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  • Writer's pictureEllen

Starting Seeds with Soil Blocks

Updated: Sep 19, 2023

We are always looking for ways to reduce our plastic use on the farm. A few years ago we were starting all of our seeds in plastic plug flats, as many people do. We were constantly frustrated by these plastic flats, however: we never seemed to be able to water them evenly, the plants almost always became root bound, and the flimsy plastic cracked and split after a few uses. After throwing too many plastic flats in the garbage, we knew we needed a better way.

Enter soil blocks! We first experimented with soil blocks two years ago, and quickly fell in love. We now start almost all of our plants in soil blocks. Not only do they produce healthier plant starts, but they have also allowed us to cut way back on our plastic use.

The idea behind a soil block is simple. Wet seed starting mix is compressed into a metal mold, which forms the block. The mold creates a small divot in the top of the block, which is the perfect depth for most plant seeds. After seeding, you sprinkle the flat of soil blocks with loose soil, and you're done! After seeding our soil blocks, we usually place them in our germination chamber until they sprout. Although we have seen many types of trays used for soil blocks (including cafeteria trays and plastic nursery trays), we use 1/2" thick wood boards to hold our blocks. This further reduces our plastic use, and we can make the boards for free on the farm.

Left: Two sizes of soil block molds used on our farm. Right: Petunias started in 3/4" blocks are potted up into larger 2" blocks, made using the special square dibble attachments.

Soil block molds come in several sizes. We start most of our vegetables, such as tomatoes, peppers, cucumbers, and melons, in larger 2" blocks. These plants can stay in the 2" block until they are ready to transplant. We use a smaller 3/4" block maker for very small seeds that take a long time to germinate, such as herbs and flowers. Once they have sprouted, these smaller blocks can be potted up into the larger 2" blocks using a special set of square dibbles. Recently, we were starting so many pepper plants that we ran out of room in the germination chamber. We decided to start the rest of our peppers in the small blocks, then immediately potted them up to 2" blocks when they germinated. So far, they are doing great!

One big benefit of soil blocks is that they allow the plant roots to air prune. Roots don't like to be in contact with air; they prefer to stay safe inside the dark, damp soil. When plant roots reach the edge of a soil block, they stop growing. This prevents the plants from becoming root bound and reduces transplant shock. Our spring weather in upstate New York is very unpredictable, and it's not unusual for planting to be delayed by several weeks due to snow, late freezes, or torrential rains that turn the fields into a muddy soup. If our transplants have to spend a couple more weeks in the greenhouse than planned, they are much more likely to survive and thrive in soil blocks than they would be in root bound plastic plug trays.

Do you want to try soil blocks? Here are a few tips:

  • Get a good seed starting mix with a high percentage of peat, which helps the blocks stick together. We love the Ag Blend from GreenTree Ithaca. You can use regular potting soil from the hardware store in a pinch, but the blocks don't hold together as well and come out a bit misshapen.

  • Make sure your potting mix is really wet. We usually aim to have a bit of water pooling on the top as we make our blocks. If the mix is too dry or isn't evenly saturated, the blocks don't unmold well. This is especially true for the small 3/4" blocks and for the larger 2" blocks when using the square dibbles.

  • Water the blocks gently, especially at first. High flows can blast them apart. After the plants have grown a bit and the blocks are filled with roots, they are more stable.

  • You may need to water the soil blocks more frequently than you would plastic trays, as they are more exposed to air.

Local farm fans: we will be selling six packs of plant starts in soil blocks again this year! They should be available later this month at our farm stand. We have a wide variety of plants including heirloom, slicing, and cherry tomatoes, sweet and hot peppers, flowers, herbs, and more. Subscribe to our mailing list on our website so you don't miss them!

Stay tuned for our next blog post, where we will be sharing some more ways we reduce plastic use at Little Salmon Farm!

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