Soil blocks

Soil blocks are small compressed blocks of special sowing mix (this is not actually soil) used for starting seeds. They are one of the most ecologically sound method of raising seeds as they don’t require any kind of container, just something to sit them on for transportation.

The biggest advantage of soil blocks is that there is no root disturbance at all; once the plant roots fill the block it is planted straight into the ground. Because there is a relatively large volume of growing medium they are less prone to drying out and contain more nutrients so you can leave them longer before transplanting. They are especially useful if you are going to give a lot of plants away, as you don’t lose a container too.

There are a couple of drawback to soil blocks, the biggest being that you need a special soil block making press to make them (these come in several sizes, but the 2” one is most useful). Another is that the process of making them is relatively slow. 

I haven’t used soil blocks for starting seeds for a while, because I find recycled six pack containers simpler (I always go for the easiest option). However I made some recently because I wanted to take some photographs of the soil block making process.

The mix

Begin by mixing a batch of soil block sowing mix, which differs from other mixes in that it has to be cohesive enough to hold together when ejected from the mold. They also usually contain more nutrients. Here are three “recipes”:

2 parts peat (+ lime)

1 part soil

1 part vermiculite

1 part compost (or leaf mold)

1 part soil

4 parts peat (+ lime)

2 parts sand

1 part soil

1 part compost

Mix the ingredients thoroughly in a plastic tub or bucket, and then add one part water for every four parts of dry ingredients, to create a mix with the consistency of mud. The moisture level is critical, if you don’t add enough water, the dry mix won’t fill up the mold properly and won’t stay together when ejected from the mold. If you add too much water the mix may slop out prematurely, or may collapse when ejected. It’s not a bad idea to save a little dry mix for the not unlikely event that you add too much water initially. 

Making the blocks

Fill the mold by pressing it into the mix 2 or 3 times. You will know when it’s full because surplus liquid will ooze out of the top. If you put the mix in a 5 gallon bucket it can be deep enough that you can fill the mold with one press by rocking from side to side and pressing hard. Scrape the full mold across the top of the container (or with a trowel) to remove excess mix, put the mold on the chosen base and eject the block carefully. Rinse the mold in a bucket of water as necessary to stop the blocks sticking to them. Place the blocks close together to minimize hiding places for slugs and earwigs, but not so close that they are touching (which may cause roots to travel from one block to another). Don’t try to move the newly made blocks as they aren’t very strong and may disintegrate if roughly handled.

Planting  the blocks

Just drop the appropriate number of seeds (usually 2 - 3) into the indentation on top of the block and cover with a little soil mix (or don’t cover them at all – in which case you must be careful they don’t dry out). The blocks don’t really need watering for several days, but I like to mist the surface to wet the seeds thoroughly.

The blocks get more cohesive as the growing roots of the plant (or plants) bind it together and eventually you can handle it quite roughly without it falling apart. If the plant gets too big for the block before you are ready to plant out, simply plant them in a 4” pot.

Tomatoes germinating 

The same plants a couple of weeks later. The blocks are green because they have been somewhat wetter than is ideal.