Soil Block Recipe: A Complete Guide to Soil Blocking

If you have started seedlings indoors, you have probably used seed starting trays, plastic nursery pots, or a mix of containers. All of these are good options, but what if I told you that soil blocks are a better, cheaper, and simpler way for quick growing crops?

Soil blocking is a great way to grow seedlings and reduce the use of plastic, getting healthier seedlings in the process! In this blog post, I’m sharing how to make your own soil blocks, the benefits of soil blocks, and the best easy soil block recipe.

Why I Love Soil Blocking

I love soil blocking and I honestly think that it is one of the best seed starting methods for the at-home gardener! This soil block recipe makes it even easier because it holds together without crumbling.

I am always looking for ways to work smarter, not harder in the organic garden. Soil blocking allows me to reduce costs and reduce waste while growing healthier seedlings.

And I want to help you do the same! That is why I’ve created this guide and others like How to Transplant Seedlings, DIY Seed Starting Soil, Cold Stratification, Winter Sowing, When to Start Seeds Indoors, and How to Make Compost to help you grow your best garden ever.

What Is Soil Blocking

Soil blocking is a method of seed starting that uses compressed soil instead of containers. Using a soil blocking tool, you literally make containers out of the soil.

The soil blocking tool compresses the soil into a firm block that holds its shape, just as if it were surrounded by a plastic pot. This allows the plant to grow in a soil cell until it is ready to be transplanted into garden soil.

Pros and Cons of Soil Blocking

Normally plastic pots and seed starting trays are used for indoor seed starting. These are great and definitely have their uses, but soil blocking can be a game-changer.

Here’s a summary of the advantages for the home gardener to start seeds in soil blocks:

• Saves Money - Let’s say you buy one 48-cell plastic seed starting tray that costs about $2. This soil blocker tool is $30 so it will pay for itself when it saves you from buying 15 of those plastic trays. After that, every block you make will be money back in your pocket!

• Plant Health - Even if you inadvertently overwater (which is a common, easy-to-make mistake), the seedlings will be healthier in soil blocks because water can escape from all sides. Plastic pots hold onto moisture because it can only evaporate out of the top or the drain hole. So if you overwater in plastic pots the seedlings can suffer from dampening off, rot, mold, and poor root development.

• Air Pruning - Air pruning happens naturally in fabric pots and in soil blocks! When the plant roots grow to the side of the block and touch the air, the tip dies off and forks, making two new roots. Plants will not become root-bound in soil blocks. That means you are doubling the plant's root mass and creating a far healthier root system that leads to a far healthier plant.

• Reduces Transplant Shock - Because soil blocks lead to air pruning, the roots experience less transplant shock, which means they can recover from transplanting much quicker.

• Ease of Transplanting - Soil blocks reduce the risk of causing root damage to the seedlings that you’ve worked so hard to grow. Just pick up the soil block and plant it in the garden when you’re ready to transplant. There’s no tipping or squeezing trying to get the plant free of the container. (I know I certainly have dropped and ruined seedlings trying to get them out of plastic cells. It’s not fun!)

• Minimal Waste - Soil blocking eliminates the need for reusable plastic pots or cells making it more eco friendly. While plastic pots can work great, they eventually crack or wear out and need to be thrown away and replaced. 

Even with all these great pros, we shouldn’t ignore the fact that soil blocking has some cons as well. The most notable are block fragility, drying out faster, and the time it takes to make the blocks.

Unlike plastic pots, soil blocks will start to degrade over time because of watering and other factors. If the soil block falls apart, the seedling can be damaged or ruined.

What Crops Can Be Started in Soil Blocks

The options of seeds that can be started in soil blocks are pretty limitless. Soil blocks even allow you to plant things that you wouldn’t be able to start in containers or cells, such as corn, carrots, radishes, and other root vegetables. These plants do not become root-bound in soil blocks because they are air pruned.

I will say, though, that quick turnaround crops are the best option. The longer you have before the soil block goes into the garden, the more the block will degrade. For that reason, I still grow pepper and tomato seedlings in plastic pots.

So, the sooner you can transplant the seedling, the better. Quick turnaround crops would include lettuce, broccoli, cabbage, carrots, beets, and other plants that can be transplanted in 3 to 5 weeks.

DIY Soil Block Recipe

This super simple soil block recipe creates a quality seed starting mix at a fraction of the cost of store-bought. Making your own seed starting mix is surprisingly quick and easy.

To make this recipe, soak the coco coir in water, stirring it with your hands until it is moist and fluffy. Then add the perlite and vermiculite, each at 10-15% of the overall volume. Mix it together and you're done!

You can buy seed starting mix instead of making your own, but it will be more expensive. Many varieties are available, and all claim to be the best.

I recommend purchasing Pro-Mix Premium Potting/Seed Starting Soil or Happy Frog Potting Soil by FoxFarm if you don't make your own. These are both high-quality and will give you great results.

Supplies and Ingredients Needed To Make Soil Blocks

Here's what you need to make soil blocks:

• Seed Starting Mix - It is very important to use a quality seed starting mix or potting mix that contains some perlite and vermiculite. The finer the mix, the better, because we want it to compact well. The soil block recipe above will give you a quality mix at a fraction of the cost.

• ¼ Inch Mesh - I recommend using ¼ inch mesh to sift the starting mix.

• Fertilizer - A well-balanced, slow-release fertilizer like Trifecta+ provides essential nutrients for your seedlings when they need it. You may also want to add vermicompost (worm castings) which is full of beneficial microbes, good bacteria and nutrients to aid plant growth.

• Water - Water will help the soil hold its shape when it is made into blocks.

• Soil Blocker - MIgardener’s Soil Block Makers are made from heavy-duty stainless steel, which makes them super durable so they will last a lifetime of seed starting. We offer two sizes to make either 4 cells or 8 cells of 2”x2” blocks, both with a pre-formed dibbler for seed sowing.

• Bucket or Washtub - You will need a bucket or some other container to prep your seed starting mix in.

• 2 Trays - Choose sturdy trays, like these propagation trays or shallow trays.

Instructions for Making Soil Blocks

1. Sift the Seed Starting Mix - Use ¼ inch mesh to sift the seed starting mix. Pro-Tip: Don’t skip this step! Sifting the soil mix makes it finer, gives it more surface area, helps it hold moisture, and removes the big chunks allowing the roots more room to grow. Sifting the seed starting mix has made a dramatic improvement in my seedlings!

2. Add Fertilizer - Mix fertilizer into your seed starting mix now if it doesn’t already contain it. I use Trifecta+ at a ratio of 3 Tbsp of Trifecta per gallon of soil mix plus 2 Tbsp of worm castings per gallon as well. Fertilizing now eliminates the need to fertilize after your seedlings have emerged. Pro-Tip: Most store-bought seed starting mixes will not contain fertilizer. You need to add it!

3. Mix Water into the Starting Mix - Add water to the starting mix in a bucket, wheelbarrow, or other container. Add enough water to saturate the seed starting mix until it feels muddy and so that you can squeeze excess water out of it.

4. Put the Soil in a Tray - Move the soil into one of the trays (if it is not already in a sturdy container that you can compress blocks in). Leave the second tray empty for now.

5. Compress the Soil - Take the soil blocker in your hands and, with a firm grip, press the blocker into the soil repeatedly to compact the soil. The plate in the blocker tool will rise as it fills up with soil. Stop once you cannot get any more soil in. Pro-Tip: Make sure the starting mix is well packed into your soil blocker. You want the cubes to be thoroughly compressed so that it comes out nice and clean, giving you a good block.

6. Place Your Blocks - Place the soil blocker in the second tray where you want the soil blocks to go.

7. Remove the Soil Blocks from the Blocker - While applying consistent pressure down with your palms on the top of the soil blocker, slowly pull up with your fingers to release the soil blocks from the blocker.

8. Repeat - Repeat steps 5, 6, and 7 until the tray is filled with soil blocks.

9. Spacing - Make sure there is a little bit of space between each block. If the blocks are touching, the roots from one seedling may grow into the soil block next to it. If this happens, you will need to take a serrated knife to cut through the roots between each block later on.

Now you are ready to sow your seeds! Place your seeds in the pre-formed indent at the top of the block that was made by the dibbler.

Tips for Growing In Soil Blocks

Set yourself up for success by following these tips:

• Start seeds that will germinate at the same time and grow at the same rate together in the same trays. Otherwise, your seedlings can shade each other, slowing their growth.
• Place grow lights 2-3 inches above your seedlings to avoid leggy plants. Raise the lights as the plants grow higher. If you are in the market for grow lights, I love this linkable grow light system!
• You may put the tray on a heat mat if the seeds you planted need that. But remember, soil blocks will dry out much faster than soil in plastic seed trays because they are exposed to evaporation on all sides. Water regularly to keep consistent moisture in your blocks.

Pro-Tip: A word of caution, don’t leave the seedlings on the heat mat too long. Once the seedlings begin germinating, remove them from the heat mat! It is no longer necessary or beneficial.

I hope this soil block recipe saves you tons and helps you grow bigger!