Soil blocking began in Holland as a means of starting garden seeds indoors before the spring frosts had abated. Essentially, soil blocking is a method of piling dirt onto a tray in blocks and planting seeds in those soil blocks. Soil blocking grew in popularity in the early 2000s, and continues to be a growing trend in seed starting. For good reason!

Seedlings started in soil blocks are healthier, stronger, and more capably transplanted into the garden after spring frosts have passed. You’ll never go back to seed starting trays after you learn the soil blocking method!

What is Soil Blocking?

Starting seeds in soil blocks typically involves compacting a tiny piece of dirt which germinates the seed. After the seed’s roots are established enough that they’re visible on the sides, the gardener transplants the seedling inside another larger parcel of dirt. The larger block typically has a hole in the center to accommodate the smaller block. This process may be repeated several times depending on how early seeds are started, how much root base certain plants need, and how long the winter lasts.

I first learned about soil blocking from author and gardener Eliot Coleman in his book The Winter Harvest Handbook & Year-Round Vegetable Production (affiliate link). He mentioned it as an efficient method of starting seeds without being beholden to the fickle nature of spring weather.

With the added dirt surrounding its roots at each added block, the plant extends its reach organically and forms a solid root base. Combined with hardening off the seedlings, this solid root base ensures a successful transplant into the garden after spring frosts have passed.

Benefits of Soil Blocking

Soil blocking is similar to the commercially available seed starting trays and single-use seed starting pots. However, there are a lot of benefits that more modern seed starting methods don’t have.

Increased Air Flow

Roots need air too! Because 5 sides of the soil block are exposed to the air, the plant’s roots have the greatest ability to pull air in. This reduces the possibilities of root rot and means greater growth for the young seedlings.

No Root Binding

Modern seed starter trays bind young seedlings into tiny squares or circles and don’t allow them to venture into their neighbor’s soil. This is good because it keeps the plants separate and stops them from stealing nutrients from one another, but it also harms the growth of the seedling.

Plastic makes plants fight to let them through and ultimately stunt the growth of the plant. The plant thinks it can overcome any physical obstacle. (Have you seen what tree roots do to concrete on sidewalks?!) So, it won’t course correct itself when faced with an immovable object like a mesh of plastic or the edge of a seed tray.

Soil blocks, however, allow plant roots to wander throughout the dirt until they encounter air. When plant roots encounter air, they perform a process called air pruning. When air pruning, roots either clip themselves off to preserve the moisture of the plant or they redirect themselves in search of soil that hasn’t been investigated yet.

Starting from a small, but compact root ball, plants tell the gardener when they’re ready for more soil. Multiple roots will appear veined at the four vertical edges. When you place a larger soil block around a smaller soil block with a proper root ball, the roots recognize there’s more dirt to burrow into. This results in a stronger root base.

Eliminates Problems of a Late Spring Start

Many gardeners know the pain of starting their seeds early indoors, only to be thwarted by a late spring frost. In fact, I had this very problem last year when I was ready to transplant my seedlings outside. Not only does a late spring derail plans, but it means the seedlings continue to grow while you wait for the weather to cooperate.

If left too long in a seed starting tray, even a successful seedling will attempt to break out of their plastic prisons looking for more ground. Finding none, they’ll start to wither in want of true garden soil. This results in hampered growth all season long.

The soil blocking method means you can continue to add dirt around your seedlings, increasing their root base indefinitely. Or at least until the weather finally cooperates. They’ll continue to take in nutrients from the continually added soil and grow, rather than stay held in an uncertain stasis.

Downsides of Soil Blocking

Let’s be honest, it’s all about cost. Commercial soil blockers for sale can get expensive really quickly. When starting seeds, you may start with a 1 inch by 1 inch by 1 inch square block. This tiny segment of soil essentially holds moisture and warmth near enough to the seed to commence germination. This initial 1″ soil blocker purchase is usually fairly inexpensive. The cost comes from the fact that you need multiple soil blocking tools to achieve the full method.

That 1″ soil block must transplant into a 2″ soil block. So you need a 2″cube tool to form the blocks. That 2″ soil block then requires a 3″cube or 4″cube to transplant into. This requires another tool to create those blocks. Also, each successively larger soil blocker increases incrementally in price. For a full set of soil blockers, you could be looking at over $200 USD.

Yes, soil blockers are a one-time purchase. You will use them year after year after year if you wash them, store them, and care for them regularly to prevent or remove rust. But that’s a lot of money if you’re not yet sure if soil blocking will work for you and your garden.

Happily, if you don’t want to shell out the cash (or just want to see if this method is for you first), I have a secret for you. Commercial soil blockers are not required for soil blocking. Read on for a no-fail, no-soil-blocker-purchase-necessary method!

Blocking Soil Without A Soil Blocker

Yes, you can use the soil blocking method without a soil blocker. The Dutch have been doing it for thousands of years. Guaranteed they didn’t have access to fancy commercial tools to make everything just so. They did what gardeners all around the world do. They got their hands dirty. And isn’t that the fun of gardening?

What You Need

Hint: Not a soil blocker. But you will need a few handy items to get started. You probably have all or most, if not all, of these about your house.

Seed Starting Soil

Let’s start with the obvious. If you’re going to make soil blocks, you need soil. It’s right there in the name. However, what kind of soil you should use to start seeds is an age-old question. Everyone has a different answer and opinion. If you ask 10 gardeners what to use to start seeds, you’ll probably get 10 different answers.

Here’s the thing: seeds are self-contained little powerhouses. They have everything they need to germinate except for water, light, and heat. No amount of fertilizer in your seed starter is going to give them a better start than what’s contained inside their little husks. That said, there are a few things that I insist on when starting my own seeds. Soil blocking is no different.

My Seed Starting Mix

• Soil – It may come as a surprise to learn that many seed starter mixes don’t contain any soil at all. This is because, as stated above, the seeds themselves are enough at the very beginning. But if your seed starter medium doesn’t have some soil in it, it will fall apart when soil blocking. That said, make sure your soil is light and airy. Tiny seedling roots are tiny, and need gentle soil to burrow into at the onset.
• Coir – The fibrous hairs of a coconut. It serves as an excellent binding agent and provides structure in soil blocks. Old coconut hanging basket liners are absolutely perfect. Rip them apart and add those fibers to your purchased seed starter if it doesn’t have it already. (Hint: like the grocery store, read the ingredients).
• Some form of tiny stones or course sand – You’ve probably heard of perlite and/or vermiculite. At the most basic level, these two work to create pockets in the soil which capture air that your seedlings need to build strong roots. Avoid seed starters with plastic beads that also serve this same purpose. Who needs more microplastics in their life these days?

A Sturdy Tray

In an attempt to be fully environmentally friendly, I first attempted to create a tray from sturdy cardboard boxes. However, I found the cardboard leeched away water from my soil blocks. Not ideal. So, note to self (and to you!) don’t do what I did!

Some seed starting trays that come as pre-packaged seed starters are reusable. If you’ve saved these trays in the past, give them a good scrubbing with soap and water (if you didn’t wash them when you put them away last year). These trays will break down over time, but they’ll give a good few years as soil blocking trays before they do.

You want a tray that is…

• Not water absorbent, so it doesn’t steal water from your plants.
• Sturdy enough to hold your soil blocks, so it doesn’t collapse and make a mess.
• Deep enough to hold some water, so it doesn’t leak when you water your seedlings.
• Wide enough to account for the growth of your soil blocks over time. (Or multiple trays that can accommodate the extra girth of your soil blocks.)

I found that metal cooking sheets work very well. Just make sure the edges are sufficiently beveled and the cooking sheet isn’t too bowed from the heat of the oven. As noted above: you don’t want water to leak off the sides when you water your soil blocks.

A Large Bowl or Bucket

You’re going to need a container to mix your seed starter and water together in to make the mud that will make up your soil blocks. A large mixing bowl works if you don’t mind getting it dirty. (You can wash it!). A large bucket or gallon ice cream tub would work great too. Heck, if you’re making a whole garden of soil blocks, use your wheelbarrow as the container! Whatever you choose, you want enough space in your container that you have enough room to dig in, but won’t get mud everywhere while you’re forming your blocks.

A Mold

One thing commercial soil blockers do really well is form several perfectly sized blocks at once. Depending on the brand and the size, they may make up to 20 small portions of dirt for seed starting. This is great if you’re planting a large garden and efficiency is a concern. But, like I’ve said, you don’t need soil blockers to make soil blocks.

For the sake of starting all your seeds on the same level, you may want a mold as a guide. That way you’ll have little uniformly-formed blocks to work with. This mold can be anything. Use your imagination! Your chosen mold should have a few properties.

• Open on both ends (like a cookie cutter). This way you can pack soil into your mold on one end and push the formed block out the other side onto your tray.
• Deep enough to make soil blocks at most an inch tall.
• Not wider than an inch. You want your soil blocks small to start.
• Not so tall that you can’t reach your fingers in all the way. If you can’t reach, pushing your soil block out of the mold will be tricky.

A few months before seed starting, I started saving toilet paper rolls. Cut in half, they worked brilliantly! They did dissolve in the water after creating about 3 blocks for each half-roll. But that was quite enough for my purposes.

Hint: You don’t have to take the “block” part of soil blocks literally. You can even invite your children (or your own inner child) to the task and make little mud patties with your hands if you’re so inclined!

Preventing Soil Blocks from Falling Apart

The most common complaint of people who have tried soil blocking is that soil blocks are prone to falling apart. This is a legitimate concern, but easily dealt with. Soil blocks largely fall apart if there is an absence or an abundance of water.

Think about the seed starter you’re using. How much coir, perlite and/or vermiculite does it have? These ingredients are great webs in the fabric of your seed starter, but they also dehydrate their surroundings. Dehydration in the soil is great for air flow. But soil blocks already have air flow on 5 sides, so you don’t need as much dehydration as modern seed starters. Essentially, if you add too much non-soil medium, you won’t have enough mud to make your mud pies. And that’s where your seedlings grow.

The opposite is also true. Soil blocks that are too wet will also fall apart. Making soil blocks is a lot like making cookies. Too much water and the dough sticks to your fingers and doesn’t form right. Too little and it crumbles in dusty chunks. The middle ground, as in most things, is key.

How to Plant Seeds in Soil Blocks

Now that you have your soil blocks, how do you plant your seeds? You don’t want your soil blocks to fall apart when planting, so you need to be careful here.

Commercial soil blockers create soil blocks with a divot in the center of the soil block that is perfect for dropping your seed into. If you’re making your own soil blocks DIY style, you have to make that divot yourself.

No problem! You probably have tools in your kitchen that can do this work for you. Toothpicks, bamboo skewers, beverage stirring sticks, or even chopsticks are all great options to create a small hole big enough to drop a seed into. If you have help, you might ask one person to gently hold the sides of the soil block while you make your divot.

Once you’ve made the hole in the soil block, it’s a simple matter of dropping your seed in the hole. If your seed doesn’t require light to germinate (some do!) fill in the hole with a small sprinkle of seed starter that didn’t make it into your soil blocks.

Transitioning to Larger Soil Blocks

Once your plants are a few weeks old, they’ll start to complain about their small surroundings. Their roots will show on the sides where they’re air pruning. They may even send out root tendrils in an attempt to lay claim to their neighbors’ soil. When you see these signs, it’s time to add dirt and give them more room to grow.

You’ll need to call your trusty bowl or bucket back into service, just as you did when creating your initial soil blocks. Make up another round of your seed starter and get it good and wet so that you can form patties with your hands.

Add Cinnamon

Given that your soil blocks have been wet and out in the air for the past couple weeks, you probably want to add some cinnamon to your seed starter mix. Wet dirt is a prime location for white mold and other fungi that cause problems such as dampening off. Cinnamon is a natural anti-fungal agent, so adding it to your seed starter is an excellent deterrent to mold. This stage of the soil blocking process is an excellent time to tell the fungi who’s boss.

Mounding Dirt

Once you have your mixture ready to go, you’re ready to give your seedlings the real estate they’re yearning for. Gently pick up each small soil block that’s ready for more, and place it into your bowl. While you should be gentle whenever handling plants, your soil block isn’t likely to fall apart if it’s showing roots on the sides. The roots themselves are providing some structure to the soil block at this point. Once the existing root ball is in the bowl, it’s pretty easy to pack new soil around it from all sides.

Don’t forget the bottom! Your seedlings are growing in size three dimensionally. It’s easy to see that their leaves and stems are growing taller and broader, right? Likewise, their roots aren’t just growing laterally, but deep, too. So give those roots some soil to burrow into. Also, don’t compact the soil too tightly. Remember that plant roots need oxygen too!

Keeping Track

You’re probably planting multiple kinds of plants when you start your seeds. While you’re adding more soil, be careful that you don’t mix them up! Keep a careful, cognizant awareness (or take notes!) of which plants you’re moving where and try to place them back onto their tray in the same order as before, if you can.

Don’t get confused during this part of the process and forget which plants are which! Are those tomato seedlings the cherry tomatoes that require trellising, or the beef steak tomatoes that don’t?! At this stage of growth, they all look the same. Losing track could mean confusing your whole spring garden plan.

Home Again, Home Again

When you’ve finished adding about an inch of soil around all sides of your seedlings’ roots, place them on their tray and back under their grow lights. You’ll probably have to adjust the grow lights to account for the additional height of the soil.

You may also need an additional tray to account for the extra girth of the new soil blocks. Which means you may need an additional grow lamp too. Make sure you have their new temporary home ready to go before you give them new ground to explore.

Taking Turns

Not every one of your seedlings will be ready for more soil at the same time. Different kinds of plants grow faster and need more room for their roots than others. Even plants of the same type can grow at vastly different rates.

Our arugula grew so much faster than anything else we started in soil blocks this year and desperately required new soil much earlier than any of the others. Likewise, both varieties of tomatoes germinated and grew far faster than the Thomas Jefferson cayenne peppers.

Observe your seedlings every day and they will tell you when they’re ready for more. More water, more light, and, yes, more soil. If you really pay attention to them, plants are chatty little creatures!

Repeat the Process

Once you’ve added soil to your soil blocks once, you’ll be an old pro at it going forward. Repeat the addition of soil as many times as you need until the spring frosts have passed and they’re ready for the garden.

Like all seedlings started indoors, you’ll still have to harden off plants in soil blocks before transplanting them into the garden. But once hardened off, once your garden is prepared for spring planting, and once the danger of frost is over, your seedlings will be ready to transplant into the garden.

Good Luck, Gardener!

I really do hope you give soil blocking a try. After trying multiple different methods of seed starting, soil blocking is hands down the best start my seedlings have ever had indoors. Let us know if you tried the soil blocking method (DIY or otherwise) and if it worked for you!