Over the past few years I’ve been hearing a lot of talk about soil blocks, they have become very popular with market gardeners and were recently featured by Monty Don on BBC Gardeners World. Over the past few weeks I have been trying one out for myself to see if they are really as good as people claim.
So what is a soil block?
Essentially soil blocks are a replacement for plastic pots, roottrainers and peat pots. Soil blocks are blocks of compressed compost, which you can use to germinate seeds or transplant seedlings.
To create soil blocks, you need a special “soil blocker” tool. The tool is pushed into the compost mix until compact and then the soil blocks are just ejected from the tool and you are ready to plant. I watched this video which really gave me an idea of how this works and the consistency of the soil to use.
You can’t use regular potting compost or soil, due to the way the soil blocks are made. The compost is watered and mixed to create a paste like consistency.
To achieve this, the compost mix needs fibrous material and be able to hold water, as the soil blocks don’t have nonporous walls. BBC’s Monty Don suggests the following recipe for a soil block mix:
- 1 part garden soil
- 1 part leaf mould
- 1 part garden compost
- 1 part grit or vermiculite
- 4 parts coir
Many recipes for blocking mixes suggest adding peat. However, using peat is not good for the environment, because peatlands, such as bogs and fens, are important habitats for a variety of plants and animals.
Because peat grows very slowly, harvesting it will cause huge damage. Peatlands also help to fight climate change, because they trap carbon dioxide. So it is important that we protect our peatlands and one thing we can do is not to use peat in our gardens.
Advantages & Disadvantages Of Soil Blocks
Time – Making soil blocks does take more time than just filling a few pots but is ideal if you grow a lot of seeds and the mixing can be done in bulk.
Cost – The tool to create the soil blocks is an outlay but will last a lifetime. Roottrainers are a similar alternative but are very expensive and have a relativly short (3-4 years) life span. I think long term soil blocks are good value for money.
Space – If you have only limited space, then these soil blocks are ideal as they don’t take up as much space as plastic pots.
Environment – Using soil blocks rather than plastic pots is more environmentally friendly and will get rid of a lot of plastic in your garden.
The Mix – I used the soil block mix suggested by Monty Don, but when researching the topic I did find other recipes too. But what I would say is, that you want to buy good quality compost. If the compost you buy is of low quality and contains uncomposted woodchips I don’t think it would work for soil blocks.
Germination – In my trials growing sweetcorn, lettuce, peas, and beetroot I have found the germination to be as good as any alternative. It’s hard to overwater soil blocks which I think does help with germination.
Plant Quality – Using plug trays or pots can quickly lead to the plants becoming rootbound, with soil blocks the roots are air pruned which gives a far better quality of plant when it comes to transplanting. Plant quality is really the big advantage to using soil blocks over plug trays.
No transplant shock – Using soil blocking makes transplanting so much less stressful for the seedlings which means they will grow stronger and healthier. And it also means that you can start more sensitive plants such as squashes, cucumbers or courgettes from seed.
Starting earlier – Because you avoid transplant shock, you can start earlier indoors and give your plants a strong start, which means you can plant them outdoors earlier and enjoy an earlier harvest.
How to use soil blocks
Generally, soil block makers make different soil block sizes, maxi, mini and micro blocks. Depending on the different equipment you own, you can make holes to fit the smaller blocks into the big ones, small indents to plant seeds or small holes to plant seedlings.
The micro blocks are the smallest size and could also be called germination blocks. They are ideal for growing your vegetables from seed.
Unlike with plastic pots, where the roots will start to circle around in the between the pot and the soil, in a soil block the roots will stop and wait when they reach the edge of the block ready to continue growing once the plant has been transplanted.
This will help with good root growth.
And when you come to transplant your seedling, there is no risk of the roots being damaged, which will lead to stronger roots in the adult plant.
If you have seeds that will need longer before they can be transplanted, you might want to use the next block size up to give your seedling the chance to grow strong enough to be transplanted before it runs out of nutrients.
Transplanting seedlings is very easy with soil blocks, you just take the micro block with your seedling and put it in the mini block. This will avoid transplant shock and protect the roots.
Once the plant is big enough to be transplanted again, you just put it in one of the larger blocks (or maxi blocks) or if you are planting your vegetables outside, you can plant them directly into your bed.
It’s important that you don’t leave the transplanting too long. If the seedling has absorbed all the nutrients from the soil, the roots will start to grow out of the block and potentially into another one if one is near enough. General advice on when to transplant seedlings is when they have grown their first set of adult leaves (the second set of leaves that emerges).
However, when using soil blocks, you can transplant your seedlings as soon as the first leaves appear, as you don’t have to take the seedlings out of their soil.
There is no risk of damaging the roots.
I really do believe that soil blocks are a far better way to grow seedlings for transplanting. For vegetable gardeners in particular it allows a lot of plants to be grown cheaply, in large quantities and prevents any sort of transplant shock.
Overall I would highly recommend anyone growing a number of seeds every year to invest in a soil block kit.
My Recommended Soil Blocker For UK Gardeners:
- Soil blockers are fun and easy to use.
- Environmentally friendly by reducing use of plastics
- Provide indentations for seed sowing, eliminating pricking out
Anyone who is new to soil blocks would be best starting with the “gift set” which contains the standard soil block size as well as the smaller Micro 20 (ideal for small seedlings like lettuce) – this set contains everything you need to get started!
Frequently Asked Questions
Didn’t find what you were looking for? Find out more about using soil blocks in our FAQ section.
Can I use the same compost mix for all growing stages?
Seeds need a soil mix that is finer, lighter and contains few to no nutrients, as they get all their nutrients from the seed itself. Only once the seeds have germinated do they need nutrients.
If you make your own blocking soil, make sure you keep this in mind.
The soil mix recommended above by Monty Don is suitable both as seed starting mix and potting mix.
But if you use it for germination, make sure you remove any stones or wood pieces from the compost and grumble up any larger pieces of compost. This will make it easier for the seeds to navigate their way through to the top.
You can buy special seed starting compost, which is especially mixed for growing from seed. I have used GRO Sure Seed and Cutting Compost before and it worked well.
- Improves germination by 25% for the best start.
- Super fine grade Improved aeration & drainage.
- Seaweed & nutrients for healthy growth.
Do I need to water soil blocks differently?
Because the soil blocks are held together by the compressed soil, watering them from above, could destroy them. So watering from below is a much better way. Put your soil blocks in some sort of tray, such as a baking tray or a food serving tray, anything that is deep enough to hold water and your soil blocks.
Arrange your soil blocks so that you have space of about 2 inches (5cm) on one side. Carefully, pour water into the tray on the side you left empty until there is standing water. The soil blocks will absorb the water. If all the water is absorbed, pour in a bit more. Continue to do this until the soil blocks have stopped absorbing any water.
Carefully drain off the water that is still in the tray. Doing it this way, will ensure that the soil blocks are thoroughly watered. If you use larger blocks, you might need a deeper tray as you will need to pour in more water to give them a chance to soak up enough to get thoroughly wet without it taking ages. For micro or mini blocks the tray doesn’t need to be too deep.
How often should you water your soil blocks?
Well, this depends on how warm and ventilated the room you keep your blocks in is. I would say to water them every day, but make sure you drain off any excess water in the tray to ensure that the soil blocks don’t stand in water.
How do I prepare my blocking mix?
Mix all the ingredients together according to your recipe. If you are sure you will use all of it, then you can mix the water in with the ingredients. However, as it can be a bit difficult to gauge how much of the blocking mix you will need, I would recommend mixing the dry ingredients together first and then wet the mix in batches. This way, you don’t end up with a mud heap that you don’t need.
Can I re-use the blocking mix if I made too much?
Yes, just put the excess mix in a container with a lid. You can then keep it until you do more soil blocking. Make sure the compost mix is not wet though, otherwise you might get moss and fungi growing on it.