Potting Soil of the Gods

By Rick Gush, Urban Farm Contributing Editor

Friday, August 27, 2010

Photo by Rick Gush

Using a soil sifter, I sift my dirt and compost for my potting mix.

I cleaned out the main compost bin this week and am really happy with the results.  I’m doing some seeding and repotting these days, and the compost has allowed me to prepare what I think of as the potting soil of the gods. Plants and seedlings grow particularly well in this mix, and I even give my friends bags full of the stuff. 

My potting soil recipe is simple: equal parts of sifted dirt and sifted compost mixed thoroughly.  The only way to make this stuff better is to let it mature a bit. Two weeks after it’s mixed, the resultant soil is fully chemically active and alive with all the microorganisms that make soil fertile.

Garden supply centers often sell bags of planting mix, but those bags usually contain only organic materials like ground up tree bark. The planting mixes are sterile, which can be good for some seeding situations, but otherwise they are a poor substitute for the real thing. The reason they contain only organic materials is mostly because that material is relatively light in weight, whereas dirt is pretty heavy. If the potting mix manufacturers put dirt in their mixes, the shipping costs would jump considerably, and they’d need to charge more for their product. 

Photo by Rick Gush

I have four areas in my garden, such as this compost bin, where I harvest compost.

Commercial nurseries also often use organic-only potting mixes because they are sterile and because they are much less expensive. When I worked in a snooty upscale nursery in California where they grew a lot of their own plants, they never skimped on this detail. There was a big area out back where dirt was sifted and sterilized before being mixed with various organic materials to prepare the potting mixes used in their growing operation.
 
When dirt is mixed with organic materials it provides not only a mineral fraction, but also an important electrochemical capacity that makes the flow of nutrient materials with the soil solution more robust. When I don’t have suitable compost to use, I do buy bags of planting mix, but then I always add a bunch of dirt to the mix before I use it.

I have three places I make compost with our kitchen waste and another pile where I put the big weeds and branches from the garden work. For a month or two, I pile in all the kitchen refuse and lots of the small garden weeds in one place, and then I switch places and pile in one of the other locations. The down time, when I’m not adding material, is the main cooking phase, and I usually turn the pile a few times during the next two months to allow oxygen to enter the pile.

Then, when the resulting compost is really cooked and dry, I am ready to sift it. I use the sifted out bigger pieces around the bases of the fruit trees and the fine sifted material is used in making potting soil and to enrich the soil in planting beds. I think the secret of compost is the high amounts of humic acids that are produced when organic materials like old plant parts are decomposed in an aerobic manner.

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