Making a Soil Block Tray

To start all of the seedlings I need for my garden and the Farmer’s Market I need a dozen seed trays, but I don’t like the price of trays in the catalogs, I don’t want to use plastic, and I don’t want either water tight trays that can flood seedlings or the open mesh trays that help soil blocks dry out.

Eliot Coleman (The New Organic Grower) provides dimensions for a wooden three-sided tray sized to fit soil blocks. The standardized tray has interior floor space of 18-3/4″ by 8″ inches. That’s large enough to hold over 540 of the 1/2-inch blocks,  or 60 1-1/2 inch blocks, or 36 2-inch blocks, or 18 3-inch soil blocks. The 3-sided character of the tray makes it easy to slide a putty knife under the blocks when transplanting. And a 4th side can be put on to keep the outside row of soil blocks from drying out.

Using Coleman’s basic measurements and construction suggestions, I have manufactured my own wooden trays. My goal has been three-fold: to keep my cost to a minimum, make construction quick and easy, and get a long life out of each tray.

I found the perfect manufacturing stock in 3″ by 1″ fir strapping, bought in 12-foot lengths for $1.87 per board at my neighborhood Home Depot.

Actual dimensions of the milled lumber are 2-1/2″ by 3/4″, which turns out to be perfect. Using 3/4″ thick wood for the sides of my tray, my outside dimensions need to be 20-1/4″ by 9-1/2″ to create the inside dimensions Eliot recommends.

Also, as it turns out each 12-foot length of 3″ by 1″ strapping yields one complete tray. That’s $1.87 per tray. Add in 19 1-1/4″ long #8 screws for durable construction ($0.95), and the total price is $2.99, tax included. Because of the long life I expect, that fits my price criterion. I can also use these trays to display my seedlings at the Farmer’s Market, eliminating the need for separate display trays or shelving.

Construction is fairly easy. Here’s how I made each tray.

Add in blade width...

...& cut between the lines

(1) Cut 6 20-1/4″ lengths. Don’t forget to calculate the width of your saw blade. My rotary blade is 1/8″ thick. Add in blade thickness; you want your finished pieces exactly 20-1/4″ long.

(2) Cut 2 8″ lengths of strapping. Again, remember to account for the saw blade.

Mark: 3/8" from edge, centered

(3) Take 4 of the 20-1/4″ lengths. For each length, measure in from the end 3/8″ and make a mark. Using that mark line up a ruler and draw a line parallel to the end. Along that line measure in from one side 1-1/4″ and make a cross mark. You should have one cross mark in the middle of the width of the board and 3/8″ in from the end. Drill a small pilot hole through the thickness of the board at the cross mark (use a 1/8″ diameter drill). Do the same on the other end, and repeat this step for all 4 boards. Set these boards aside.

(4) Take a 5th length of 20-1/4″ board and measure in from the end 3/8″, just as you did for the first 4, make a mark, and draw a line parallel to the cut edge. Along that line, measure in 1″ and make a cross mark. Drill a pilot hole through the thickness of the board at that mark. Note that the hole you just drilled is not in the center of the width; it is offset 1/4″ toward one side. At the other end of the board, you want to offset your pilot hole to the same side. Set this board aside to use in Step 11.

(5) On the 6th 20-1/4″ long board measure in from the end 3/8″, just as you did for the other 5, make a mark, and draw a line parallel to the cut edge. Along that line, measure in 3/4″ from the  side of your board and make a cross mark. Measure another 1″ and make a second cross mark. Now you should have two cross marks, each 3/4″ in from the side and 1″ apart. Drill a small pilot hole through the thickness of the board at the cross marks. Repeat on the other end.

Setting screws. Note paint cans and vice free my hands to line up wood and handle drill fitted with screw bit

(6) Take the long board with two pilot holes at each end from Step 5, and both of the 8″ short lengths and lay them on their edges to make 3 sides of a rectangle. Use 4 #8 screws (1-1/4″ long) to attach the long board to the ends of the short boards. Now take one of the 4 long boards with one centered hole in each end and secure it to the open ends of the 8″ short boards with screws.

Step 7: Mark the midpoint of the 8" piece

The result is a rectangle with an interior dimension of 18-3/4″ by 8″. The long board on one side is attached to the 8″ width boards with two screws on each end, and the long board on the other side is attached by one screw at each end.

(7) Measure to the middle of the 8″ short boards and make a mark. This is the midpoint.

Align 1st bottom board to the mid-point marks & screw in place...

...abut 2nd board & secure; 2 center bottom boards in place

(8) Line up another of the long boards so one edge abuts the midpoint marks. Using screws, attach the long board to the short edge of the frame. Place a second long board against the first board, on the other side of the midpoint mark, and screw it in place. Now the two center boards should be secured to the 8″ sections of sidewall.

Secure 3rd bottom board over the side length that has 2 screws

Step 10: Secure bottom board to rear frame edge

(9) Be very careful in this step. Identify which of the two long frame pieces has two screws on each end. Work with that side. Place a third length of board against the already-secured center board and screw it in place. You will see that the outside edge of this board overhangs the long side wall by 1/4″.

(10) Measure in from the side 1/2″ and make a mark. Using that mark, draw a line parallel to the long side of the board. Then, measure in 2″, 6″, and 10-1/8″, 14-1/4″, and 18-1/4″ and make cross marks. Drill pilot holes through the thickness of the board at each cross mark, and into the edge of the frame. Use 5 screws to attach the bottom board to the side wall.

Note screw is not quite centered - to avoid splitting end of frame

(11) Take care in this step. Take the board you prepared with an off-center hole in Step 4. Set it to cover the remaining portion of open frame with the off-set hole toward the other bottom boards. Screw the bottom board into the 8″ end boards.

(12) Flip the tray over. Remove the long side board that is secured with one screw on each end.

Result: What you have is a 3-sided tray properly sized for standard soil blocks. With the long sidewall removed, it is easy to remove soil block seedlings when you are in the field, transplanting. Screw the long sidewall in place when you are growing your seedlings to prevent the exposed soil blocks from drying out.

Completed soil block tray

Wall off to easily remove seedlings when transplanting

The absorbent wood and the seams along the bottom  help excess water drain from your tray while keeping the edges of the soil block moist.

Below is a photo of two soil block trays in use. To help conserve moisture I placed a few extra pieces of 8″ board along one sidewall, and filled around my soil blocks with loose potting soil. I also found it easy to identify my seedlings by writing their name on the wood in erasable pencil (magnify image to see the writing).

Note removable "filler" boards along left-hand sidewall

May 13, 2010 Addendum

I have found that the 18-3/4″ x 8″ interior dimension is great for the 2″ cube soil blocks, but not ideal for 4″ blocks.

In theory the 18-3/4″ length is room for rows of nine 2″ blocks, with a little left over to allow for the block maker to drop the last set of blocks without damaging those beside it. However, since blocks made with 512 Mix potting soil tend to contract a bit as they dry, I have taken to placing 10 blocks in a row by slightly compressing the blocks, which tend to be just a tad under 2″. I think that does a better job of keeping the blocks damp. Also, when I have extra space in a tray, I thow in some potting soil to avoid leaving a soil block side wall totally exposed to evaporation.

However, when the trays are used for 4″ cube blocks, the 18-3/4″ length leaves a 2″ space at the end. That’s a lot of evaporation. I have used that extra space for some younger plants at the 2″ block size, but now that all of my seedlings are becoming more mature, I don’t either need or want the extra space.

So today I made a new set of trays that are 2″ shorter. With an interior length of 16-3/4″, they are sized to fit 2 rows of 4″ cubes, 8 cubes per tray. There’s still enough extra space to allow the soil block maker to drop the last block in each row without damaging the block beside it. But it’s important to load the blocks with the side wall removed. Replacing the side wall will slightly compress the blocks – which tend to be just a bit over 4″ on a side at their base.

In this image I’m dropping a 4″ block into a tray, with the side wall removed. Note the slightly tapered sides which creates a wider block bottom than top – and allows the block to slip out of the form.

Also note the compressed look of the block on the far left. That’s made with 512 Mix from Johnny’s Seeds, and has contracted a bit as it’s lost moisture since being made 4 or 5 days ago. You can also see that the new blocks stick out a bit. They will be compressed when the side wall is replaced.