Making a Soil Blocker





Introduction: Making a Soil Blocker

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!

Step 1: 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.

Step 2: Cut Out Disks

Cut out 3 disks, each a bit smaller than the inside diameter of the pipe. Cut 1 first to check fit. Remember, the wood will swell a little when wet. It will swell even after sealing it. Don't make it too tight. The top disk can fit tightly in the pipe top. This piece won't move when installed. The plunger needs to fit loose just so that it can push the soil block out without too much effort.

Step 3: Drill Holes

Layout for a 3/4" hole. Use carpet tape to secure the disk to the drill press top. Line up your marks and drill. You need to get perfectly centered. There is no center punch to guide, so this is why you need to carefully lay out your 3/4" hole on cross hairs and boundaries on top. Drill 2 disks all the way through. Drill the disk top handle about 1/4" deep only.

Step 4: Dry Fit and Assemble

Cut 3/4" dowel to 6" long
Glue in 3/4" dowel to the handle disk
Cut the pvc pipe to 3.5" long.
Put top disk in flush with the top. Drive a screws into opposite sides to secure.
Seal with polyurethane (3-4 coats)
Put handle rod assembly into hole
Attach plunger disk flush with bottom of rod for a solid soil block. Secure disk to rod with a screw.
Or attach plunger disk 3/4" up from bottom of rod to make a soil block with 3/4" hole to accept a 3/4" soil block transplant.
Put some candle wax on the 2" plunger disk if tight to help smooth the stroke.

Step 5: Finished Assembly

Here the finished soil blocker with tomato plant that was started a few weeks ago.

Step 6: Soil Block Mix

3 buckets brown peat (standard peat moss, use a premium grade)
1/2 cup lime. Mix ingredients together thoroughly.
2 buckets coarse sand or perlite
3 cups base fertilizer (equal part mix blood meal, colloidal phosphate, and greensand). Mix.
1 bucket garden soil
2 buckets well-decomposed compost. Mix ingredients together thoroughly.
* From The New Organic Grower, by Eliot Coleman

Or you can use any good organic potting soil. I found a bag of Master Nursury Gardener's Gold Organic potting mix and it works great. I did make my own blend as above too, but it was expensive and time consuming trying to locate greensand, colloidal phosphate (gave up - could not find), etc.

Add 1 part water to 3 parts soil mix. Mix to a soft putty or wet cement (not soupy). A soil block needs to have the soil packed tight. Rinse out blocker in bucket of water after each block is made. This will help release each new block made.

Remember that soil blocks can dry out faster than soil in flats or pots. Keep an eye on them. Good luck!

Step 7: V8 Can Prototype

First I made a 2" blocker from a V8 can. It did not hold up very well and I would not recommend using. The aluminum is too flimsy.



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    I made one using an old aspirin bottle.

    Awesome work...if your not good in wood working, you can also use a small round sardines the top with
    a can opener, keep the round piece to use later or make a plastic round piece
    or look for a washer which would fit in the sardines can and drill a
    hole in the other end, insert a threaded carriage bolt and a spring(optional) and
    voila...round sardines can soil block maker

    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.  

    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!

    5 replies

    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.

    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

    1 reply

    Cool!  That is a great idea.  Thanks.  I will look up your website. - Dave

    How about attaching these things to the drill-press? Would make pressing easier :)

    1 reply

    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.

    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.

    1 reply

    Thanks for the input. Great idea!

    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!

    1 reply

    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.

    Hi Dave, Thanks for this. I needed to get off the couch. Jody

    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.

    1 reply

    It is 1/2 cup. Sorry about that. I went back and fixed the text. Thanks for the feedback!