Raised Bed Mix

tomat0(10a)December 14, 2011

Hello! I'd like some opinions regarding a raised bed mix.

I figure a roughly 75% mineral soil, 25% organic mix is sound but I'm open to suggestions.

I have access to the following ingredients from other planting uses and such. Will probably only add compost/micros as needed.

- topsoil, screened

- Turface

- sand

- 3/8" mini bark

- peat moss

- compost (quality horse manure/wood bedding)

- cow manure, OMRI/composted

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Many people think "topsoil" is a good soil to use, but since "topsoil" is simply the top 4 to 6 inches of soil from someplace it can be anything. My conversations with some people tells me that most everyone thinks "topsoil" is going to be loam and it probably will not be. Do not just get some "topsoil" but take a good look at what you propose purchasing first to see if it is what youu really want.
What you want to end up with is a soil that is well drained, but retains soil moisture well, is well endowed with organic matter (that is in the 5 to 8 percent range), and has balance plant nutrients with a pH in the 6.2 to 6.8 range. A really good soil, loam, is about 45 percent sand, 25 percent silt, 25 percent clay, and about 5 percent organic matter. However, I am never going to get my Lake Michigan beach sand to be loam, but adding enough organic matter to that sand (about 8 percent) turns it into a very good soil that grows strong and healthy plants for me.

    Bookmark   December 15, 2011 at 7:43AM
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gardenlen(s/e qld aust)

g'day tomat,

we sue almost exclusively spent mushroom compost from the farm check our presentation:

these new gardens soon to be built when house is built will use a technique we have used from the beginning now popularly know as hugelkultur or hugelbeet, still with mushroom compost.


Here is a link that might be useful: lens straw bale garden

    Bookmark   December 15, 2011 at 4:02PM
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tapla (mid-MI z5b-6a)

Tomat0 - I think you're being very realistic in your thinking that a RB soil comprised of a mineral fraction around 75% topsoil and the rest organic ingredients is right on and will be very productive - more so after the first year. I wouldn't be too concerned (at all) about what the topsoil 'is', as long as it's not clay. If it happened to be silty, you can incorporate some sand into the 75% mineral fraction. I would try to include some of the pine bark fines as at least 50% of the original organic fraction. You'll find that in subsequent years, all you'll need to do is mulch, & nature will do the rest.

You'll likely need to supplement N in the first year. My beds started out with a much higher organic fraction 75-80%), and have always been extremely productive. Over the years, I've continually added a soil I use in considerable volume for containerized plants (the gritty mix), leftover after repotting. This has eliminated the only real drawback I can see to a RB soil whose predominant fraction is pine bark, that being shrinkage.

As the inert mineral fraction and the organic fraction reverses itself and the mineral fraction becomes predominant, I see no change in productivity .... but I'm now running out of room to use the old soil from repots, as the beds are now all topped off.

Make sure the compost is finished, or the need for additional/considerable N supplementation is assured, which is why I like the pine bark so well - little in the way of N immobilization.

FWIW - be careful with mushroom compost. It's usually very hot, very high in soluble salts, and gasses off very quickly.


    Bookmark   December 15, 2011 at 4:35PM
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wayne_5 zone 6a Central Indiana


You seem to have a fairly good start there. I have added equal parts of medium/coarse sand and peat moss. The peat moss loosens the soil structure and holds moisture. The sand also loosens the soil when mixed with peat moss and counterbalances any sogginess. I love it.

    Bookmark   December 15, 2011 at 5:16PM
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Tapla, I know you are a walling, talking book when it comes to container garden.
But I did not know you used raised beds, too.

    Bookmark   December 16, 2011 at 9:22PM
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