I have this recipe that I found from my Granddad's place for making potting soil, for which I need 120 gallons (or for a 16 cubic foot raised box). The recipe calls for the following ingredients:

  • 5 parts compost
  • 4 parts lava sand
  • 3 parts peat moss
  • 2 parts cedar flakes
  • 1 part soft rock phosphate
  • 1/2 part something
  • 1/4 part something 2

Could someone please help me with how much each of these ingredients are to make 120 gallons or 16 ft cubed? Thanks.

  • 3
    I think it's a reasonable question. If you haven't worked with a potting soil recipe before, you might think that "parts" are some kind of gardeners' jargon. Leaving a question like this intact, with a good answer, can provide a basic framework for others searching for this kind of info.
    – bstpierre
    Jun 28 '11 at 11:22

As Tea Drinker said, a "part" is some arbitrary unit based on whatever you're using to measure: a bucket, scoop, etc. (I used a 5 gal pail as my "part" when I'm mixing potting soil.)

To figure out how to make X amount of total soil:

  1. First add up the parts: 5+4+3+2+1+0.5+0.25 = 15.75 then round to make it even (makes the rest of the math easy). Your recipe has 16 parts. (Potting soil recipes don't really call for exacting precision.)

  2. Take the total amount you need, and divide by the number of parts. In your case, this is trivial: you need 16 cubic feet, so each part should be 1 cubic foot (or about 7.5 gallons).

  3. If you need to go out and buy raw materials, you'll need to multiply the parts by the measure you determined to know how much to buy. For example, you need 5 parts of compost at 1 cu ft/part, so you need to get 5 cu ft of compost (or get 5 x 7.5 = 37.5 gallons, about 8 5gal pails).

If you're filling a raised box, I'd make more than you think you need -- the soil will settle pretty quickly over time, and when you're watering, with rain, etc.

As an aside, Google is helpful at converting units. If you search for "1 cu ft in gal", it will give you the answer: 1 (cu foot) = 7.48051948 US gallons.

  • 1
    The part about filling a raised box was important for me. Thank you for the extra information.
    – user370
    Jun 28 '11 at 15:52

It's just a set of proportions. If you take any measure, say a scoop, then the recipe says to use 5 scoops of compost, 4 scoops of lava sand etc.

In your case to make just over 120 gallons use 1 part = 8 gallons, resulting in:

  • 40 gallons compost
  • 32 gallons lava sand

    etc ...

  • 2 gallons something 2

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