Find out the pros and cons to using soil blocks for indoor seed starting. I will tell you about my experience with soil blocks and if I plan on using them again in the future.
Why Soil Blocks?
This was my first year using soil blocks. I had heard about soil blocks before, but I always thought it would be too much of an investment for the blockers and it seemed like making soil blocks was a lot of work.
Over the winter, I read Eliot Coleman's The New Organic Grower. Eliot Coleman uses soil blocks to start his plants. This finally convinced me it was time for me to try soil blocking. I tried to follow what Coleman does with which size soil blocks to use to start each seed as well as using a version of his soil block mixture.
There are a lot of things about soil blocks that I loved. I also found that there were some things with soil blocks that I found a little more difficult to work around.
This post contains affiliate links which means at no extra cost to you, we make a tiny commission from sales.
Making Soil Blocks for Indoor Seed Starting
The soil block makers I have are:
The ¾" micro block maker. This one has 20 cells and is good for germinating seeds.
The 2" block maker.
I believe there is a 3" block maker that I had trouble finding anywhere.
The 4" block maker is the biggest blocker and is good for plants like tomatoes and peppers.
You will also need some blocking mix. There are lots of recipes out there. I will tell you about my experience with this as well.
Pros to Using Soil Blocks for Indoor Seed Starting
Ability to start a lot of seeds
One of the advantages to using soil blocks was that I could start way more seeds than I normally would. I think this would sound like a positive for most of us! If you are starting seeds in the smallest ¾" blocks, you can fit about 300 of those in a 1020 tray.
The one thing to keep in mind is that if you start that many seeds at one time, you are going to have to pot up to bigger pots or blocks. You will need a lot of space for that many seedlings.
Less soil waste
By starting seeds in the smallest ¾" blocks, there is less of a possibility of creating waste. If nothing germinates in one block, you have not used a lot of soil. If you are starting seeds in plastic seed starting trays, you will generally use more soil.
Healthier for the plants
The roots do not become root bound in soil blocks which is healthier for the root system. It also causes less shock when plants are transplanted.
Transplanting to the garden
Soil blocks make transplanting to the garden so much easier. It is so much more efficient to grab a soil block and put it in the ground than it is to try to pop seedlings out of plastic pots without disturbing their roots. It's less cleanup, too!
Cons to Using Soil Blocks for Indoor Seed Starting
There are a few things that I didn't like about using soil blocks or found cumbersome or difficult for my situation.
The initial cost of the soil block makers is kind of high. But if you think about what you will be saving in buying plastic pots, over the long run, it seems like a good investment.
Timing the transplant
The seedling from the ¾" block needs to be transplanted to a larger block pretty quickly after germination.
The 2" block has a pin that can be used in the soil blocker to form the exact size and shape hole for the ¾" block to fit into. This is a great feature. You just have to be prepared with enough of the 2" blocks to transplant the germinated seedlings.
The 4" blocker
I found the maxi 4" blocker to be a little cumbersome for me. It took a lot of effort to get the soil mix into the block and pushed down enough to make one block. And making one block at a time just seemed like it wasn't a time-saver.
My tomato planting method
I tried using the soil blocks for tomatoes, but I think I like the way I usually plant tomatoes and will probably revert to that in the future. I like to be able to get tomato plants as deep as possible into the soil. Take a look at my tomato repotting method here.
I like to start tomatoes using a dense planting technique where I start a bunch of seeds (up to 20 or so) in one cell of a plastic seed starting tray. I transplant the individual seedlings to 3.5 inch containers where I plant them as deeply as possible so that they can grow roots all along the stem. After the tomatoes outgrow the 3.5 inch pots, I transplant them to 5.5 inch pots.
I have found this method is what I prefer for tomatoes, but definitely try all of the options to see what works best for you. I will say that the 3.5 inch pots and 5.5 inch pots that I bought last year for tomatoes have held up exceptionally well. They are easy to work with and show no signs of wear. If you are going to invest in plastic pots, I highly recommend these!
Another thing I noticed was that the soil blocks dried out very quickly. This was especially true for the ¾" blocks. These blocks have to be sprayed at least a couple of times throughout the day with a water bottle to keep them moist. They dry out even faster if they are under lights or if a fan is running.
Important Things I Learned About Soil Blocks for Indoor Seed Starting
Soil block mixture
I started out using a variation of Eliot Coleman's soil block mix that I found online. It includes peat moss, compost, green sand and phosphate powder. I found that it took me a really long time to screen the compost. It took me way longer than it should have, and I probably didn't transplant seedlings soon enough because of the effort it took to screen the compost to make bigger blocks.
I think for the future I will try some different soil mixes instead of using the one I used this year. I just read about a mixture of half peat moss and half potting soil. This mix seems easy to make and can still provide nutrients.
I've also heard of some people just using potting soil. I don't know how well this would drain and if the moisture content would be right. It's something to try out and see what works best!
Moisture level in making soil blocks
Another thing I noticed when making soil blocks is that I started out with not using enough water in my mix. My soil blocks fell apart easily, were hard to move and weren't holding together.
I started adding a lot more water to the mix to the point that if I pick up a handful, I can squeeze water out and it feels spongy. I found that it's better to add too much water than too little. The blocks will dry out and don't have to be planted right away, but once they are made with too little water, they are much harder to handle.
Transplanting soon enough
Even though the roots of the plants don't become root bound in the soil blocks, they still need to be transplanted to the garden before they get too big. I thought that by the plants not getting root bound, I had extra time before transplanting. I noticed that if I let the plants sit too long without transplanting, they don't thrive. It is better to get them to the garden so they can have the space they need.
I learned that when you are using something non-sterilized for seed starting, like compost, there is more of a risk for things like fungus gnats. At first, I didn't think anything of the gnats flying around and didn't realize how detrimental they could be. Fungus gnats lay eggs in the soil. I noticed some of my plants weren't doing well, but they weren't being overwatered and didn't have signs of damping off.
The best thing to do is try to prevent the gnats. I read that cinnamon sprinkled on the soil can help to mitigate gnats. I tried this method which seemed to have some beneficial impact. Another thing I did was hung up some fly strips to catch the gnats.
Keep in mind that 300 seedlings in a 1020 tray is going to need a LOT more space when moved up to 300 seedlings in 2 inch blocks or even larger pots. You will have to either have enough space or be okay with throwing away some seedlings.
This is important to keep in mind if you plan to start spring crops and summer crops and have some overlap in between before they are ready to go to the garden.
Transplanting ¾" blocks
I found that when I was transplanting the ¾" blocks to the bigger sizes, they would fall apart or I would mess up the ones around them as I stuck my fingers in to grab the blocks. I found that this offset spatula used for cake making worked great for getting the small block up without disturbing anything around it. There is a garden tool that looks something like this, but I found this in my kitchen drawer, and it worked in a pinch.
Final Thoughts on Soil Blocks for Indoor Seed Starting
If I had to start over with the soil blocks, I probably would have started out only buying the ¾" micro blocker and the 2" blocker. I would have waited to buy the maxi 4" blocker.
Soil blocks are a great way to start seeds. They make healthy plants with good root systems. Soil blocks also save you from having to buy all the plastic pots all the time.
Give soil blocks a try for indoor seed starting and let me know what pros and cons you find.