year for various projects and to confirm or deepen our knowledge.
The recipe is calculated in parts. I use a 2 quart yogurt container as my “part” and my lime addition is proportional to this volume:
3 parts coconut coir or peat moss
2 parts coarse perlite
2 parts compost
1 part garden soil
1 ounce lime
We have diverged from Eliot Coleman’s recipe in that we are transitioning to coconut coir instead of peat moss, and we don’t add the extra nutrients he recommends to our soil block mixture. Instead, we use a diluted fertilizer once true leaves appear.
I use a large, shallow plastic bin I purchased at Ikea for the purposes of mixing. I mix all the ingredients together dry, and then add as many buckets of water as needed to super saturate the mixture. I end up using at least half as much water as volume of dry ingredients to get it to the moisture level I prefer. And then, it marinates.
Once mixed, I let it sit for a few hours up to overnight to allow the mixture to fully saturate and blend. The ideal moisture makes the perfect block. I rock the blocker front to back while pressing firmly down to ensure it is a very firmly packed block. Next I inspect the underside of the block and knock off any extra soil, keeping the bottom of the block as flush as possible. If the blocks are bulging, they won’t sit flat in the tray. You can use a knife, but I like to get my hands dirty and simply knock off and smooth off any mounds I find. I usually find mounded bottoms so this is an important step to getting the blocks just right.
The blocks are pushed out into a soil block propagation tray. We don’t just make them and drop them into unlined trays, we weave cotton rope through the base of the tray and line the propagation tray with a wicking material to help water the soil blocks from below. This provides the base for a passive watering system and we find this system works best for us.
Despite being head over heels in love with this process, there are still some vegetables we don’t start in soil blocks. Things like onions and leeks whom I sow in 4” pots from the start just go right into their 4” pot where they hang out until they make it into the garden. Other crops with large seeds like peanuts and squash I start in 3” pots because I want to give their delicate roots as much room as possible before transplant, as disturbing roots can be really detrimental to these vegetables.
Coconut Coir or Peat Moss
I would be remiss if I didn’t discuss the use of peat moss in this recipe. This is the recipe found in the book and online when you search for soil block recipes. Since this recipe first came to popularity, much research has been done comparing coconut coir to peat moss with very promising results.
Coconut coir is a by-product of the food industry in southeast Asia, and utilizing it in the garden is a viable way to reduce plant material from being dumped in a landfill while contributing similar properties of peat, the previous darling of mitigating heavy soils. It almost sounds too good to be true.
Peat moss, on the contrary, takes hundreds of years to mature, and while some claim its management is sustainable, in this global economy with increasing pressures placed on the environment, if a renewable resource offers similar properties to the home gardener (minus the acidity), it seems like we should all be transitioning to using coconut coir. The transportation costs of shipping versus trucking are similar, even though the distances are dissimilar, so the shipping costs are negated in the argument.
The next time we mix up soil blocks, we will be using coir instead of peat moss.