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Making soil blocks for germination and transplanting.

After reading the recommendations of Elliott Coleman in his books The New Organic Grower and Four Season Harvest, an experiment using soil blocks was in order for my spring transplants this year.

My usual method for starting plants is to germinate seeds in a heated flat, then to transplant them into various sizes of plastic pots or inserts depending on the growth rate of the plant. The pots are placed in standard 1020 growing trays for easy watering.

The theory behind soil blocks is to dispense with the transplanting and the plastic pots and to allow the plants to grow in compacted soil mix which forms a cube. With careful watering, the cube maintains its shape and serves to air prune the roots of the young plant. When the plants are ready, the entire cube is transplanted into the garden.

Coleman says that various sizes of soil block makers are available from a variety of sources, but I found them somewhat difficult to locate. Smith and Hawken carries block makers in the 3/ 4 inch, two inch, and four inch sizes for $23.50, $28.50 and $98.00 respectively. I purchased the 3/4 inch block, maker from them, along with a set of inserts for the two-inch soil block maker. The inserts create 3/4 inch impressions in the two inch cubes, allowing the smaller cubes to be transplanted into the larger ones.

The 3/4 inch block maker produces 20 cubes at a time. I found that I could make 300 cubes and fill a 1020 flat in about five minutes.

A two-inch block maker, which makes four blocks at a time, was purchased for $14.95 from Gardener's Supply. Due to the high price the four inch block maker was not purchased. Instead an APS-6 insert was purchased from Gardener's Supply for $7.95 which can be used to make four inch blocks. I made my own two inch inserts from scrap wood and placed them in the bottom of the APS. The soil mix was hand packed tightly into the APS insert, resulting in perfectly formed cubes that were much less expensive to produce than with a four inch blocker.

The name "soil block" is a misnomer because little or no soil is used in the mix. A one gallon pot was used as a measure and the mixture consisted of four pots of peat moss and two pots of vermiculite. To the mixture was added one-half cup rock phosphate, one-half cup greensand and one-half cup blood meal. Compost was substituted for one pot of the vermiculite with some of the mix, but no difference in results was noted. The blood meal was not added to the mixture that was used for the 3/4 inch germinating blocks nor for some plants such as impatiens that do not need the extra nitrogen.

Slowly add approximately two gallons of water to the mixture until it forms a slurry. Experiment until you get the mixture just right. If the mixture is too wet or too dry the blocks will not be formed properly and will fall apart. A good test is to squeeze some of the mixture in your hands. It should stick together in a ball and you should be able to squeeze a small amount of water out of the ball.

In theory, all but the largest seeds can be started in the 3/4 inch cubes by placing one seed in each cube. As soon as they germinate, transfer this small cube into the larger two inch cube which has been made with the 3/4 inch insert. Larger transplants, such as tomatoes, can later be transplanted into the four-inch blocks as they get bigger.

In practice, the results of the 3/4 inch cubes were very disappointing. Since the blocker makes these cubes 20 at a time, they are spaced closely together. Usually the seeds in the middle of the group tended to germinate sooner than those on the outside. It was very difficult to extract the cubes as soon as necessary. The alternative was to wait for a majority of seeds in each 20 cube group to germinate. This meant that the roots of the earliest germinating seeds had quickly reached the bottom of the small cube and quit growing. These plants were usually set back for quite a while when transferred into the two inch cubes. A small flexible putty knife is the best way to remove the small cubes from the group and transfer them to the larger cubes.

The only advantage of using the 3/4 inch cubes is being able to germinate a large number of seeds in a small space. The 3/4 inch block maker was easy to bend, which kept it from properly forming blocks. This is the only time I have ever been disappointed by something sold by Smith and Hawken.

The most successful method I found was to germinate my seeds in a heated germinating flat. Use a mixture of equal parts of finely ground peat moss and vermiculite for germination of all kinds of seeds. The flat allows more flexibility in timing the transplanting of the seedlings to the two-inch cubes. For this method 1-1/2-inch long inserts were made out of a 7/16-inch dowel for the two-inch block maker. The resulting block had a fairly large hole in which to place the transplant. This allows use of some of the germinating mix around each transplant and causes no damage to the roots.

All plants were started under fluorescent lights and within a day or two after transplanting them to the two inch block were moved under a 1000 watt metal halide light. Despite this intense light source the blocks did not dry out. Coleman recommends bottom watering of the cubes but I experimented with top and bottom watering and did not notice a difference.

Fifty cubes were packed into each 1020 flat and when it came time to transplant them into the garden, the roots of a few plants had grown together. These were easily separated with a knife and did not result in any damage to the plants. The cubes are easier to work with if they are hardened somewhat by skipping a day of watering prior to transplanting.

In conclusion, I would highly recommend the two-inch and larger cubes. I transplanted over 500 vegetable and flower plants using this method and am very satisfied with the results. The uniform size of the cubes is a great advantage and the slight inconvenience of making the blocks is more than offset by not having to deal with a variety of containers and pots.
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Author:Morgan, Frank
Publication:Countryside & Small Stock Journal
Date:Sep 1, 1993
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