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Connecting to Our Food Web

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Seed Blocking Mix
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Welcome to My Jungle - March, 2019


Over the years I have collected a number of planting media recipes, and each has its own characteristics and usefulness in the garden. Eliot Coleman, author of New Organic Grower, developed a blocking mix a number of years ago that is basically 3 parts peat for structure, 1 part perlite for aeration, 3 parts compost/garden soil, lime for pH correction and a base fertilizer for nutrient needs, which is made up of blood meal, rock phosphate and greensand. This mix works better than standard potting mix when making soil blocks (growing transplants without a pot). A blocking mix needs to be more fibrous so it can stand up to wetting it to the consistency of brownie mix, then being formed into blocks using a soil blocking tool (purchased or made). Using Eliot's recipe, I usually make a batch in a wheel barrow, then dry store the finished mix in a large lidded garbage can for convenient access. The recipe can easily be scaled back though if a smaller amount than two bushels is needed. And just like in the kitchen, I have been known to make substitutions when I don't have all the called for ingredients. In general I prefer perlite over sand because it's so much lighter and easier to mix, but sand works just as well in terms of drainage if that is all you have at the time. I have used all compost instead of adding garden soil and that seems to work just as well too. Substituting fertilizers though takes a bit more consideration. This recipe calls for organic fertilizers, which in general are low in analysis and slower in their release of nutrients to the plants. If synthetic fertilizer(s) are substituted, one of the slow-release fertilizers like Osmocote® may be the best option to avoid the potential for plant injury due to over-fertilization/salt burn. Follow the label for the appropriate amount to add to the volume of planting media being mixed. Because this recipe is intended to meet organic standards, the amount specified for lime would be for agricultural lime (calcium carbonate) rather than hydrated lime (calcium hydroxide), which is not allowed as an organic input. If hydrated lime is substituted, less is generally needed because of its higher neutralizing value. I compared two Bonide® brand limes which are readily available to home gardeners, hydrated lime and agricultural lime. After comparing the analysis on these two specific products, the hydrated lime turned out to be 14% more reactive, meaning you would need to use 14% less of it to get the same effect compared to the Bonide® brand agricultural lime. Remember though, just because you use less does not mean it is the cheapest. I have found that Eliot's advice on mixing order to be really sound, particularly if you are mixing the full recipe or larger. It is tough to mix it all at once, especially if you used sand instead of perlite…unless you are blessed with one of those small cement mixers. It is so much easier to get a uniform blend of the lime and fertilizers if you mix it in the peat moss first. Ignore the picture I took of the individual ingredients in the wheel barrow before mixing…I did that just for demonstration. Start out by adding the lime to the peat moss and mixing thoroughly. Next add the perlite (or sand) and fertilizer, then mix again. Finally add the garden soil and/or compost and one last mix. Then to make soil blocks, add water until the mix is like paste. Soil blocking tools (metal expulsion mold) of various sizes can be purchased, or someone handy could devise their own from re-purposed materials.

Blocking Mix Recipe from Eliot Coleman's The New Organic Grower, 1995

  • Using a 10 quart bucket as the unit of measurement
  • 3 buckets brown peat
  • ½ cup lime (agricultural) Mix
  • 2 buckets coarse sand or perlite
  • 3 cups base fertilizer Mix
  • 1 cup blood meal
  • 1 cup soft rock phosphate
  • 1 cup greensand
  • 1 bucket garden soil
  • 2 buckets compost Mix

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