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What Kind of Soil Should I Use in My Seedling Flats?

Question:   I was at your November workshop this past year and have moved to Vermont and am starting a small farm using biointensive techniques and want to know if there is any thing wrong with using the soil from my double-dug beds vs. potting soil?  I have a local worm castings guy, and plan on using his castings for nutrients for my seedling mixed with the ground soil. I have heard some things about soil-borne pathogens in seedlings grown in non-treated outside soil. I want to know what your thoughts are on all of this.

Answer: Thanks for the good question!

First of all this is a multi-part question and we will break out the different questions to answer separately.

1. Using treated vs. non-treated outside soil. Using sterilized soil is not optimal because soil treated this way contains no beneficial soil microorganisms, and can be more easily permeated by harmful pathogens. Sterilizing soil is similar to taking an antibiotic that kills the good bowel flora and sets up the environment for the bad to flourish. In a garden, harmful insects, such as aphids, are beneficial because they are a food source for beneficial predator insects, such as yellow jackets. We want a healthy reserve of beneficial predator insects in the garden to manage and over-growths of harmful insects. Similarly, when properly maintained, healthy soil contains a balanced population of organisms that build a complete mini-ecosystem in each shovelful. The objective is to have a wide spectrum of microbial life in living soil in your flats to raise seedlings, so non-treated soil is the best.
   
   
2. Using soil from double-dug beds for flats.  Bed soil is a good component in a soil mix for raising seedlings in flats. One good recipe is to have 50% soil from the growing bed and 50% cured compost. Or, you can use 1/3 soil from growing bed, 1/3 cured compost, and 1/3 from previously used flat soil that you have saved in a bin to protect it.
   
   
3. Using purchased potting soil.  Many commercial potting soils contain non-optimal elements including wood fiber, vermiculite (which we have been told may contain asbestos), and bone meal, which can contain Mad Cow disease proteins, so be careful which mix you choose to use. Even better is to make your own mix, as in 2) above.
   
   
4. Using Worm Castings. These can be used as a soil amendment. However, the important thing to understand about this process (vermiculture) is that the worm castings make nutrients super-available for the plants, but available nutrients cannot be easily retained over time. Cured compost made from plants, on the other hand, is a slow time-released nutrient supplement and is much better to rely upon. Earth worms (different from compost worms) are very important in the soil, as are microorganisms. When you create good soil, using cured compost, all these elements will be there.
   

In addition, worm castings are going to have a comparable inefficiency to using animal manure. See the following excerpt from the FAQ Time Needed to Grow a Complete Diet:

While it is fine to make use of animal manure that is already available, it is not optimal to overly depend on it, because ultimately it is more a part of the depletion problem than part of the solution. This is due to the fact that you would have to grow an acre of fodder to feed one cow. The fodder itself contains 1 giga unit of carbon. Once it goes through the cow the resulting manure only contains 2/3 of a giga unit of carbon, and once the manure is cured it has lost an additional 1/3 giga unit of carbon. Both the metabolism of the cow and the microorganisms in the manure release CO2 into the atmosphere and so the carbon isn’t getting into the soil where it is most needed. In addition, the fodder has taken up minerals from the soil that are now tied up in the cow’s bones, tissues, milk and meat so many of the minerals don’t get back to the soil either.

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