We started using soil blocks to start garden seedlings several years ago, and really think they help us grow strong, stocky transplants that get off to a super start in the garden. Soil blocks were first developed in Holland as a more efficient method for starting seedlings or transplants, and extensive research in European countries has shown transplants grown in them are superior to transplants grown in containers.
Soil blocks are made using a seed starting soil mix that is compressed into a block by a forming tool. When the blocks are formed, a slight depression is left in the top of each soil block, and one or two seeds are placed in the indentation for each transplant desired. The big advantages to using soil blocks (over traditional container grown transplants) are:
Less expensive and faster – there are no containers needed for the individual transplants thereby saving the cost of containers and the hassle of dealing with them. Further time is saved during seeding because a depression is already created in the block for the seed (you usually have to make a hole or depression in other systems), and in transplanting because you don’t have to remove all the little pots.
No root shock – Seed block grown transplants don’t suffer root shock like transplants grown in smooth walled plastic containers or rough peat containers. Transplants grown in smooth-sided containers send their roots to the container walls, then the roots start circling the pots – when they are then set in the garden these roots are “shocked” by their new environment and growth is inhibited. Similarly, transplants in peat pots send their roots to the walls but start adhering to the rough surface as they circle the pots; when the pots are pulled away the roots are often damaged, and they suffer “shock” when set in the garden. Roots grown in soil blocks grow to the outside and then turn back inward – they avoid the air surface of the soil blocks. As a result, there’s very little shock when they’re set in the garden, because the roots can gradually adapt to the environment.
Pre-watered – With other methods of starting transplants, the pots or containers are filled with dry mix, then the seeds are added, and then you have to get the mix uniformly moist. This is often difficult when using seeding mixes which are initially quite resistant to absorbing moisture. With soil blocks, the seeding mix must be thoroughly wet before making the blocks or they won’t hold together properly, so there’s not even a need to water the blocks for several days after the seeds are added.
More Varieties – Plants that are normally difficult to start indoors (because they die from root shock) can typically be successfully started with soil blocks. These include cucumbers, squash, watermelon, cantaloupe, corn, beets, and other “sensitive” or tap-root type crops.
Quantity independent – Just a few soil blocks can be started or many soil blocks can be started, it’s not necessary to fill a whole tray of whatever system with seedlings.
More Space Efficient – More transplants can be fit per square foot using the soil blocks as opposed to pots.
The tools for forming the soil blocks do require an investment which is not insignificant; however, if they’re used for many seedlings over many years, the cost quickly becomes miniscule. Actually making the soil blocks consists of the following steps:
1) Put the growing medium in a suitable tub and add water. It’s best to wait for about an hour to let it soak in and then stir well. A thick oatmeal-like consistency is about right – there’ll probably be some trial and error before getting the right consistency.
2) Push the blocker into the soil several times to fill the chambers, scrape any excess mix off on the side of the tub, and set the blocker onto the tray that will hold the seedlings.
3) Hold the “T” shaped plunger in place while pulling the frame up. That’s it – soil blocks! Shown below are the 3/4″mini-blocks and it only takes me about 15 minutes to make a whole tray full.
Soil blocking tools come in several sizes, but the most popular for backyard or market gardeners are the ¾”, 2”, and 4”. The system has been designed so that things that are seeded into the ¾” mini-blocks (like lettuce or spinach) can then be transplanted into 2” blocks (that have been made with a ¾” square depression in the top) by just dropping the mini-block into the 2” block. Similarly, things seeded into the 2” block can be dropped into a 4” block made with a 2” depression in the top (see graphical depiction below).
There are several different soil mix recipes to use with soil blocks, and we use the recipes given here. You can use any flat trays that you have to put the blocks on, I used the trays from the seed starting system I used previously (which aren’t perfect because they’re not completely flat), and one tray holds 420 mini-blocks. For market gardeners that start many seedlings, there are also soil block makers that are used while standing up, but we’ve always used the hand held ones and they’ve been fast enough for the quantities of seedlings we start. This is the best method we’ve found for getting our seedlings off to a great start!