Potting Soil

Although it is still spitting snow in this cruddy Pacific Northwest La Nina Spring, it is time to get tomatoes seedlings started.  Last year I had a major disaster using commercial potting soil.  There is virtually no regulation of what can be sold as potting soil, and the quality of commercial potting soils, especially at the low price end of the market, is incredibly variable.  Even for reputable commercial producers of potting soil, quality control can be problematic. Commercial potting soils typically contain compost made from home yard waste.  Home gardeners are notorious for using huge quantities of herbicides such as Weed-n-Feed.  If your potting soil contains significant quantities of Weed-n-Feed, guess what – your plants are going to die.  I experienced exactly that last year when every tomato seedling transplanted into 4 inch pots containing a particular batch of commercial potting soil died.  I lost 500 plants. Disasters like this could probably be avoided, or at least minimized, by buying premium brands of potting soil, but the price for these products is prohibitive.

Enough of that, from today forward I am producing my own potting soil.  This isn’t a simple matter, however, because recipes for potting soil are hugely variable, and there is almost no reliable information available about how they compare in performance.  Comparing about 20 recipes from different sources, I have come up with a consensus recipe.

Potting soils generally have three types of ingredients.
1) Organic matter, to provide aeration and moisture retention.  Good sources of organic matter include sphagnum moss, coir (coconut fiber) and compost.
2) Materials to promote drainage, including vermiculite, perlite, and sand.
3) Lime.  Organic matter, and especially sphagnum (peat) moss is extremely acidic (typically pH 3). Addition of lime is required to bring the pH up to near neutrality (pH 6-7).
4) Fertilizer.  The main requirements for fertility are nitrogen, phosphate and potassium, typically given as a nitrogen-phosphate-potassium percentage ( for example a 4-10-1 fertilizer is, by weight, 4% nitrogen, 10% phosphate and 1 percent potassium.  A good ratio of the three components for vegetable is 1:2:1. I am not a stickler for staying strictly organic, but I like to go the organic route when I can. Blood meal is a widely available organic source of Nitrogen.  The stuff I have is 13-0-0. Bone meal is commonly used for phosphate.  My supply is 3-15-0.  Wood ash is a good source of potassium, and since I sometimes heat my house with a wood-stove, it is free.  Wood ash typically has a nutrient composition of 0-1-3.

An optional additional ingredient is garden soil.  This will increase water retention, and will provide some additional fertility, especially in the form of micronutrients that may be lacking in the other components. 

Here is my recipe-

1 part soil (sterilized)
1 part sand
1 part composted horse manure (sterilized)
1 part peat moss

And for each 5 gallons of this mix, add 1 cup lime, 1/2 cup blood meal, 1 cup bone meal and 3 cups wood ash.
The garden soil and the composted manure are sterilized by baking in my home oven at 200 degrees, until a temperature of 180 degrees F is reached.  You may worry that baking manure will smell, but my horse manure has been aged for 2 full years, so it is completely odorless. The sterilized manure and soil is passed through a coarse sieve to remove rocks and sticks.
The ingredients are thoroughly mixed in a wheel barrow using a hoe, and then stored until needed.  Because I raise horses, I have a large supply of bags that originally contained 50 lbs of grain.  The bags are high-density polyethylene – perfect for storing potting soil.
Here is garden soil baking in my oven.

One comment

  1. I’ve always wanted to do this but am forbidden from using the oven for gardening.I am also forbidden from using the airing cupboard for cooking after some fermenting rice/lentil mix decided to walk all of our clean clothes 🙂

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