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Soil Blocks for Seed Starting

Soil blocks are great a great option for starting seeds. They help promote a good root system by a process called air pruning. This is where the root grow thru the soil block and when they reach the surface of the block that is in contact with the air it causes the root to stop and branch off into the rest of the soil block material.

They are a good way to start all seedlings that do generate a deep root system as seedlings such a sweet pea and help prevent the transplant shock that occurs by the transplanting of bare root seedlings into the garden.

They lessen the use of plastic in indoor seed starting. There’s no need to purchase seed starting containers each year or sterilize re-used ones.

The soil block tool compresses the potting mix lightly enough that the potting mix stays in a formed cube for the length of a seedling’s growth.

The tools to make soil blocks come in different sizes.

There is the mini soil blocker. Making four, two-inch cubes, the mini soil block is perfect for starting almost every seed indoors.

They are big enough to allow the seedlings to get off to a good start.

The micro soil blocker creates twenty, 3/4-inch cubes. These are best for starting a large volume of seed. Most seedlings grown in the micro soil blocker will need to be “potted up” into a larger pot within a couple of weeks after germination

In the press plate of the unit there is a plastic cone that creates the depression for the seed to be placed.

These smaller the size of the blocks used the more important that the proper potting mix be used to form the block and the seed depression properly.

Poting Mix Materials

The smaller the size the block in the press the more important it is to use a finer texture potting mixture to form the block. If the depression does not form you can gently use a pencil to create a depression to place the seeds in.

Here are some pictures showing the differences in mix's

Seedling Potting Mic

Potting Mix ( not potting soil)

You can improve the potting mix’s structure to support the formation of the depression by removing the larger course components contained in the mix by sieving it thru a 1/8 mesh wire screen

If you would like to make you own soil block mix you can use the following formulation

  • 60 cups peat

  • ¼ cup lime

  • 40 cups coarse sand or fine perlite

  • 3/4 cup balanced fertilizer

The soil blocks created will be pressed out into shallow trays with rims that allow you to water from the bottom, which is critical for soil-blocked seedlings.

You can use the standard 10-20 seedling trays

You can use plastic trays

You will also need a cover of some type, such as plastic wrap, to cover the blocks prior to germination to help retain moisture while the seeds germinate.

Making Soil Blocks

To make the blocks you will first take your block mix in a container and thoroughly wet it with water

You will add water to the point that the mix is wetted only such that it will not drip water when squeezed in your hand

With the mix properly wetted you will then then you press the block unit down into the mix to pack the cells with the wetted mix.

You can check if the blocks have been proper filed by pressing on the mix in the block with your finger to ensure the cell is filled and tightly packed

You can remove the excess mix from the bottom of the cells by scraping it off on the side of the container or a metal ruler.

You can test your potting mix condition and technique by pressing the block out into the container to see how it looks

When happy you can now fill up your tray with soil blocks

If any do not come out the way you would like just to gather up the material and return it to you bin to be mixed back in for your next attempt.

Planting the Soil Blocks

Place your seeds into the center of the soil blocks.

To aid in germination you will want to make sure that the seeds have good contact with the potting mix surface. You can aid in this contact by lightly spraying the surface of the block with water.

it is important you do not allow the soil blocks to dry out. This stress’s the seedling and rehydrating the blocks can be more challenging than using regular potting soil in a traditional container.

To allow the seeds to have the best change of germination you will cover the tray of seeded soil blocks with a sheet of plastic wrap. This will help keep the moisture of the mix at a constant level while the seeds germinate.

The moisture of the blocks will create condensation on the plastic showing that there is enough moisture to allow the seeds enough moisture to germinate.

You should not need to water the seeds in the soil blocks before they germinate. But if you need to provide moisture to blocks that are showing signs of drying out then with a spray bottle just mist the top of the block as needed.

Once the seeds germinate and the leaves form you can remove the plastic sheet covering the tray

All watering from this point will be just by adding water to the tray holding the blocks and not watering the tops of the blocks.

As the seedlings are growing in the blocks you want to water from the bottom only. Capillary action in the mix will wick the water from the bottom of the seed block to the top. You can tell this has occurred by the change in color of the soil from light to dark as the water rises in the blocks.

You never want your blocks sitting in standing water in the tray. This can cause root rot and will exacerbate the growth of mold. If you add too much water and the water in the try has not been absorbed by the soil block you can remove the extra water with a turkey baster or medicine dropper. Not to worry if you do not get every little bit, you just want to remove as much as you can.

You may need some seedlings that outgrown their blocks to be “potted up” into a larger container prior to transplanting in the garden depending on how early you have started you seeds indoors and your growing conditions.

When transplanting you no longer need to use the fine potting mix and can use any standard potting mix of your choice or just transplant out in the garden

Plants can be transplanted into the garden when they have several sets of true leaves

Be sure to harden off all plants (place them outside for small amounts of time each day for a week, increasing the time each day) before planting them outside to get them use to the sunlight and temperatures

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