If you've been starting seeds for a few years, you may have heard of soil blocks—those little formed squares of seed starting mix. Like many things in gardening, their popularity rises and falls, but they're worth checking out even if the fad may have passed. If you're wondering whether switching to soil blocks is worth it, here are a few factors to consider.

What are Soil Blocks?

Soil blocks are small cubes of seed-starting mix pressed together with enough force to retain their shape. They're a grown-up version of making mud pies. While we don't normally start seeds indoors in actual soil (seed-starting mixes are a sterile mixture of other ingredients), we often refer to it as such. 

Soil blockers, the tools designed to make soil blocks, typically are sold with block sizes of ¾ to two inches across, with some up to a mammoth four inches. Larger blocks may have an indention to make an imprint for a smaller block to fit into, aiding in transplanting up to larger blocks as the seedlings grow. For example, planting an ¾ inch block into a hole in a 2-inch soil block as the plant grows. Soil blockers make several cubes at one pressing. A two inch blocker may make four blocks per run, while a ¾ inch tool commonly makes runs of 20. Soil blocks can be made with homemade tools too.

The process is similar, whether a homemade DIY blocker or one purchased from a manufacturer. A tub of seed starting mix is wetted until damp—more so than for normal planting. A handful, when squeezed, should drip a bit of water. Then, the blocker is either pressed down with force into a pile of soil or filled in the case of a homemade blocker. Excess soil is brushed off, the plunger is pushed firmly but quickly to expel the soil blocks, or the mold is flipped upside down like emptying an ice cube tray. 

With the proper mix, which we'll discuss below, the small blocks retain their square shape even when they begin to dry or get rewetted. Blocks should hold their shape and be neat and trim. Sloppy blocks will fall apart as they dry and rewet. 

Soil blocking shares some similar techniques to other seed starting methods. They’ll need to be in some sort of tray to hold them. Seedlings will need light, and just like with trays, watering is best done "from below," meaning add water to the tray and let the blocks soak it up. Don't water from above on top of the blocks. Soil blocks will often change color as they begin to dry, providing a clue that it's time to water. As the seedlings get larger, you'll need to water more often, just as with cell trays. 

Benefits of Soil Blocking

I use soil blocks in combination with other methods every year for several reasons. A caveat here is that my favorite size is the mini or ¾ inch soil block. The learning curve was pretty easy, and now I find I’m much faster, both in starting seeds and in transplanting, and my seedlings look healthier.

Air-Pruned Roots

Each soil block is surrounded by a little bit of air, and the blocks aren't quite touching each other. The air gap encourages a process called air pruning of the roots. Basically, when the growing points of the roots hit the open air, they stop, and a signal is sent to start up a new root farther back toward the plant. 

It's similar to how pruning a shrub will cause newer, denser growth. Instead of circling around the plastic pot looking for somewhere to go and getting rootbound, the air-pruned roots become more numerous. Of course, problems can still occur if the plant is held too long in a small block.

Space and Soil Savings

For folks with larger gardens, starting seeds indoors in spring can grow to take over an entire room. Standard trays full of seedlings require space under the grow lights and a lot of soil (seed starting mix). However, a fantastic number of seeds can be started in the smaller sized soil blocks in very little space. 

As an example, with my ¾"  blocker, I make batches or presses of 20 soil blocks at a time. Once my tub with the premoistened mix is ready, a batch of 20 soil blocks takes me less than a minute to make. Two presses make forty blocks, which fit easily on a small foam tray. I reuse foam trays like the ones chicken is sold in at the grocery store. For larger batches, 160 seedlings (soil blocks) fit on a standard baking sheet—the ¼ sheet size—which is cheap and works great. Since I start anywhere from 500 to over a 1,000 seeds every spring, space is at a premium.

While it is true that those seedlings in small blocks will need to be potted up if they grow larger, I find that adapting my start date so they won't need potting up works quite well. This allows me to start nearly 200 little plants, grow them for 2–3 weeks, and then transplant them outdoors all in the space of a standard 1020 tray. I also use less seed starting mix than filling multiple larger cell trays. 

Less Transplant Shock

Air-pruned roots in soil blocks are not disturbed as much—if at all—at transplant time. Seedlings grown in plastic cell packs often have circling roots around the edges of their pot. Rootbound seedlings grown in cells or small pots need to have their roots trimmed and roughed up when transplanting to encourage new growth. Seedlings started in soil blocks establish more quickly, giving your plants a week or two headstart over those grown in plastic trays.

Less Waste

Plastic waste is seemingly hand-in-hand with gardening these days. Plants from the garden center come in plastic pots. Trees and shrubs are sold in larger black plastic nursery pots. Bagged soil and fertilizer are packaged in plastic bags. For gardeners who wish to reduce their plastic footprint, soil blocking allows the use of reusable trays (or fun homemade wood trays) and fewer cracked cells and 1020 trays headed to the landfill. 

While I still use some cell trays and plastic pots, the amount of plastic I need to use to start all those seedlings mentioned above is greatly reduced. I don't have to buy, sanitize, or take them to the landfill after they crack or break.

Why some gardeners hate soil blocks

Not everyone is on Team Soil Block. While they are beneficial in several ways, there is a learning curve. The moisture level and quality of the soil mix strongly affect the ease or frustration experienced when making soil blocks. When the mix is right, the tool and process used greatly determine the speed of the task. Many new soil blockers decide the method is slow and go back to plastic cell trays after a year or two. 

If you plan on using only 1.5 or 2-inch blocks, there isn't a significant space saving over regular cell trays. However, you'll still experience the other benefits of soil blocking. Many gardeners also struggle with getting the mix right, which leads to a frustrating time filling and compacting blocks.

Choosing the right soil block mix

Many soil blockers insist on making their own custom medium of several components to form the best soil blocks. Fortunately, we’re usually quite willing to share our recipe. Common ingredients include compost, peat or coco coir, sand, perlite, fertilizer, loamy soil, and more. 

When preparing your mix, you'll likely need to sift it through a screen. A homemade tool using ½" hardware cloth works well. Compost and potting soil often contains shredded bark which has yet to break down. A piece of bark or a small stone under the soil blocker will prevent it from filling properly when pressed against the bottom of the tub. 

If the seedlings will be grown larger before transplanting, like a tomato, a mix with fertilizer included is needed. While seedlings have the energy they need for the first week or two within their seed, longer-term growth needs are supplied by nutrients taken up by the roots. Sterile seed starting mixes are great for germinating seeds without any soil-borne disease issues but don't typically have much for nutrients.