Many home gardeners are familiar with plug trays. These rectangular plastic trays with dozens of tiny pots are widely used to start seeds in a more controlled environment before transplanting into the garden.
At Good Dirt Farm, transplanting seedlings is very important. It gives our plants a head start out in the field, allows us to better control germination and early growth stages, lets us get started early in the spring, and more.
However, we don’t use the standard plug trays to do this. Instead, we use soil blocks.
What Are Soil Blocks?
What are soil blocks? It’s fairly simple! A soil block is precisely what it sounds like, a compacted cube of potting soil.
In order to create these blocks, we use a tool called a soil blocker. A soil blocker is basically a hand-held press that compacts wet potting soil so that it sticks together and forms a solid cube. Ours makes five 1.5 inch cubes at a time.
These blocks are then assembled into a tray similar to a standard plug tray, called 1020 trays. Instead of containing individual plugs, these trays are totally open, with good airflow out of the bottom. Each one holds 72 of our soil blocks.
The soil blocker creates a little indentation on the top of the block. We simply drop seeds into that hole, sprinkle a bit more soil on top, and that’s it! With proper temperature and humidity, those seeds will germinate in a few days.
Why Use Soil Blocks?
There are a huge number of advantages to soil blocks over conventional plastic plug trays.
Reduce Plastic Use
First, the blocks use no plastic, so we don’t worry about fragile plug tray plastic that breaks apart quickly. The 1020 trays we put the blocks in are fewer in number and built to last several years, making them more sustainable. We look for any opportunity to use less plastic at Good Dirt Farm.
More Space for Roots to Grow
The blocks are also bigger than a normal plug, giving plant roots more room to grow before transplanting. Speaking of transplanting, when the time comes to put those plants into the ground, we simply pick them up and put them in. No need to spend time trying to gently pop the root ball out of its plastic enclosure!
Air Pruning
Probably the biggest advantage of soil blocks is a concept called air pruning. Basically, when we make the block, we create a smooth wall on each of the sides of the block. When a seedling’s roots encounter that wall, instead of wrapping around the block, they simply stop in their tracks and wait for more soil to appear in front of them.
If you’ve ever seen a store-bought seedling pop out of a plastic container, you know that the root mass can get quite messy. Transplanting seedlings with a huge wrapped root ball doesn’t do the plant much good, because those roots take more time to re-orient to the soil, and you likely have to damage them to get them to un-ball in the first place.
Soil blocks not only eliminate that problem, they also accelerate the plant’s transition to the field. The air-pruned roots, upon being placed in the soil, immediately continue their growth directly into the soil, unlocking the soil’s nutrients and water.
To put it simply, soil blocks result in a stronger transplant.
Disadvantages Of Using Soil Blocks
The main disadvantage to using soil blocks is easily time. While it takes seconds to fill a plug tray with soil, it usually takes me 5 minutes or so to make a full tray of blocks. This is a sacrifice we gladly make at the farm, because the advantages still outweigh that extra time.
Another thing to consider is the cost of good seed-starting soil. Our seed-starting mix is purchased, and it’s not cheap! Soil blocks use a good bit more soil per transplant than plug trays, so that is also something to consider if soil blocking seems worth a try.
How Can You Get Started?
If you start more than a few dozen transplants per year, we think you should definitely consider soil blocks! Getting started is relatively cheap, and very easy.
All you really need is a soil blocker and something to put the soil-blocked seedlings in. Using some old plug tray bottoms is good enough to hold the blocks. You can buy a soil blocker and plug trays pretty much anywhere you can get gardening materials.
Once you have these materials, your seeds, and some quality seed-starting soil, you’re ready to give soil blocking a try!