Starting Seeds in Soil Blocks Part 1, Making the Mix

Soil blocks - just a cube of compressed dirt - have advantages for gardeners who grow their vegetables and flowers from seed.

40 lettuce seedlings in 3/4" soil blocks
in a scavenged plastic deli tray

There is no plastic except for the reusable, perhaps scavenged, trays for the blocks.  The micro blocks shown here take a very small amount of space compared to other seed starting methods. If a seed fails to sprout, the block can be tossed into the garden or compost bin. When well-sprouted, after a couple of true leaves have developed, the seedlings are ready for planting out or moving into a larger soil block or a pot.

The Block Makers

The best-known brand of soil block makers for home gardeners is Ladbrooke, sold by several seed and garden supply web sites. They make several sizes.

Micro 20, Mini 5, and Mini 4 block makers

• Micro 20: makes 3/4" cubes for starting small seeds.
• Mini 5: makes 1 1/2" cubes for medium seed.
• Mini 4: makes 2" cubes for medium seeds or for potting up seedlings in the 3/4" blocks
• Macro (not shown): makes 4" cubes for big seeds, or the mini cubes can be inserted into this for potting up.
• They also have various sizes you can use while standing up

The block makers are not expensive if you compare them to the expanding peat pellets (10 to 20 cents each).  If cost is an issue, you could share the cost with other gardeners and periodically have a block-making party. 

Soil Block Mix Ingredients

There are a lot of very complicated recipes for soil block mixes out on the Internet.  They are so complicated they scare gardeners away from using soil blocks.  All you need is some sort of growth medium and something with short fibers to hold the blocks together.
Here's my simplified version:

• Bag of commercial seed starting mix or "light weight container mix"
• Coconut coir "brick" of a brand that is sifted to give lots of 1/2 inch fibers
  (Botanicare Cocogro is one brand that has long fibers)
• Water
• The usual buckets and scoops

Making the Soil Block Mix

The block mix works better if it has at least 24 hours to fully hydrate, so mix this up on a Friday evening if you want to make blocks that weekend.

1. Rehydrate the coir brick according to its package instructions.  Break it up and fluff it well.This can be stored dry in a covered bucket.
2. Sift the potting mix to remove any lumpy bits of bark. I use 1/8" hardware cloth and sift an entire bag and store it
3. Mix 2 parts potting mix and 1 part coir together in a bucket or other container.
   
   NOTE 1: A "part" is one measuring scoop. whatever size scoop you are using.
   NOTE 2: These proportions are not critical. If you have problems with the blocks falling apart, use more coir. I have seen recipes with one part potting soil and 4 parts coir that reportedly worked well.
   NOTE 3: You can store the dry mix and add water to small batches of it as you need more blocks.
   NOTE 4: The larger blocks hold together better with more coir in the mix. I ended up with about 1 part potting mix and 1 part coir when I started making large blocks.
   
   
4. Add water and mix it well.  Add about 1/2 part water to start, stir it and see what the texture is. Add more water if it is needed.
   
   NOTE 5: The mix should be wet, like runny cement or fresh cow manure.  You want an excess of water, enough to see a bit of puddling if you dig toward the bottom of the container.  The mix should hold the shape of the trowel.

   

   Properly wet soil block mix, showing puddle

5. Cover the bucket and let it rest and hydrate overnight.

How much mix will you need?  The quantity shown above almost filled a salvaged laundry detergent bucket when it was dry, and now is about half a bucket of wet mix. It made several hundreds of 3/4" blocks, and a few of the larger blocks.

Should you add fertilizer?

Should you add fertilizer to the mix or not?  This is a tricky issue, because seedlings are easy to kill. I firmly believe that it's better to under than overfertilize.

How long do you plan to leave the seedlings in the blocks?  Because I plant the seedlings out into their permanent garden spot when they have their first true leaves I do not need to add fertilizer.  If you will be growing the plants in the blocks for longer than that, the seedlings will probably need fertilizer.

Some of the complicated recipes called for a variety of dry pulverized mineral and nitrogen sources. I'd take the easier route and use a fertilizer meant to be dissolved in water. Using diluted liquid fertilizer in the water for hydrating the mix would be one way to fertilize.  Watering the blocks occasionally with the liquid fertilizer would be another.