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Soil at a new house in Seattle

Posted by plantslayer 8 (My Page) on
Sun, Jun 9, 13 at 1:05

Hello everone,

My wife and I are buying a new house in NE Seattle. One of the cool things about the new place is that the front hard is a few feet below the street, and adjacent to the (not busy) street there is are three levels of terraced beds made from poured concrete, maybe around 300 sq ft. We really want to plant some ornamentals next to the street and vegetables on the bottom two terraces.

Right now, the soil in there terraces is some kind of hard grey stuff a fair amount of gravel in it. I assume the previous owner bought this stuff somewhere since it does not really look like native soil. I was wondering, does anyone know what this is? Is it just the cheapest fill dirt he could find, or might it actually be OK for growing stuff?

I am prepared to till it and remove it with a grain silo scoop in order to replace good topsoil in there. How much of it do I need to remove and replace with "real" soil in order to have a decent garden? If I leave it as a subsoil under a top layer of good topsoil, should that be OK?

Thanks for your help!


Follow-Up Postings:

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RE: Soil at a new house in Seattle

Does it drain?
dig a hole, fill with water and time how long it takes to drain.

there are sure to be other considerations as well, but this would be a start.


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RE: Soil at a new house in Seattle

That would have been my first question as well :-) And closely followed by is there anything growing in that area now? If yes, that's a good indication the soil is at least adequate.

If decent drainage, all you may need to do to encourage both ornamentals and edibles is add some organic matter. A couple of inches of compost worked into the soil is often all that is needed to improve questionable soil quality.


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RE: Soil at a new house in Seattle

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Sun, Jun 9, 13 at 14:13

A local soil type that is frequent is Alderwood Glacial Till. It has a high percentage of fine particles and stones, looks like concrete. It was left behind by the Vashon Glaciation, with its mile thick sheet of ice compressing and grinding the earth.

You can even grow plants placed in topsoil laid over asphalt if the water drains off when it hits the base layer. If instead you try to blend organic amendments with an existing deposit of heavy soil with much slit and clay in it - such as Alderwood Till - it takes huge amounts of amendments to balance all the fine particles, which go a very long way. When there is a lot of clay and much mixing during the amending process the clay will coat the amendment particles and maintain its influence.

I once saw a presentation by some alpine plant specialist growers where they had a segment on trying to master the container cultivation of a tiny, high altitude bleeding heart native to rocky places. They started with a mostly coarse mix but continued to have problems with the plants rotting out until they finally ended with a mix that had effectively no silt or clay in it at all.


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RE: Soil at a new house in Seattle

I haven't actually moved in yet, so I don't know if the stuff drains, or even how deep it goes. I think Bboy's description of Alderwood Glacial Till is pretty spot on, so I am guessing this is what it is.

I think maybe the company that staged the house for selling may have poured this stuff on top what was already in the beds in order to make a nice, tidy-looking beds. There is indeed some pampas grass and marigolds "growing" there, but that is only the selling agency arranged to plant some cheapo flowers to make the house look nice for showing.

They also poured a thick coat of cedar mulch on top of the dirt. So, I'll have to break this stuff up and scoop it all out with a grain shovel or something, I guess.

Another problem is that the beds are made out of stamped concrete that is all one piece, which could be bad news if it doesn't drain. I guess if that is the case I might have to drill some holes or something? Hope this is not necessary...


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RE: Soil at a new house in Seattle

FWIW, terraced or raised beds seldom use indigenous soil unless they were constructed at the same time the house was built. If that's the case then the glacial till is spot on. Otherwise aftermarket landscaping or filling of terraced/raised beds could be anything from 'fill dirt' to rubbish to decent soil. btw, glacial till is a perfectly adequate soil if somewhat low in OM - generally friable and well-draining. Some are even very sandy.

Raised or terraced beds tend to drain well just by virtue of their elevation - gravity is a wonderful thing! Generally, back drainage with small retaining walls or terraced planting areas is quite adequate. The water percolates down to the base then out underneath. Larger walls may have weeping tiles or a gravel backfill that addresses drainage. Sometimes weep holes are necessary but not always. It may make sense for you to hire a qualified landscape contractor (one experienced in retaining walls) to check out this terraced construction. Has it been there long or is it recent construction?


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RE: Soil at a new house in Seattle

Perhaps the staging grass is a common ornamental grass, rather than Pampas grass. Marigolds makes sense.

A recessed front yard relative to the street can present drainage problems, I would check the drainage as mentioned above before planting anything significant.

Does the entire property continue to slope down from the street, or is the front yard surrounded by higher terrain?

For vegetables, one-foot depth of good soil is fine; two feet is overkill.


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RE: Soil at a new house in Seattle

Thanks everyone for the advice!

You're right larry, it's actually ribbon grass or something that looks about 90% dead right now, I don't know where I came up with "pampas grass".

Actually, below the terrace it is just a flat yard. The street is sort on a ledge above the yard about 8 ft or so (?) and I think the previous owner just put the terrace so he could use the grade down from the street as a place to plant stuff.

The thing is, I think the stamped concrete is new, it might have been added in order to sell the house. When I look at the address in Google maps I can see that the terraced beds had large wooden posts or railroad ties or something to hold the earth in. So it's possible that the things don't drain too well, since probably no one has tried to seriously grow stuff there since the concrete walls were added.

On the one hand, I'd like to trust whoever decided to add the concrete walls or their contractor to not make a poorly drained terrace, but on the other hand there were places in the house where they did a cruddy rush-job on some things (nothing really critical, or we wouldn't have bought the place) to make it look nicer for showing.

So assuming the alpine till goes down pretty far, is it probably OK to just scoop out the top 8-12 inches or so out of the beds and replace with a nice artificial gardening soil? Assuming the Alpine till drains middling-OK, I suppose it can be fine as a sub-soil?

Everyone's info and advice greatly appreciated!


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RE: Soil at a new house in Seattle

(double post :( )

This post was edited by plantslayer on Mon, Jun 10, 13 at 0:45


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RE: Soil at a new house in Seattle

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Mon, Jun 10, 13 at 14:52

If it were me I would to plant edibles as far away from the street as possible and I would not want to plant edibles where there might have been creosoted railroad ties or pressure treated landscape timbers sitting for some years. Especially if there had been timbers treated with copper chromated arsenic!


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RE: Soil at a new house in Seattle

It is very easy to test the soil for drainage - I've attached a link that explains how to do that. Although terraced or raised beds almost always DO provide reasonable drainage, it never hurts to confirm before investing a lot - of money in plantings.

And it is usually unnecessary to remove a lot of soil and replace unless there is some serious chemical contamination going on. Provided the drainage is decent, just adding some richer, more organic material - like compost - is sufficient.

The cautions about planting edibles in front gardens pertains to their exposure to pollutants and run-off from vehicular traffic. Much of that is dependant on how busy your neighborhood may be and your own personal feelings on the subject. With the popularity of edible gardening overall, more and more homeowners are incorporating edibles into their front landscaping, often intermixing them with more ornamental plants. It is not at all uncommon to see very prolific and lush vegetable/herb/edible gardens growing in the parking or "hell" strips common to many neighborhoods.

Concerns about railroad ties or treated lumber that were used to retain planting areas leaching harmful chemicals into the soil are somewhat overstated. First, railroad ties that were recycled for home use tend to have had most of the creosote already leached out. And while some additional leaching may occur over time, it tends to be minimal and rapidly dissipated with natural soil biology and climate. And the leaching from CCA treated has been proven to be even less of a concern (read this) - minimal amounts that both move through the soil and decline fairly rapidly. And remember....both copper and arsenic are present in all soils anyway. Plus, there is really no evidence these elements are taken up or absorbed by the plants in any greater concentrations than normal.

If the coated or treated lumber is not there NOW you really do not have anything to worry about.

Here is a link that might be useful: soil drainage test


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RE: Soil at a new house in Seattle

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Tue, Jun 11, 13 at 15:06

You linked to the drainage page twice instead of linking to one about CCA the first time.


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RE: Soil at a new house in Seattle

When putting in a retaining wall, one has to allow for drainage or else the wall would fail from the weight of wet soil behind it. Usually there will be some kind of weep holes at the bottom of the wall.


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