question for Bob1016

fultonFebruary 5, 2012


Previously you mentioned one would need a lot of sand and organic matter to help alter clay soil.

I mostly see bagged "playbox sand" at the local garden centers. Would this be acceptable?

Also, in regards to organic matter, the local hardware store has topsoil, organic composted manure and organic composted peat in reasonably priced 20# bags. Would any of these qualify? Thanks.


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The issue with clay sand is kind of the same issue as pear based mixes: you need to add *a lot* of material.
You need to ask yourself, do you really want to amend it. There are plants that thrive in clays, or you can do raised beds of soil. To make a clay soil better you need at least a 4" thick layer of coarse sand (usually sold as builders sand) and another 4" of very coarse organic matter (barks work great because they are very coarse and take a while to break down); that's about a 1:3 ratio of cft:sqrft. So if you want to ammend a large area (I would not do anything smaller than a 6' x 6'), quart bags are not really going to work. Also make sure you check the pH, clay soils tend to be very alkali, the bark helps but you may need to add sulfur.
I've also heard of people using large amounts of calcium to make the clay form aggregate (calcium somehow makes clay looser), but I would not recommend that; you pH will be dangerously alkali, I doubt it would last, and it just doesn't sound like a good idea;-)

    Bookmark   February 5, 2012 at 1:47PM
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Bob, what happens if you you don't add any sand and just add coarse organic matter instead? Or if you only add a mix of coarse and fine organic matter to the clay?

(I'm asking this because I've already done this to a couple of small beds and want to know what I can do after the fact to correct it, if needed.)

And what's a good ratio of clay/sand/organic matter in a raised bed?

Fulton, I made the mistake of adding "play sand" to my clay and ended up making concrete. :/

    Bookmark   February 5, 2012 at 3:09PM
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The hard thing about clay soils is that you can't amend a small section, and you need to add a lot of the right ingredients to even have a small effect. If you just add a little sand, it might get worse.
The ideal soil is 60-30-10 (sand-silt-clay) for a sandy loam or 40-40-20 for a general loam, organic mater should comprise between 10%-40% total soil material (if it is more towards a sandy loam, the less organic matter is needed). Raised beds can be filled with a lot of different mixes, some of which have very little actual soil, but most crops do exceedingly well in a sandy loam with lots of organic mater. Lots of people make the mistake of just buying topsoil and growing on that, but a loam based mix is more natural, so plants tend to do better in it.
Silica sand between 0.1mm-2mm is the ideal (more towards 0.75mm) to amend clay. Remember, sand silt and clay are particle sizes when talking about soil types, clay is superfine, silt is medium, and sand is larger.

    Bookmark   February 5, 2012 at 6:31PM
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Sorry Bob, I think I'm not understanding fully.

So sand/silt/clay refer to particle size or the inorganic matter?

So if you have 10-40 percent organic matter (leaf mold/compost/pine bark), then you need 90-60 percent inorganic matter (sand/silt/clay)?

What's the optimal ratio of small/medium/large particles of organic matter?

(Please let me know if this doesn't belong in container forum and I'll post this question again in the Soil forum.)

    Bookmark   February 5, 2012 at 6:52PM
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Sand, silt, and clay are inorganic mineral particle sizes, 0.0625-2mm, 0.0039-0.0625, and I know it seems hard, but really you need to establish good soil, the worry about organic content; but with a clay soil it can be very hard to make good soil, so these questions are very good to ask. Also, feel free to ask your local universities, nurseries, and ag. department what they do, they will have experience with your local soil, so their solution will be more specific.

It might be best to put this in the soil forum. Container soils are a whole different group, where ideally particle size will not go below a tenth of an inch, Al has done a lot of good work with the container media world and he suggests >0.110".

    Bookmark   February 5, 2012 at 9:03PM
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Bob and Larry, I started a new post on the Soil forum. Hoping to get your opinions!

Here is a link that might be useful: Soil discussion

    Bookmark   February 6, 2012 at 9:52AM
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