Help making raised bed soil mix

fernhillhallAugust 20, 2011

I'm a first-timer with raised bed gardening, and I need your experience about soil building.

I've built two large raised beds 8" high. The soil the beds sit on was pretty compacted sandy loam, (heavy on the silt) with decent drainage, and has never been amended. I pulled all weeds/grass and turned the soil to a shovel depth to loosen it up. Then I layered on 2" shredded leaves, 2" composted horse manure, and 2" composted rice hulls. This was done in early June and left to settle. Today I dug down through the layers and found an abundance of worms, no sign of the original shredded leaves, very moist soil, manure and hulls under a dry 1" or less top layer of hulls, (we had 1" of rain 3 days ago). The beds need another 2-3 inches of fill and I'm considering what to add, when to test the soil, and whether to double dig it all to mix everything together, or just let the worms slowly till it for me kind of lasagna style.

I'm wondering if I should add something to create sharper drainage, as the soil and manure are a bit too sticky/damp. Although slightly less damp when I mixed all the layers together with the bottom soil it still didn't pass the "clump falls apart when touched" test. (I just mixed a small patch, the rest is still layered). I don't have near enough finished compost, so as an alternative should I add more shredded leaves in the fall, and/or possibly some crushed granite? I was thinking maybe 2" of leaves and 1" of granite? Should I be adding more topsoil? We don't have any extra soil so I'd have to buy it. I was planning to let the topped off beds sit till next spring and then get a soil test to see if I need to add any supplements for fertility. I'd like to grow vegies and edible flowers beginning next spring.

I've learned so much from this form about how to develop great soil. Our soil was pretty bleak when we moved here 6 years ago. Now it's teaming with life thanks to all the organic material we have double dug into it and mulched with. With the raised bed approach I don't feel confident about what to do next. I eagerly await your advice. Thanks.

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feijoas(New Zealand)

Does that top 2-3 inches allow for mulch? If not, I'd say that's what that space is for!
I think deep mulch is really important in raised beds as they tend to dry out easily.
There's always exceptions, but they're pretty...exceptional IMO.

    Bookmark   August 21, 2011 at 5:29PM
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You didn't state your zone location. In my area of western WA with dry summers & wet mild winters drainage is utmost because things rot in our cold wet soils if I don't amend soil for fall/winter crops. We need to add lime yearly due to the heavy rain leaching the soil. You could consult the county extension gardening resources or consult a local independent nursery for more information about the soils in your area.

Mixing it up now sounds like a good idea. Mulch plants after planting. Add compost after harvest. Keep putting organic matter back in on top of the soil & the worms will work it in the rest of the way. You won't have to dig through & mix it all up again. The beauty of raised beds is that you only dig once.

If you're planting transplants you could just part the layers & plant them. Seeds will need fine soil so they can break through. Chunks make it more difficult for them.

I agree about allowing space for mulch to hold the moisture & provide some frost protection if your climate warrants.

    Bookmark   August 22, 2011 at 5:20PM
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Do not dig anything in!!! Ruins the structure of the soil. Rest assured the worms take care of it.

The beds will continue to settle, so I would just plant and mulch or grow a good cover crop and the bed will supply its' own mulch.

    Bookmark   August 26, 2011 at 12:58AM
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I am putting in raised beds over clay soil that before had two big trees shading it (Douglas Fir and Sycamore). Right now ferns, Solomon's Seal and several plants I don't know the names of are growing there perhaps planted by the birds. Anyway, I would appreciate any helpful suggestions on how to make the most of this area. I am raising the beds because of the huge roots from the large trees that we needed to take out.

Thank you

    Bookmark   August 30, 2011 at 12:43PM
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The best soil for raising any plant is Loam, which is basically about 45 percent sand, 25 percent silt, 25 percent clay and about 5 percent organic matter. If you use only organic matter in the raised bed you will need to add more as the growing season pregresses and that OM "shrinks", as some people say (the soil bacteria digest it).
I have, and still do, grow very good plum tomatoes in containers where the growing medium is compost even though some here maintain that it cannot be done. Compost is organic matter. Peat moss, finely ground bark, and coor are all forms of organic matter except they do not have any nutrients in them, ie. little, if any, bacterial activity to feed the plants which is why synthetic fertilizers are often needed to grow plants in them.

    Bookmark   September 3, 2011 at 6:52AM
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