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I have a raised bed that is on concrete. My local greenhouse told me I needed to replace ALL the dirt in there every year due to pollutants. Do you know if that is true as there is a lot of dirt and I don't think I could do that every year. The bed is probably 7'x3' and about 12-15" deep.

  • 1
    Two big questions, where are you, and what will you be growing in your raised beds? – wax eagle May 23 '14 at 18:45
  • I live in Denver and was planning on some tomatoes, leafy greens and maybe zucchini – brendan May 23 '14 at 19:04
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drainage doesn't have to be straight down, if your raised bed is 15" deep and is made of stacked wood or block you probably have some drainage

I really wouldn't worry about it until you think you could have a problem, and then it would be just making drains in the bottom sides of your raised bed.

as for replacing your soil every year: I would guess that the man at the garden center gets a commission on selling soil, because that just isn't how it works.

if you are worried about a particular pollutant then you could test for that, but soil doesn't just get polluted without being exposed to pollution.

If your soil does get polluted every 2 years to the extent that you can't eat tomatoes grown in it, do not eat tomatoes that are grown in soil that has 1 year's pollution, and also... MOVE FAR, FAR AWAY NOW!

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By 'pollutants' they probably mean pathogens as well as sourness of the soil, disregarding completely anything that might be in the air. Drainage is essential, as others have said - without it, soil can easily 'sour' and various pathogens build up, and it may even develop a noxious smell. The bio diversity in soil which drains is not the same as that existing in undrained situations, so I'd guess that's what they mean, but the best thing is to check whether there is drainage and if not, put that right. Punching holes through the concrete to soil beneath at regular intervals should do it.

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Is there any way you could punch through the concrete. You HAVE to have drainage to grow anything. Pollutants...the only thing I can think of is lime leaching into the soil and bringing the pH up, or alkaline. It is relatively easy to change the pH to favor whatever kind of plants you want to grow.

Get a pH test...a soil test from your extension service will give you the pH as well as nutrient deficiencies or excess.

Are they talking about pollutants from chemtrails? The test should give you a profile on the heavy metals present.

Dig down to the concrete and use an iron bar or pick to break the concrete into pieces. Pull out as much as you can. Water it to see if the water drains. If it does, use a good potting soil since you are essentially planting in a pot. Potting soils come with mychorrizae fungi, bacteria...life that plants need to take up different nutrients. Don't use ordinary garden soil.

How is the lighting? If it doesn't get sunlight at least 6 hours a day, you need to rethink what you want to plant.

  • Thanks @stormy. It has great light. Direct sunlight all day. The bed came with the house when we purchased it, so I would dig down. Who knows, maybe the previous owners did punch through the concrete already! – brendan May 23 '14 at 19:30
  • I hope that is true but concrete isn't that tough to break-up...unless it's reinforced with rebar. Send a picture, sounds as if it would be big enough for year 'round shrubs as well as vegetables. – stormy May 23 '14 at 21:42
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Essentially you have a large pot with soil with an impermeable bottom layer. If you have any slope at all to the concrete you'll achieve drainage. If not, you'll still achieve drainage as the sides won't contain all the water but you'd be better off with the base of the bed layered with small branches.

As for whether you need to change the soil each year, that's clearly impractical and not what happens in nature. When you grow things in a bed, certain pathogens do accumulate for a season or so, and some plants might preferentially extract some nutrients rather then others unbalancing the soil. The best approach is what has been done for centuries, and is to practice crop rotation. Normally that's done on a bed basis, but perhaps you could divide the bed into 5, and rotate each year through the 5 minibeds. At least one bed should be planted with a green manure crop.

Each year you should layer down some compost over the whole bed to replace the nutrients you have removed with the plants you've grown and removed. There is no need to mix it all in. As it decomposes further, nutrients will be released to the new plants.

Replacing lost nutrients is the law of return.

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