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Raised beds or boxes on a sand and gravel pit?

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Raised beds or boxes on a sand and gravel pit?

Postby terrynew » Jan 15, 2011 2:03 pm

Hi, can I get some advice on how deep a raised bed needs to be for growing vegetables over a sand and gravel base? I'm planning to build a new vegetable garden on land that was formerly a gravel quarry. It's very soil-less soil, just packed sand and gravel and pit stone.

I will obviously need to truck in tons of new soil, and I believe I'll need an inch or two of clay trucked in as a base layer to prevent water drainage from the new soil into the existing high-drainage sand/gravel. Does that make sense?

More importantly, how deep should the trucked in soil be for a 'standard vegetables' garden? Steve Solomon's "Gardening When It Counts" has some drawings showing most common vegetables have roots that can go down 4-6 feet or more, but obviously I can't build a raised bed that high.

The simple answer is to remove much of the existing sand/gravel and truck in a few feet of subsoil before putting in the garden soil. But I'm looking at a large garden, perhaps 1000 sq ft, so that's a lot of heavy sand/gravel to truck away and replace with subsoil. I can't just dump the subsoil then garden soil on the existing sand/gravel, as it's in my yard just two feet below the level of the house foundation, so I can't raise the yard level more than a foot.

I'm considering building a bunch of 5' x 16' raised beds/boxes by using two or three levels of rough-cut 6"x6" timber (I'm in an eastern Ontario timber area) to form the walls. But would a 12" - 18" depth of good soil be enough to grow veggies in? I know it can be done in container gardening, but is it practical in large-scale beds like this? Would such a shallow system lead to heavy watering needs? Would the roots go down thru the thin clay layer and slowly convert the sand/clay soil into useful subsoil?

Thanks for your thoughts,
...Terry
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Re: Raised beds or boxes on a sand and gravel pit?

Postby Eeyore » Jan 15, 2011 2:37 pm

18 inches should be good for most typical veggies. Even potatoes should do well in that depth of soil. Depending on how often you plan to water and the method I don't think you need to be concerned about a clay layer. 18" of soil should hold enough water to last 2-3 days even in hot weather.
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Re: Raised beds or boxes on a sand and gravel pit?

Postby B_BQ » Jan 15, 2011 4:14 pm

These raised beds were built directly on top of a lawn, (which itself is on clay and rock!), 5 years ago. They're about 20" deep. After building the frame I lined the whole thing with landscape cloth so that the soil would not leach out every time it was watered. At the bottom of the beds I put a very thick layer of cardboard, (we'd just moved), and as many leaves as I could rake up from the garden in November. I then had topsoil trucked in, which was a very heavy and difficult job to transfer from the driveway to the raised beds! (Two strong backs and one fairly strong back were needed!). For 5 years now I've added, amended, added compost, kitchen scraps, sheep manure, (weed free), and leaves to it, slowly improving the soil. They're absolutely great. Fortunately they're watered with a sprinkling system, but I do give the tomato plants extra deep watering every few days.
~BBQ
Edited in to add that these beds are approximately 8' x 6' each. This size enables me to be able to reach all parts of the bed, without having to climb up on to the bed, to do any weeding! Important for those of us with back problems! :)

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Re: Raised beds or boxes on a sand and gravel pit?

Postby houseinthecountry » Jan 15, 2011 9:00 pm

wow, great beds bbq!! I don't have photos, but last summer built 3 3x6 beds of cedar. They're 24" deep, but I didn't fill them to the brim - $$$!

In the fall I filled them with compost, garden clippings and leaves, and sprinkled some soil over top. I'm hoping they're busily composting away under the snow, so I'll have great soil in the spring.

Anyway, my veggies did very well in about 18" of soil. Plenty of depth.
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Re: Raised beds or boxes on a sand and gravel pit?

Postby terrynew » Jan 15, 2011 10:29 pm

Those are great patio-stone beds, BBQ. They look like they're made from the Lee Valley kits -- am I right? Have you found them sturdy over the years? Do the lower 2x4s rot? Is it hard to weed the edges where the soil meets the patio stones? (Would that dull any hoe?)

The kits (metal connectors to join the patio stones to upper and lower 2x4s) are expensive, though -- $70 for a 2x4x8 raised bed. That's not practical for 1000 sq ft, which would take 30 of them and $2100 plus the stones and 2x4s. Has anyone found a home-made way of framing patio stones for raised bed walls?

As a compromise, I could bury the 24x24 patio stones in a 6" trench and get two 18" x 4'x8' raised beds from one kit, just using the hardware and 2x4s at the top. That would also reduce the wood rot problem.

Any other ideas out there for inexpensive raised beds?
Thanks,
...Terry
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Re: Raised beds or boxes on a sand and gravel pit?

Postby B_BQ » Jan 15, 2011 10:50 pm

Hi:

Yes, they are made with the Lee Valley raised bed kits. I really don't have any memory of how much they cost - I didn't think they were exorbitantly expensive! But I didn't pay for them!!!!! :roll: :lol:

We used cedar. They're now 5 years old and the wood is not showing any signs of wear or tear at all.

Weeding is absolutely not a problem; it's so easy.

To me they're worth their weight in gold!

Hope someone can jump in and answer your other questions though. Some of our regular veggie gardeners are taking a bit of a break, but hopefully they're lurking and will see your post.

I would love to have another raised bed, but I'm not getting any volunteers to build it!

~BBQ
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Re: Raised beds or boxes on a sand and gravel pit?

Postby CdnChelsea » Jan 15, 2011 10:52 pm


Last summer, my husband surprised me by building us a raised patio.

Love it...love it...love it! :D We ordered a dump truck full of rocks. Very inexpensive rocks called "shot rock" for $125.00 .
With the left-over rocks, he also built me another garden 15' wide by 20' in length. Love my new garden :)
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Re: Raised beds or boxes on a sand and gravel pit?

Postby CdnChelsea » Jan 15, 2011 10:59 pm


Oops....I forgot to show a picutre of the new patio.
patio under construction.JPG


So.....both the patio which is around 400 square feet and the new garden were built with $125.00 worth of stone :D
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Re: Raised beds or boxes on a sand and gravel pit?

Postby houseinthecountry » Jan 16, 2011 8:37 am

oh my goodness, I love this place. You guys are very inspirational - and I'm impressed by the ability to put up photos for everything!!

terrynew - my cedar beds were cheap to build. The planks came in 6 foot lengths (2"x6', I think) and we just screwed them to corner cedar posts cut into 2 foot lengths. Couldn't have been more than $75 for the 3 beds.

I eyed up the lee valley kits, but once I bought patio stones, etc it was cost prohibitive for us.
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Re: Raised beds or boxes on a sand and gravel pit?

Postby OGrubber » Jan 16, 2011 10:32 am

terrynew wrote:More importantly, how deep should the trucked in soil be for a 'standard vegetables' garden? .......common vegetables have roots that can go down 4-6 feet or more, but obviously I can't build a raised bed that high.

But would a 12" - 18" depth of good soil be enough to grow veggies in? I know it can be done in container gardening, but is it practical in large-scale beds like this? Would such a shallow system lead to heavy watering needs? Would the roots go down thru the thin clay layer and slowly convert the sand/clay soil into useful subsoil?


Terry, hi!
Fascinating project.
I would guess that the roots of any veggy will stay within the boundries of where they find moisture and nutrients so the 4-6 foot depth isn't as big an issue as it first seems. However, you will need enough depth to allow for anchorage and feeding needs. So.... I would target 18 to 24 inches. There are a lot of shallow rooted veggys that won't need that much depth, [lettuce, onions etc] but all the plants with large canopies will [tomatoes etc,]
To start, I would yes, put in a layer of clay subsoil to help with water retention and to keep your soil from washing into the sand/gravel base. In a pinch, a decently thick layer of newspaper or cardboard would do as well. But, I wouldn't use landscape fabric or plastic with drainage holes, or you'll restrict roots and prevent the natural development of your sub soil.
You will probably need to "top up" annually with soil or other ammendments because yes, your gravel/sand base will slowly convert [and deepen] to actual sub-soil.
My guess your watering needs would be a little higher than a bed placed on soil for the first season but then get better in successive seasons.
I have areas in my growing land base where soil over bedrock was initially, only about 6 inches and is now about 12.
I'd go for it.
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