How to Start Seeds with Soil Blocking This Season

Soil blocking is a method of seed starting that involves pressing a soil medium into small blocks that will accommodate seedlings until they’re transplanted into a pot or directly into the soil. One of the primary benefits of soil blocking is that you get so skip using plastic seed trays.

This idea behind this method has been around for quite some time, but the champion of soil blocking and the person behind its popularity today is Eliot Coleman, author of The New Organic Grower.

So, if you’ve decided that you’d like to give soil blocking a try, you’ve come to the right place! Continue on to find out all you need to know about soil blocking and how it’s done.

Advantages of Soil Blocking

Proponents of soil blocking tout many benefits to the system. Some of these benefits are:

• Less plastic waste
• Less money spent long term
• Healthier plants that are more resistant to transplant shock
• Air pruning of roots (no root-bound seedlings)
• Less space needed
• Can bump up into larger blocks easily

In a nutshell, because blocks are formed with only the soil medium, there is no need for plastic cell trays that are potentially thrown away after 1 to 3 years. This means less plastic waste (and less time washing trays!). 

Roots of seedlings are said to “air prune”. This means rather than the roots continuing to form and circling in the confined spaces of a cell, the roots will naturally “prune” themselves off once they reach the edge of the block. Because of this phenomenon, seedlings are said to have reduced transplant shock.

Blocks can be squished close together, too, so you can fit more seedlings per square foot or inch compared to the fixed spacing between cells and trays.

Disadvantages of Soil Blocking

While there are many wonderful benefits to soil blocking, there are a few drawbacks.

• Blocks can dry out quickly and, therefore must be monitored closely.
• You need different size soil blockers for different types of plants.
• You may have to adjust your seed-starting time.
• There is a learning curve to mixing and making soil blocks.

Starting Seeds with Soil Blocking in 7 Steps

By now, you’re likely intrigued by soil blocking, so let’s dive into how you can get started with soil blocking today.

Step 1: Choose Your Block Size

There are three commercially available soil blockers on the market today, and these are the ¾ inch, 2-inch, and 4-inch blockers.

Which one should you choose? Well, that depends on what you’re planting.

Small Seeded Plants

As a general rule of thumb, small-seeded plants should be planted into ¾-inch blockers, while large-seeded plants should be planted into 2-inch blockers. The 4-inch blockers are mostly used for bumping up seedlings that have outgrown their 2-inch blocks.

Small-seeded plants that will do well in mini blocks (¾-inch blocks) are oregano, eggplant, peppers, basil, love-in-a-mist, lavender, and any other small seeds that like bottom heat. 

Large Seeded Plants

Large-seeded plants like sunflowers and squash are great for starting in 2-inch soil blockers. 

Most of the time the 2-inch blockers are used with special inserts that create ¾-inch square indentations at the top of the 2-inch block.

This allows seedlings sown in ¾-inch blocks to be easily bumped up into 2-inch blocks by nestling the mini block inside the perfectly sized indentation created by the insert. Inserts are usually sold separately and have to be snapped into the 2-inch blocker.

Picking Your Block

There is debate on if you have to choose one blocker over the other (because they are a little pricey upfront). Which should you choose? 

From experience, I would say I use a ¾-inch blocker more than a 2-inch blocker. The caveat is that I usually transplant my seedlings sooner because a ¾-inch cube does not have very much space for root growth. For this reason, I push my seed-starting date back and just plant my ¾-inch block directly into the field.

Other people will suggest that the 2-inch blocker is the most versatile because you can plant small AND large-seeded plants in the blocker.

You also may not need to adjust your seed starting time, because the blocks have more space for root growth. In my experience, you will need just as much space–if not more–for 2-inch soil blocks than if you were using cell trays.

Step 2: Choose Your Surface Tray

You’ll need a solid surface to place your blocks on, and ideally this surface will be able to hold water. 

I like to use plastic lunch trays. Yes, the cafeteria kind (I purchased the 10-inch by 14-inch trays). I can fit 180 to 200+ mini soil blocks on a standard-size lunch tray. Sometimes I use the 1020 plastic trays that are used underneath cell propagation inserts.

The problem with these is that there are ridges in them, and sometimes the mini blocks won’t stay level on those surfaces. With the 2-inch blocks, I find that these 1020 trays are a little flimsy when it comes to supporting all the weight of the blocks because the trays are made of thin plastic. But they can work!

Some people like to use disposable styrofoam meat trays because they’re cheap and a nice size. I don’t like to use them because, again, they’re disposable. But they work great for soil blocking.

You can ultimately use anything you’d like that holds water. What about using old cookie sheets or casserole pans? Sounds like a great idea to me. I have even used plastic lids to gallon ice cream tubs for small batches of seedlings. 

Step 3: Acquire and Mix Soil Amendments

There are a ton of different soil-blocking recipes, but here are some of the basic ingredients:

• Peat or coconut coir
• Perlite or coarse sand
• Sifted compost
• Greensand
• Blood meal
• Phosphate
• Lime

The main idea behind a good soil-blocking recipe is the ability for the block to stick together. Ingredients like compost, peat, and coconut coir will help with that.

Some growers use a basic seed starting mix to form blocks. Others will say that compost is needed to create a perfect, sticky block. 

If you’re looking for a recipe from Eliot Coleman himself, then follow this recipe from his book The New Organic Grower. This recipe makes 2 bushels of soil-blocking mix:

• 3 buckets of peat
• 2 buckets of compost
• 2 buckets of coarse sand or perlite
• 3 cups of base fertilizer (a combination of blood meal, greensand, and colloidal phosphate)
• ½ cup of lime

Another recipe from Cornell College of Agriculture is pretty simple:

• 1 part coconut coir
• 1 part peat moss

Some online retailers are starting to sell soil-blocking mix if you’re afraid to mix your own (which you shouldn’t be). 

A tip for creating your soil-blocking mix is to make sure you sift your peat and compost. If you’re making mini blocks, you don’t want large chunks of wood floating around in your mix that can get stuck in the blocker or not allow the blocker to fill properly. 

Also, you want to use a medium that will absorb water. Sand and perlite will NOT absorb water, and you’ll notice if you use too much of these components, then your blocks will fall apart.

Step 4: Wet Your Soil Medium

Once your soil medium has been mixed thoroughly, you’ll need to add water to form blocks.

Most people recommend warm water, but cold water will also work. I prefer warm water because I feel like it permeates the soil medium better and, it’s not as shocking as cold water on my hands!

To form your blocks, you will need to add enough moisture that the soil medium holds together but that it’s not an overly soggy, loose mess. I like to describe it as having more moisture than you would typically add to your germination mix when starting seeds in cell trays or pots. It may take a little practice to get the moisture level right.

I mix my soil-blocking medium around until I can squeeze together a handful, and it stays together. Once you have the moisture level correct, it’s time to start blocking!

Step 5: Pack Your Blockers

The important thing about packing your blockers is that you make sure the cavities are filled up. If you don’t pack them properly, then your block will be misshapen and could potentially fall apart. 

I usually hold the blocker (hole side up) in one hand and fill up the blocker with the mix with my other hand. I periodically use my palm to smush the blocking mix in tight, then add more as needed. If you’re using a mini blocker (which has 20 cavities), it’s especially important to make sure your outside cavities are packed well. 

Another method for filling blocks is to pile up your moist blocking mix in a mound and then press your blocker straight down into the mix, using your upper body and rocking the blocker back and forth to ensure all the cavities are filled tight.

Then, performing a scooping motion so that the cavities are pointing up, flip the blocker up and make sure that everything is packed tight.

Step 6: Make Soil Blocks & Sow Seeds

Place your blocker cavity-side-down onto your tray or pan. Press firmly. You’ll notice a little bit of water starts to seep out. This water will soak back up into the blocks when you release them. Squeeze your hand to engage the blocker, still pressing down firmly, then release. 

Congratulations! You’ve made your first soil blocks. It may take a little bit of practice before your blocks are perfect, but no worries. Simply throw the batch back into the mix and try again if your blocks are misshapen. 

Soil blocks are great for small-seeded plants because, with the small indentation on the top, it’s easy to place your seeds on the block surface. Dust with vermiculite and spritz with water. Then keep the blocks moist until germination by adding a humidity dome or Saran wrap over the top of the blocks.

For larger seeds, you might want to sprinkle some more blocking mix on top of the seeds, then dust with vermiculite and spritz with water. Add a humidity dome or Saran wrap to keep blocks moist until germination.

Step 7: Provide Proper Maintenance & Care

It’s very important to keep blocks moist (but not sopping wet) after you’ve removed the humidity dome. The soil blocks will dry out quickly, so make sure you check on blocks at least once a day.

It’s okay to let them dry out between waterings. You don’t want them to dry out to the point that the roots and leaves of your seedlings shrivel.

Water from the side instead of watering over the top of the blocks, especially when seedlings are just starting to germinate. If you water overhead, your blocks will fall apart.

I usually pour in the water and wait for the blocks to soak it up before I decide whether to add more water. Again, you don’t want to saturate your blocks so that they fall apart.