Starting vegetable seedlings indoors is a tried-and-true method for getting earlier production and increased yields from our urban farms. However, traditional seed-starting systems can be discouraging.
They are often messy, costly and time-consuming. And they produce seedlings that suffer from transplant shock and stunted growth when moved to the garden.
Fortunately, there is a better way. Soil blocks are an easy method for starting transplants, and they produce healthier seedlings that grow faster.
Eliot Coleman, author of The New Organic Grower, wrote: “It is always satisfying to find a technique that is simpler, more effective and less expensive than what existed before. For the production of transplants, the soil block meets those criteria.”
Soil blocks were developed to be a more efficient way to start seedlings. Transplants grown in them are superior to plants grown in flats or containers.
You make soil blocks by using a soil mix compressed into a block by a forming tool. Create a small depression in the top of each block, then place one or two seeds in the depression to grow each transplant.
Individual containers or pots are unnecessary when using soil blocks for starting seedlings. They offer other exciting advantages over traditional seed-starting systems, as well.
Prevent Transplant Shock
The roots of seedlings in plastic or peat pots grow to the edge of the pot and then begin circling the container walls. When transplants leave the pots, the roots suffer shock as they rip away from the container; air exposure and garden setting compound the condition.
The roots of seedlings grown in flats suffer similar damage because they also grow outward and are shocked as they are pulled apart and planted in the garden. In both methods, plant growth is arrested as plants try to recover.
Roots of seedlings in soil blocks initially grow toward the outside. But when they hit the outer edges, they turn inward to avoid the air at the surface. (This is called “air pruning.”)
Consequently, the roots are not ripped away from a pot or each other. They are not damaged, and there is little or no shock when they are set into the garden.
Rebecca McKinney, master gardener and founder of the South Carolina Organization for Organic Living advises, “Soil blocks are great for starting vegetables, and I love the fact that you have no transplant shock. You don’t have to unpot or uproot the plants. You just drop the blocks into your planting holes.”
Less Expensive, More Sustainable
You don’t need peat pots, plastic containers or flats for starting transplants when using soil blocks. The only cost is the initial expense of the forming tool.
These forming tools are very durable and last for years. Over time, the cost per soil block becomes minuscule.
There is also no need to disinfect pots prior to starting seeds, clean them after transplanting or find storage room for them from year to year.
Faster & More Efficient
Soil blocks take less space in plant trays, so you can grow more seedlings.
Block creation is fast because the blocking tools make multiple blocks at a time. Seeding is quick because the blocker automatically creates a seed depression. Transplanting is swift because you need not remove seedlings from pots.
Success with More Vegetable Varieties
Some vegetables are difficult to successfully transplant because they are so sensitive to root shock. However, you can transplant many of these difficult varieties using soil blocks because of minimal shock.
These varieties include:
- other “sensitive” or tap-root crops
In Northern or short-season climates, this means you can grow crops that were previously impossible.
Convenient “Potting On”
Soil-blocking tools come in several sizes so transplants can move into larger blocks over time for even hardier seedlings. This technique is referred to as “potting on.”
Popular sizes for urban farmers are 3⁄4-inch and 2-inch, but a 4-inch size is also available.
The system is designed so that seedlings started in the 3⁄4-inch mini-blocks can easily transplant into the 2-inch block. Transplants growing in the 2-inch block can move into the 4-inch maxi-blocks.
You do this by using “inserts” when forming blocks. These inserts allow smaller blocks to drop into the larger blocks thereby promoting continuous seedling growth and robust transplants.
To make soil blocks, you must invest in a blocking tool or tools. The 2-inch blocking tool (or “blocker”) is the most versatile and is the size typically recommended for beginners.
There are stand-up and hand-held blocking tools. But the hand-held versions are less expensive and more appropriate for urban farm use. (The stand-up versions are suitable for market gardeners.)
The 2-inch blocker makes four blocks at a time, starts nearly any type of seedling and costs about $30.
The 3⁄4-inch blocker makes 20 blocks at a time, starts many transplants in very little space and costs about $25.
The 4-inch blocker costs about $100. It makes only one block at a time—not very versatile. You’ll typically use it to “pot-on” 2-inch blocks to grow large transplants of crops such as tomatoes or peppers.
Soil Mix for Blocking
Critical to creating successful soil blocks is the right soil mix. Not only must the soil contain the proper nutrients and retain water to support the seedlings, it must also be highly compressible and fibrous to maintain the block structure.
Typical commercial potting mixes or garden soil do not meet these criteria, so making your own mix often makes sense. The mixes are easy to make and use common garden supply ingredients.
Making the Blocks
Making soil blocks takes a bit of practice, but after a few tries, they are quick and easy to produce. Just follow these three steps:
- Put the soil mix into a tub. Add warm water at a rate of approximately one part water for every three parts blocking mix. I like to let the mixture sit for about an hour to allow the water to soak in and then stir well. A thick, oatmeal-like or wet cement consistency is what you’re trying to achieve.
- Mound the soil mix to about 11⁄2 times deeper than the height of the blocker, then push the blocker into the soil several times to fill the chambers. Scrape any excess off the bottom of the blocker and set the blocker on your seedling tray. (Any flat tray with a lip around the edge will work well.)
- Push down on the T-shaped plunger while pulling the blocker frame up. You should now have soil blocks. Rinse the blocker in warm water, and continue making blocks until your tray is full or you have the desired number for your seedlings.
To start transplants in the blocks, add one or two seeds in the depression on top. Follow the germination directions (usually on the seed packet) for thinly covering the seeds with soil (or not, some seeds germinate best uncovered).
The blocks contain lots of water after being formed, so abstain from watering for the first two or three days. But you should not allow them to dry out.
Mist the seeds frequently to keep them moist, and supply bottom heat via a soil-heating mat to enhance rapid germination.
McKinney warns that, “I often hear that the blocks aren’t fragile, but that depends on how they’re handled. It’s important to handle the blocks as little as possible after seeding. If you squeeze them just a little too tightly when you pick them up, they’ll fall apart. This problem lessens as the plants’ root structures begin holding the blocks together.”
Once germinated, soil block seedlings just need plenty of light, water and room for continuous growth to produce lush transplants.
Initially, you’ll encounter a small learning curve to using soil blocks. But you won’t go back to pots or flats afterwards.
In Europe, it’s common for farmers and gardeners to buy transplants grown in soil blocks rather than pots or flats because, as McKinney explains, “flats or pots are messy. Soil blocks, on the other hand, are versatile and can be used in a variety of ways that fit your individual system and needs.”
Most importantly, they also produce healthy, robust and faster producing transplants.
Sidebar: Soil Block Mix Recipes
It’s easy to make your own soil blocking mix with just a few commonly available ingredients. You’ll find these soil block recipes in The New Organic Grower by Eliot Coleman. The publisher has permitted their reprinting here.
3⁄4-inch Mini-Block Soil Mix Recipe
Use this recipe, which that isn’t as rich in nutrients, for the very smallest soil blocks .
- 4 gallons peat
- 1 cup colloidal phosphate
- 1 cup greensand
- 1 gallon compost
Combine all ingredients, and mix well. For this recipe, it’s very important to use finely screened peat. Otherwise, the mix won’t work in the tiny mini-blocker.
2- or 4-inch Standard Soil Block Mix Recipe
Note: Use a 10-quart bucket as the measurement unit for the bulk ingredients in the recipes. Use a standard cup as the measurement unit for the supplements.
- 3 buckets high-quality peat
- 1⁄2 cup lime
- 2 buckets coarse sand
- 1 cup blood meal
- 1 cup colloidal phosphate (22 percent)
- 1 cup greensand
- 1 bucket soil
- 2 buckets compost
To make the blocking mix, measure the peat into a large mixing container, add the lime and mix. Then add the sand, blood meal, phosphate and greensand. Mix again.
Add the soil and compost. Mix again.
It’s a good idea to use finely sifted peat, or the mix will need to be screened through 1⁄4-inch mesh to work well in the blockers. This recipe makes about two bushels of soil-blocking mix.
This article appeared in Hobby Farm‘s Urban Farm 2019 annual, a specialty publication produced by the editors and writers of Hobby Farms magazine. You can purchase this volume, Hobby Farms back issues as well as special editions such asBest of Hobby Farms and Living off the Grid by following this link.