Picture of Making a Soil Blocker
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What are soil blocks? I first learned about soil blocks after watching The Real Dirt on Farmer John movie. He uses them to start seeds and then I saw that Eliott Coleman recommends them. A block of soil is made and with the right mix will hold together during plant root growth. Once the roots grow into it, it will be more solid. It eliminates the need for flats, peat pots, paper pots and any other pot which can get root bound. In the soil block, roots will air prune and not go beyond outside. It is a messy operation, but for those who like to tinker with things in their workshop, I made a 2" and 3/4" blocker to try out. A soil block is supposed to help minimize transplanting shock. Remember that soil blocks can dry out faster than soil in flats or pots. Keep an eye on them.

This is good for the home hobbyist. If you experience good success and want to try more, I would invest in a professional soil block maker that can do multiples at a time.

See this site for some great advise and ideas on the soil block method. I am not affiliated with it. The gentleman who runs the site is very knowledgeable and helpful in returning emails. He also has supplies and square soil block makers at reasonable prices. I learned a lot from all of the content he shares.

Good luck!
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Step 1: Parts and tools

Picture of Parts and tools
These are the parts and tools you will need to make:
2" inside diameter PVC pipe
3/4" wood dowel
circle cutter for use in drill press only
3/4" forstner drill bit
carpet tape
hand saw - not shown
Polyurethane - water based or similar to seal
3/4" wood screws
3/4" thick wood scrap (I used Pine)
wax (candle)

The drawing shows the idea of the soil blocker. You can start off a seedling in a 3/4" block then transplant to the the 2" block, then a larger block or pot. No shock to roots.
The soil touches the PVC for like 5 seconds when making a block.  Even it touched it for 5 hours it wouldn't off gas enough to introduce anything in the soil that would be harmful.  Then if it did, the chances of the plant absorbing that and it getting back to you is probably nil.  
chinookbar6 years ago
I wonder if there's a special purpose to the pvc part of this? Such as being smooth so the soil won't stick? The reason I wonder is that I'd like to make a 4" soil blocker to accommodate my commercially produced 2" soil blocker which makes square blocks. We have a farm and square is nice since we do thousands of starts and the squares fill a large flat without wasting space. The commercial 4" blockers cost $100! We might have to go back to using plastic as our tomato and pepper starts get bigger unless we can figure out a functional design for a large square blocker. Ideas and suggestions much appreciated! Thank you!
If I were looking to make 4 inch square soil blocks I would look at square pvc such as plastic down spouts and pvc post covers uses with pvc fencing.
YES, PVC fence posts also come in 4"x4", but several things to consider:

Most pvc products intended for outdoor use, including fence posts, contain LEAD which is used as a stabilzer/UV inhibitor.  "country estate" brand PVC fence products, made by Nebraska Plastics, is the only one I know of which is lead-free. 

All plastics are known to off-gas and to leach chemicals, as noted in prior posts.  

PVC fence components are typically rather thin and will not hold their shape with heavy use as well as aluminum.  The exception is the HD gate post that some mfrs (such as CE) offer, which are available in 4" or 5" only. 

For these reasons I prefer the aluminum fence posts.  It will be harder to cut, but much stronger and safer; well worth the extra initial effort.

Good luck to all.  I have run out of scraps due to popular response to my offer.

-Keith R 
Put the end of a length of PVC pipe into a vice, tighten it up, then heat with a torch. Continue tightening while heating, then turn. Keep going until you have somewhat of a square. I have done this before (not for this though). I am not sure what fumes or toxins are released while doing this so make sure you are in a ventilated area, also do at your own risk.
'When PVC is heated it releases clorine gas
A chimney flue block (they are fired ceramic hollow squares with rounded corners) might work for a square blocker. I'm not sure if the come in 4" though.
flatwound475 years ago
I build a lot of fences, and of course end up with some scraps on hand.  I made soil blockers from fence posts.  Most residential aluminum fencing (like Jerith or Delgard) uses 2" square posts.  Perfect for the 2" blocks.  Square aluminum posts also come in 1-5/8" , 2-1/2", 3" and 4".  It's light & smooth and isn't PVC, for those concerned about chemical leaching.  If you're not in the fence & gardening business like I am, I bet you could ask nicely & get permission to raid the dumpster of a fence company near you and get what you need.  If you're in central NJ, I usually have some scraps I'd gladly give you for nothing.  Contact me through
DaveNJ (author)  flatwound475 years ago
Cool!  That is a great idea.  Thanks.  I will look up your website. - Dave
Ranie-K6 years ago
How about attaching these things to the drill-press? Would make pressing easier :)
DaveNJ (author)  Ranie-K6 years ago
Ranie, No. It is just easy enough to pack the soil in by hand. It is a messy operation and you need to dunk in water after each one. This is really for the home hobbyist too where huge quantities are not needed.
mcenerny6 years ago
Terrific instructible - Just what I was looking for. One suggestion, though. To make it easier to line up the 3/4" hole in the wood circles, first start the disk by using the smaller bit to just mark the center. Then you can use that to line up the 2" hole cutter bit and cut the circles with the center 3/4" hole already marked and started accurately.
DaveNJ (author)  mcenerny6 years ago
Thanks for the input. Great idea!
Jason_G6 years ago
This is great! Regarding the soil mix, I wouldn't use peat moss. It has anti-microbial properties (good organic soil is full of beneficial microbes) and a lot of energy is expended with harvesting and shipping it from Canada. There are some additional environmental negatives that I can't recall right now. Other than that, I'm definitely gonna give this a shot. Thanks!
DaveNJ (author)  Jason_G6 years ago
Jason - You are right. You can substitute coconut coir which is a peat substitute and a completely renewable material. I have heard though that in order for a soil block to hold together, you cannot use it alone with the compost and perlite. It would have to be mixed 50-50 with the peat. thanks for your comment.
cam81226 years ago
Hi Dave, Thanks for this. I needed to get off the couch. Jody
nolte9196 years ago
How many cups lime is that in step 6? All I see is a little question mark instead of 1/2 or 1/4 or 1/3 or whatever.
DaveNJ (author)  nolte9196 years ago
It is 1/2 cup. Sorry about that. I went back and fixed the text. Thanks for the feedback!
Kryptonite6 years ago
My mum & I just grow seadlings in small circular pot, plastic of course, then when time to put in the garden we just take the whole "block" out & bury it. Good work on the instructable, it's rather well set out & quite clear.
Uncle Kudzu6 years ago
a fine and timely instructable! i'll soon be starting some vegetable seeds indoors for later transplant. Good Stuff, Dave!
This is a great idea. I think I will try this and replace my current method of using toilet paper rolls to hold the soil.