How to Make a Soil Blocker




About: I have a compulsion to make stuff, all kinds of stuff. I'm glad to be here...

It's seed starting season.  Yea!!
You can start seeds in a variety of containers.  People use everything from empty egg shells to the plastic six packs last years plants came in to just about any container laying around.  There comes a point at the beginning of the season when you have a lot of plants going and containers can get scarce.  Imagine if you didn't have to deal with storing mounds of little plastic pots and just did without containers all together!  Well you can and the answer to your prayers is the soil blocker.  It compresses soil into...well...blocks.

Now you can buy a soil block maker.  They cost about 30 bucks from Johnny's seeds but what's the fun in that?   I decided to make my own. 
You can too .
 It's not that hard.

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Step 1: The Materials and Tools Part

Well, what else were you expecting?


About 2 feet of pine board 3/4" thick  x 2" wide

An 8" piece of 4x4

About 2 feet of aluminum plate 1/8" thick x 2 3/8" wide

12 wood screws

1- 3/8" carriage bolt  4" long 

1- 3/8" nut & washer

Clear acrylic or varnish


Drill and bits

Table saw

Thin file

3/8"  tap

Step 2: Start the Wood Frame

Cut the 2" wide stock into 3 pieces one at 8" and 2 at 6".

Cut a 45°angle on one end of each of the 6" pieces.

Drill a hole in the center of the 8" piece big enough to let the 3/8" bolt pass through easily.

Step 3: Now the Metal Frame Sides

Cut two 9 1/4" long strips off the aluminum.
You can use the table saw for this.  It won't harm the blade but go slow and be very carefull.

Drill and countersink two holes for the screws on each end of the pieces.

Mark a 1" line at 2 5/8", 4 5/8" and 6 5/8" on the long pieces.
Each line is centered on the width which leaves about 5/8" space on either side.
Use a center punch to make a series of divots along the line.
Drill a series of 3/16" holes along the line.
Use a file to turn the holes into a rectangular slot.

Step 4: The Dividers

Cut three pieces at 2 3/8" wide off the remaining aluminum.
Notch the ends so that they have tabs sticking out of each side.  
The table saw and a file works well for this but again be careful.
The tabs are ~1/8" deep and 1" wide.  They should fit loosely into the slots.

Step 5: Assembly

Screw one of the metal strips to the two wood sides.
The tapered ends should point down and face out.
Turn it all over and stand the dividers up in the slots of the lower metal strip.
You'll need some bits of wood or another pair of hands to hold them up.
Place the other metal strip on top.  Make sure the tabs are aligned with the slots.  
Screw the upper side to the wood sides.

Step 6: The Plunger

Now we make the plunger from the piece of 4 x 4.
Make sure it fits loosely between the two wood sides.  If not make it so.
Cut one side off so that it is a little less than 2" wide.  Again we want it to fit loosely inside of the cells of the blocker.
Cut 3 slots in it where the dividers go.  Make the slots wide enough to accommodate the dividers.  
2 saw blade thicknesses should do it.
The slots should be about  2 3/4" tall.
When you're done the plunger should guessed it...loosely in the blocker.
Test fit it to make sure everything slides smoothly.

Step 7: Finish the Assembly

Place the plunger in the blocker and tip it up.
Put the top bar of the frame on top of the plunger and use the hole in it as a guide to drill a hole for the 3/8" bolt in the plunger.
Don't go to deep.
Tap the hole in the plunger.

Screw the top bar of the frame into the top of the sides with two screws on each side.

Put the carriage bolt through the hole in the top bar.
Slide on the washer and screw on the nut leaving about an inch of the bolt sticking out.
Screw the bolt into the plunger and tighten the nut against it.
Slide the plunger up and down.  
Is it working?  

Step 8: Clear Coat

Take it apart and spray all the wood parts with a couple layers of the clear coat of your choice,
Once it's good and dry put everything back together.

Step 9: Use It

The boc choi in the picture was planted last fall in a cold frame.  I just pulled it out of the ground and the block is still pretty much intact.  Pretty cool.

Now you've built this contraption.  How do you use it?
It's easy but you need a few supplies:

1. Moist potting soil / seed starting mix.  Don't use garden soil.  Get the stuff in a bag.  Put it in a plastic bin and add water till it holds together in a ball when you squeeze it in your hand.

2. A shallow container.  I use a kitty litter pan.

3. A waterproof tray.  Something to hold the blocks in.  You want it to be waterproof so you can water the blocks from the bottom.

4. A spatula.  Good for moving the blocks around in the tray.

5. A pencil.  Works well for making holes in the blocks for seeds.

Making Blocks

Put some soil in the pan.  Make a mound about 1 1/2 times the height of the blocker cells.  It needs to be deep enough that it will compress as the blocker is pressed down but not so deep that the blocker can't hit bottom.

Press the blocker into the soil and push down firmly all the way to the bottom.  Twist back and forth a little.

Take the blocker out and put it in the tray.  Press the plunger down as you pull the blocker up leaving the blocks behind in the tray.


You may find that you need to periodically clean the blocker as you are using it if it gets hard to work the plunger.
Now get out there and start planting!

If you like projects (who doesn't?), we've got plenty more at our site- Mike and Molly's House where we chronicle our Mighty Projects on our Mini Farm (aka our backyard)


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    19 Discussions


    2 years ago

    I have a 3'piece 0f aluminum but could only find one 2" wide. Wi.. this work? Will I have to modify something to make it work? Thanks

    1 reply

    Reply 2 years ago

    It will be fine. Your blocks will just be a little bit shorter. You can totally adjust the measurements to fit your materials.


    Reply 7 years ago on Introduction

    Cutting aluminum with a circular saw isn't bad, just don't use your best blade, and wear long sleeves. The chips are hot! I've heard if you run the blade backwards it is even better. Next time I do it I'm going to have to try that out for myself.

    Once I was trimming the edge of one of my workbenches with a hand circular saw and I noticed all of these sparks flying up out of my cut line, I kept on going (I figured the damage was done already so why stop) and when I was done sure enough I'd sawn right through a steel bracket under the wood. This was some pretty thick steel too. I was using this nasty blade made out of "dynite" carbide, which was claimed to be 50 times better than plain old carbide, uh huh. I don't know about 50 times but I have to admit I was impressed with the job it did!

    Slitting saws on milling machines look for all the world like little circular saws and they work too.


    Reply 5 years ago on Introduction

    I can vouch for the efficacy of running a circular saw blade backward for cutting corrugated iron: I have cut a lot lengthwise as retaining for shallow ditches. I think the idea is that the teeth do not get caught in the steel as they might if run forward. The blade in question had tungsten-tipped teeth, so I don't know if a regular blade would work as well.


    Reply 5 years ago on Introduction

    When you run a blade with a positive tooth rake angle backwards you turn it into a blade with a negative tooth rake angle. Some processes work well if you run an entire saw backwards too. Then you are changing from conventional to climb cutting. I've seen a lot of people climb cut masonry.


    6 years ago on Introduction

    Excellent instructable. Using these directions I made one last year and it works great! I've made hundreds of blocks. I added a small piece of wood at the top of the bolt to have something to push against when releasing the blocks. Also, I didn't have aluminum so I used galvanized sheet metal flashing that I had (I folded it in half to make it stronger, at the author's suggestion, thanks). The flashing was easy to cut with tin snips.
    Now I may experiment with making a couple of other sizes.

    1 reply

    Reply 6 years ago on Introduction

    Glad to hear your blocker worked out! Post a picture in the comments if you have one. I'd love to see how yours came out.


    6 years ago on Introduction

    The only thing I can think to add to this is a spring between the bolt head and the block. It will keep the plunger raised.


    7 years ago on Introduction

    Nice! I have the jonnies seeds version. It looks like you design would be more comfotable to use, because of the big wooden frame. Did you send them a link?

    1 reply

    7 years ago on Introduction

    If you use a file on aluminum. rub a little chalk onto both sides of the file. It keeps the file from clogging up with aluminum particles, which is a Bear to clean.

    1 reply

    7 years ago on Introduction

    Why not add spikes to the bottom of the plunger ? That way, you don't have to make a hole with the pencil.

    3 replies

    Reply 7 years ago on Introduction

    Using a pencil allows me to adjust the depth of the hole to the size of the seed. Lots of seeds are so tiny they don't need a hole at all.


    Reply 7 years ago on Introduction

    You could Put a long Screw or bolt into the center of each block. That way, you can screw them in or out and adjust the depth of each hole...