The Southern Agrarian

Southern Agrarianism and the culture of the Old South

Soil Blocks Update

On the Starting Seeds page, I showed how to use a soil block mold for starting seeds. After much experimenting, I have made a few modifications to the process.

  • I no longer try to pack the soil mixture in tighter than I can get by just pressing down several times. I had even tried using blocks of wood to pack it in tighter. While that gave good results, it was pretty tough on the hands, so I went back to the method recommended by the manufacturer. I still try to pack it in as tightly as possible, but only by pressing the mold into the mixture.
  • I no longer use a bucket to press the mold into the mixture – I now use a stainless steel warming tray that I bought from a used restaurant supply store. This is much easier to work with and it allows me to make much better use of the wet soil mixture than I could with the rounded sides of a bucket.
  • I have added galvanized hardware cloth in the bottom of the seed trays. Previously, the blocks would get damaged when I had to move the trays. The stiff, flat bottom that the hardware cloth provides keeps the blocks from bumping into each other.

Hardware cloth lines the bottom of the tray to keep the blocks stabilized when moving the tray. This is a big help in keeping the blocks from being damaged.

The work area. From front to back: water bucket to clean blocker between moldings, stainless steel warming tray where the blocker is packed, seedling tray where blocks are placed when finished.

Packing the growing mixture into the soil blocker.

A garden knife is used to cut away excess from the bottom.

Wipe away excess from the sides using your fingers.

Four soil blocks being extracted from the mold and onto the seed tray.

The soil blocker is rinsed off between uses. Keeping it clean helps make the blocks uniform.

When the blocks have been extracted from the mold, it is not uncommon for part of the block to separate. Use a metal putty knife to gently press the block back into shape.

Use two putty knives together to separate the blocks. They need to have enough air space between them to keep the roots of one block from growing into an adjoining block.

5 Comments

  1. Enjoyed your account and the blocks look like they’d work better than the usual flats for removing the developed seedling.
    For the totally ignorant, such as myself, you might add a note that when you go to push the soil out of the block you push the handle down while raising the blocker.

    • Stephen Clay McGehee

      February 18, 2013 at 11:43 am

      Good point, Abbie! It’s easy to take it for granted and not spell it out. Years ago, I worked as an Industrial Engineer at a large defense contractor. Part of my job was to write and review the step-by-step instructions for the folks on the assembly line. It’s far too easy to skip over things like that.

      I’ve got a batch of seeds started now, and part of this experiment is comparing the soil blocks using home-made growing mixture with a commercial plastic seed starter using commercial seed starting mix. So far, the results are about even, but I have noticed one important difference: The commercial seed starter mix does not absorb water anywhere near as readily as my home-made mixture does. The commercial stuff has a lot of surface tension that keeps the water puddling on top rather than soaking in like the soil block mixture does. Just judging by the weight of one tray of soil blocks and one tray of the commercial system, the soil blocks are holding a lot more water. Looks like I will have to be very careful about keeping them well watered this time. My next test will probably be using my home-made mixture in the plastic seed starter system and see what happens.

      I’ve tried a number of different seed starting systems, but the soil blocks are still the clear winner from what I’ve seen – at least for my own situation and needs.

  2. I just came across your website and read your 2 articles on using soil blocks to start seedlings. I have a couple observations from my own experience.

    First, I think your mix needs more compost in it to keep the seedlings fed better until transplanted. I use a mix made specifically for soil blocking from Vermont Compost, called Fort Vee. Johnny’s Seeds makes a slightly different mix for soil blocking called 512 Mix. And in his book, “The New Organic Grower,” Eliot Coleman, who is largely responsible for popularizing soil blocks, provides a recipe for a nutrient rich, form-holding blocking mix that anyone can make for themselves.

    A key benefit of using one of those 3 mixes is that they hold the soil block shape very well, which eliminates the need to tamp soil into the forms or repair separations after they come out of the form. I did some tamping the first year I used soil blocks, especially on the large, 4″ blocks, but I have learned that I was not letting the water absorb fully into the mix. Now I wet my mix in a large plastic storage bin and let it sit for 20 hours or so. I have had no trouble with the mix since doing that.

    Since letting the water fully absorb, I no longer have to rinse my blocker between rounds of making blocks, either, which saves time.

    I also don’t have to scrape the excess off the bottom because I use a shallow plastic tray loaded with soil to press against, and then “twist” as I lift the block maker up, which removes almost all of the excess blocking soil from the bottom. (I also don’t mind if some of them have a bit of extra soil on the bottom, making heights a bit uneven from time to time.) That also saves time – which is important to me as my garden grows bigger and bigger.

    Second, I made some inexpensive wooden trays from strapping fir available at any big box lumber store (Lowes, Home Depot, etc). Total cost per tray is about $2.25, and they last for years. Eliot Coleman’s book provided the construction details; I have detailed my manufacturing of trays on my blog, noting adjustments I’ve made. Those trays have a removable long side, which makes it easy to remove the soil blocks with a 2″ spackling spatula when transplanting. Again, it makes for a faster process.

    A benefit of the wooden trays is that they leak enough I don’t have to worry about over watering – which is my tendency – and they fully biodegrade when they have completed their useful life (I’m expecting a decade or so of use from each – which makes them pretty darned cheap!). Their biggest drawback, imo, is that they are not stackable one inside another, so they take up more storage room than do plastic trays.

  3. I used an electric blanket under the trays of blocks to try to warm the soil. Covered the trays. However in two days, many of the soil blocks were moldy. Too much heat? Too much moisture? I have now uncovered the trays for two days and took away the heat. Starting again with heat and covered trays and watered the blocks from the bottom. New to this, can you tell? Any suggestions about how to avoid the mold or now, get rid of it? Thanks.

    • Stephen Clay McGehee

      April 6, 2013 at 9:56 am

      Nancy, my guess would not be a problem with the heat, but needing better air circulation. When I cover my trays, it is with a loose fitting plastic cover. The objective is to retain the moisture, and to a lesser extent, retain the heat. I’ve found out the hard way that having a heating pad under the seed trays without anything to retain the moisture, results in the soil drying out very quickly.

      Before we rule out the heat, though, you might try putting a thermometer in the tray to see what the temperature is. Let me know and also what kind of seeds you’re trying to sprout. My heating pad has a chart showing idea temps for various seeds.

      My soil blocks stay on the heating pad and under the loose plastic cover until the first seedlings start to appear. I try to check them at least twice a day for anything popping up. Once one breaks the surface, the tray comes off the heating pad and under the lights. I try to avoid mixing different kinds of seeds in a tray since one kind of plant may sprout a lot faster than the other. Sometimes it works OK though – right now, I’ve got a tray of tomatoes and eggplants that both sprouted at the same time. If one had taken longer, I would have had to move those soil blocks to another tray and put it back under the cover and heating pad.

      Most of the time, I do my watering using a fine mist. If I mess up and let them get too dry, then I water from the bottom. All my watering is done after the seeds have sprouted and they are off the heat and under the lights. I have never had to add any water to the soil blocks before they sprouted. I guess it’s just maintaining a balance between enough air circulation to prevent mold and still retaining the moisture.

      I think I’ve had some mold before, but it’s been long enough ago that I don’t remember the details.

      Please stay in touch and let us know what you try and what works – and if there are any questions you have or ideas you’d like to bounce around.

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