What Are Soil Blocks? (5 Common Questions Answered)


Have you ever spent weeks germinating seeds and nursing tiny seedlings, only to have them struggle or die when you transfer them to their permanent home? If so, you probably haven’t tried using soil blocks.

Simply put, soil blocks are exactly what they sound like: soil that you form into the shape of a block. They are primarily used as an alternative to seed-starting trays to sow seeds and care for seedlings. Many gardening experts who use soil blocks praise them for their ability to produce strong root systems.

While soil blocks can be extremely useful for starting plants, they aren’t a magic solution. As with most plant care methods, your success will depend on the quality of materials you use and the growing conditions you provide. 

This article will take a deeper dive into soil blocking, including how to make them yourself, and how to care for your seedlings once they’re planted. 

What Are Soil Blocks?

Soil blocks are small cubes of specially made potting mix that double as both the container and the medium for starting seeds. You can purchase soil blocks at a garden center or nursery, or you can make them.

root growth
Soil blocks are small cubes of potting mix (soil) that act as both container and growing medium for seeds.

Many growers prefer to make their soil blocks since it allows you to control the medium’s composition, the cube’s size, and the shape of the indentation where the seed rests. 

If you decide to make your own, you will need to invest in a soil blocker tool, which is used to form the mold that you will fill with soil. These devices come in a variety of sizes, are usually inexpensive, and are easily found online or wherever you get your gardening supplies. 

Are Soil Blocks Any Good?

Soil blocking has been a popular way to start seeds since as early as 2,000 years ago. There are many benefits that explain why it has stood the test of time, including:

Minimal Waste

Some gardeners save their seed trays for the next growing season once they’ve transplanted their seedlings, while others buy new ones each year, and dispose of them when they’re done.

plug plants
Soil blocks are an alternative to seed trays. They save you the trouble of getting seedlings out of the cells in plastic trays.

Either way, soil blocks cut out the need for multiple plastic seed trays altogether. 

Healthier Roots

Plants that start in soil blocks develop robust root systems because they receive more oxygen than those that form in seed trays. 

seedling roots
The root system of a plant started in a soil bock may end up healthier and more robust, due to increased oxygen.

Easy Transplants

Soil blocking is the most efficient way to prepare seedlings for transplanting to a large container, or directly in the ground. The plant’s roots grow until they get to the edge of the block, and then pause, as if they’re patiently awaiting the move to their new home.

transplanted lettuce
Soil blocks make it easier to prepare seedlings for transplant into the ground (or into larger containers).

Since you can easily see when the roots reach the edge of the soil block, you will know when it’s time to transplant. Once you plant the seedlings in a pot or in the ground, the roots can easily stretch out and quickly establish themselves in the new soil. 

No Transplant Shock

Since your plant’s roots stop growing at the edge of the soil block, they won’t encircle the cube and become pot-bound. This makes for a much easier transplant, avoiding the need to disturb the root system, thus avoiding the damage that causes transplant shock. 

How Do You Make Soil Blocks?

Learning to make your own soil blocks almost always involves some trial and error. But once you get the hang of it, it can be a fun, relaxing process. 

Materials Needed: 

Here’s the step-by-step process:

Step One: Water Your Soil Mix 

Pour your potting soil into the large container and moisten it with warm water. Warm water soaks into the soil more readily than cold water.

watering can
To make soil blocks, start by watering the potting soil mix until it is wet enough, but not too wet, to make a solid clump.

You want the mix to be damp, but not soggy. When you pick up a handful of the mix after adding water, it should feel moist and clump together, but it should not drip. 

Step Two: Fill The Soil Blocker Tool

You can either press the device down firmly into the mound of potting soil, or pack the dirt in by hand. Since your goal is to end up with solid cubes, you need to pack the molds tightly and evenly, leaving no air pockets.

Then, use your finger or a butter knife to level off the top of the mold, scraping the excess soil off. 

Step Three: Press The Handle

Release your newly formed soil blocks into the tray by pressing the handle of the blocker down. Lift the soil blocker up slowly as it drops the cubes onto the tray. 

Step Four: Repeat If Necessary

If your blocks crumble or fall apart, don’t get discouraged. In fact, it’s highly unlikely that you’ll release perfectly formed soil blocks on your first try! Simply put the mix back into the container and try again.

Ask yourself why the blocks crumbled – was the soil not moist enough? Were there air pockets in the corner of the molds? Make the necessary adjustments and press on. 

Step Five: Plant Your Seeds

There should be an indentation in each soil block, which is where you’ll place the seeds. As a general rule, only plant one or two seeds per block. When you put the seeds in, press them into the soil gently.

winter melon seeds
Plant 1 or 2 seeds per soil block. Otherwise, you will need to thin your seedlings later.

Can I Use Potting Mix For Soil Blocks?

Your soil choice is the key to success in making homemade soil blocks. Even if you have the right components in the soil, but the proportions are off, your cubes might not stick together. 

repotting plants
You can use ordinary potting soil mix for your soil blocks, and you can also make your own custom mix.

It’s difficult to achieve the desired consistency with traditional store-bought potting mix. If you do, make sure it contains peat moss/coco coir, and perlite to help with drainage. These two ingredients are arguably the most important, since they contain the fibers that hold the soil blocks together. 

Some companies also sell their own soil block mixtures, but you must ensure the ingredients are right for the seeds you’re growing. To try a pre-made soil block mixture, check out a greenhouse or nursery near you, or find a mix online. 

perlite
Perlite is one potting mix ingredient that helps with drainage.

The best way to guarantee your soil blocks will form properly is to make your own potting mix. According to Penn State’s Extension office, a good mix consists of peat/coco coir, perlite, soil, and compost. Coarse grain sand is an acceptable substitute for perlite.

(You can find a recipe for homemade potting soil here).

It’s worth noting that many plant owners are shifting away from using peat moss in recent years, due to sustainability concerns. 

How Do You Water Soil Blocks?

Just like any other method of starting seeds, you will need to be diligent about keeping the potting mix consistently moist, but not soaking. Initially, you might not need to water them for the first few days.

Once you notice the top layer of soil drying out, use a spray bottle to mist the soil blocks. A heavy stream of water could erode the blocks or cause them to crumble. 

spray bottle head
Use a spray bottle (plant mister) to keep soil blocks moist.

Many gardeners swear by bottom watering for soil blocks. To do this, you can place the soil blocks in a mesh bottom tray.

Put that tray inside a tray with a solid bottom. Then, you can fill the bottom tray with water as needed. Do not let the soil blocks sit in water once they are moist enough, or they may fall apart. 

How Do You Fertilize Soil Blocks?

If your soil blocks are comprised of a commercially-made potting mix, there is probably already fertilizer in the soil. If not, you have two options:

Option 1

Add fertilizer to the mix while you’re forming the soil blocks. Make sure the fertilizer you select is appropriate for what you’re growing.

ammonium nitrate
You can add fertilizer directly to your potting soil mix before forming it into blocks.

Some vegetables require different nutrients than others, so always read the labels carefully, and only use the recommended amount for starting seeds. Over-fertilizing will harm your fragile seedlings. 

Option 2

If your soil blocks don’t contain fertilizer, simply add a soluble fertilizer to the water during your regular watering. Again – choose the appropriate type and dose for your seedlings. 

Soil Blocks vs. Trays

Making the choice between soil blocks and trays for starting seeds boils down to personal preference. The sowing process is easier when you plant in seed-starting trays, but transplanting the seedlings is riskier. Conversely, forming soil blocks is more difficult and time-consuming, but they are a breeze to transplant. 

seed tray
Sowing is easier in seed trays, but transplanting is a little more difficult than using soil blocks.

If you’re curious and determined to hone your seed-starting methods, consider experimenting with both to see which you prefer. 

Conclusion

Any new gardening technique can be intimidating if you’ve never tried it before. Thankfully, soil blocks are a tried and true method, and there are many resources available to help you succeed. When in doubt, reach out to your local cooperative extension office for advice. You can find yours on the United States Department of Agriculture’s website.

I hope you found this article helpful. If so, please share it with someone who can use the information. If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here. Enjoy! 

About the author:
Kathryn is a plant enthusiast and freelance content writer who specializes in home and garden topics. Based in New York, you can get in touch with Kathryn at https://kathrynflegal.journoportfolio.com/.

Kathryn F.

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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