potting soil in raised bed

wolfy528(7)January 18, 2012

If I add potting soil to a raised bed what ratio of potting soil should I use?

I am considering some top soil, black kow, other compost and some potting soil. I saw somewhere 60% topsoil and 30% compost but I am not sure where potting soil fits in. I used a potting soil with topsoil in a flower bed and it worked really good but this time I am building a vegetable garden bed with cinder block. My bed will be 4x10 8 inches high and I will be buying bags from the big box stores to fill in. Long story but I cannot get a load delivered anywhere near where I am building the bed.

Any thoughts on mushroom compost? Lowes has this but I read mix messages about it on the web.

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ifraser25(z11 Brazil)

How long is a piece of string? It depends what's in your top soil and "potting soil" and how heavy/light it is. If you have bought the potting soil in bags it should say what the chemical composition is. You shouldn't have to add extra fertilizer unless you want giant pumpkins for example. I don't know how moisture retentive it is. If it dries out too quickly you may still have to work in some leaf mold. As far as I'm concerned, the main advantage of having a raised bed is for drainage and ease of cultivation rather than nutrients, so make sure it's fairly light and easy to work as well. It takes several years, however, for you to "get to know" your soil, whatever it is, and what grows well in it and what does not. Root veg is usually good in raised beds + stuff like onions. Don't use mushroom compost unless you want mushrooms.

    Bookmark   January 18, 2012 at 8:53AM
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tapla (mid-MI z5b-6a)

You CAN grow in raised beds in soils that have a very high % of OM if you wish, but you should know the downside. A high % of OM often means poor water retention and often poor rewettability. Lots of OM ensures lots of shrinkage, and if the OM you choose doesn't break down very slowly, then N immobilization can present significant issues.

In most cases, if your native soil is decent, something like 75-80% native soil or decent topsoil with the remaining fraction being OM in the form of pine/fir/redwood bark fines, finished compost, or a fraction of sphagnum peat.

The soil in my raised beds looks like this:

You can actually see that it has wonderful tilth. It's about 80% OM (pine bark), extremely productive, and full of soil life. The downside is it shrinks quickly, so the pine bark fraction needs topping off regularly, which I do by mulching. I've also been adding used container soils that have a 2/3 mineral fraction as mulch, which has significantly reduced the shrinkage with no apparent loss of productivity.

FWIW - pine bark fines will be considerably less expensive than potting soil, and because it breaks down MUCH more slowly, shrinkage and N immobilization will be much less a problem. You might wish to consider that tack.

BTW - you WILL need to add fertilizer - plan on something heavy on the N to compensate for N immobilization, especially in the first year.

Mushroom compost is often partially decomposed straw with lots of urea in it. It's often very hot and high in soluble salts, so treat it like fresh manure - probably not a good choice for something in the more immediate.


    Bookmark   January 18, 2012 at 12:33PM
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toxcrusadr Clay Soil(Zone 6a - MO)

One general suggestion: If I was buying bags of anything in that kind of volume, I would use several products to even out the pros and cons of each. For example mushroom compost has its drawbacks as stated, so maybe a few bags of that along with some other products would be a good idea for whatever final mixture you eventually decide on.

    Bookmark   January 18, 2012 at 12:52PM
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For a 4x6 bed I am considering 1 bag of black kow, i bag of mushroom compost, one bad of cheap manure compost, 7 bags of topsoil, 2 bags of MG potting soil and 2 bags of MG garden soil. I will mulch with hay and fertilize with fish emulsion if needed. I am considering perlite but have not decided. I also have plenty of leaves left over from fall to add.
Before I add the soil I plan to dig out about 2 inches down into my solid red clay and lay cardboard.
Is there anything I should add or take away or consider in the above?
I am a newby but determined to start growing my own vegies.

    Bookmark   January 18, 2012 at 7:20PM
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Potting soils are essentially Peat Moss, finely ground bark, or maybe coir, with either perlite or vermiculite added for drainage. So, basically adding potting soil to your garden means you may well be adding some very expensive material when other, and similar, less expensive material would be available.
The leaves will replace peat moss, bark, and coir and be better for your soil then any of them would be. What you need to end up with, eventually, is about 6 to 8 percent humus (residual organic matter) in your soil. For the first few years you may well need to supplement fertilize but if you work on getting the soil into a good healthy condition you may well be able to skip that, depending on what good reliable soils tests tell you about your soil.

    Bookmark   January 19, 2012 at 7:27AM
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