Garden soil organic matter = 5%, but raised bed organic matter = 50%? Why?

Asked May 7, 2019, 6:16 PM EDT

Hi there, Why is it that for garden soil it's recommended on have no more than 5-8% organic matter, but then for raised beds the recommended recipe is to use 50% compost. I understand why you want to limit the amount of organic matter in garden soil (soil can have too much phosphorus, compost is high in salts, etc). But then when look at university publications on raised beds, I'm seeing recipes the recommend anywhere from 35-50% compost. How do crops in raised beds not suffer from the same negative effects of too much compost that crops planted in traditional garden beds do? Thanks so much. Bethany

Utah County Utah

2 Responses

I don't know what you've been reading but I recommend no more than 30% compost. As far as why the discrepancy between raised beds and regular garden soil is that 1) raised beds would be much easier to leach salts out of due to better drainage. 2) the biological activity in traditional soil is much more complex and active. Regardless of how much organic matter you add to traditional soil, it quickly breaks down through biological processes resulting in a lower percentage of organic matter than would be measured in a raised bed (which is really a large version of a pot). Depending on the articles you're reading my supposition is that 5-8 % organic matter in a traditional soil is really all that is reasonably expected. The measurements between the two are comparing apples and oranges. The organic matter is measured in different forms in each.

Great. Thanks for the speedy reply! :) Your explanation makes sense. Thank you. Ya, it was a publication from 2007 put out by the Utah State University Extension that contained a few recipe suggestions for soil. One recipe recommended 40% compost, another 35% compost. I can't remember where I saw the recipe that suggested 50% compost. Then another 2012 publication from USU suggested 1:1:1 compost to soil to "drainage increasing materials", which sounds closer to the ratio you recommend. Thanks so much!