Soil Blocking: Science-Based Guide

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Soil blocking 101

Soil blocking is one of the newest ways to start seeds, and it is gaining popularity quickly. Let’s take a look at soil blocking as a tool for your seed-starting setups.

Why would you block?

  • Long run, less expensive
  • Less plastic use
  • Enjoyable process
  • allows for air pruning
  • It works with a number of different seeds.

Soil Choice

The first thing you want to look at is the choice of soil. There are many different options out there when it comes to soil choices, but simple is best. Ideally, your soil choice will be a regular seed starting mix that is homogenous in nature. This means little perlite, stems, and vermiculite.

This uniform soil medium will allow for proper aggregation. This aggregation is key to keeping the blocks together and also allowing for good seed-to-soil contact. If you intend on bottom-watering them, the uniform style is again important.

How Much Water

Water is able to force air out of the porosity space, which is key to the success of the plant. This means you cannot have too much water in the initial mixture. I like to ensure it is thoroughly saturated and to achieve this, I allow it to sit for a period of time.


For the initial tray you are planting into, you want to make sure there is a lot of porosity. I like to use the grate setups for this with a non-draining tray below. This allows you to bottom out and pivot as needed.

Blocking Size

This is determined by the size of the cotyledons. If your cotyledons hang over the sides of the block, it is too small and a larger block should be used for germination. If your cotlydons are a small spec when compared to the majority of the block, then the block is too large.


Compaction within the cells is the most important step in the entire process. This is because the level of compaction determines how quickly the block falls apart, how seeds germinate, and even the seedlings strength. When adding soil to the blocks, you want to ensure we are compressing it with our fingers and not simply relying on the push-down pressure.

When placing the blocks in the tray, you want to apply a firm, even pressure the entire time. Applying pressure during the release of soil is important for ensuring the blocks do not fall apart. This should shrink the block size by 34 to 12 of the initial block size.

Planting Your Seeds

Plant your seeds as you would in any seed-starting setup. I like to add vermiculite to avoid excessive mould formation. This addition is simply a sprinkle on the soil surface after your seeds have been added. The next step is to separate the soil block.

The reason we do this comes down to the benefits of air pruning. Added air flow between the blocks allows for proper air flow and ultimately a stronger root system. Air pruning allows the root system to become stronger and avoid becoming root-bound.

There you have it, a complete guide to soil blocking and maximising the results.