Sand/Compost Mix for raised garden?

brownmolaJune 22, 2011

I am building a raised garden and am having trouble finding good soil/dirt for cheap. However, I can get all the compost I want for free from the city landfill and I can buy bags of sand for planters for very cheap. Will veggies/herbs do okay in just a compost/sand mixture?

Thanks!

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dogwind(Z8a TX)

What do you need the sand for? Do you plan to eat anything that you grow in the raised bed? If so, check with the city and see if they recommend growing food in the compost from the landfill. If the city doesn't regularly test the compost for heavy metals or toxicity, I would stay away.

    Bookmark   June 22, 2011 at 10:04PM
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brownmola

I was going to mix in the sand to help drainage. The raised bed is going to be specifically for vegetables and herbs. I don't know if they test for heavy metals or toxins but according to their website, the compost is made from yard trimmings and food scraps. Would the heavy metals and toxins come from another source or could this compost still have these things?

    Bookmark   June 22, 2011 at 10:23PM
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Kimmsr(4a/5b-MI)

If the compost from a municipal (city) compost operation contains sewage then heavy metal contamination should be a concern, but if it is made from yard and kitchen waste there should be no need for concern.
Raised beds, as a rule, have very good drainage and that is one of the initial reasons for having a raised bed, the native soil did not drain well so the quickest solution was a raised bed. With a sand/compost mix in that raised bed I would expect the drainage to be so fast keeping the soil moist could be a real problem.
Since you have a chance to build soil for this raised bed you might want to try to make loam, about 45 percent sand, 25 percent silt, 25 percent clay, and 5 percent organic matter, unless someone around you has this for sale.

    Bookmark   June 23, 2011 at 6:53AM
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gargwarb

What you've described there, Kimm, is a 'sandy clay loam' with 50% combined silt and clay. What if the native soil is a sandy loam with a combined silt and clay content of 30%? How about 15%? How might that effect drainage at the interface of the 2 soils?
If it's a debacle, how much time, energy and money would have gone into blending that soil, installing the soil, removing the soil, then replacing that soil, which was recommended on whim with no idea of what the native texture is like?

If you have a sandy native soil, placing the sand on top would be just fine. If that's the case and you want to buy a single blended material, and the bed is 10 inches deep or less, you could use an 80% sand to 20% compost material. You shouldn't place organic amendment below that depth because it will decompose anaerobically and remain wet enough long enough to favor root disease pathogens. (not to mention the significant amount of settling over time as that much compost continues to decompose).
If your bed is deeper than 10 inches (or even if it's not and you want to put in sand and organic separately) fill the bed to within a couple inches of the top and then blend compost into the top 6 to 8 inches. A good rate would be 11 cubic feet of compost per 100 square feet of bed.

Of course, this can all change depending on the texture of the soil below the bed.

Another thing to keep in mind is that municipal "compost" can sometimes just be a pile of virtually raw yard waste. You'll want to make sure it's finished compost.

    Bookmark   June 23, 2011 at 9:45AM
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gardengal48

A) It's not all that easy to "make" loam. If you don't have access to regular, inground soils you can use, it's unlikely you can find the raw ingredients to "make" loam. You can't just go buy something labeled 'silt' or 'clay'.

B) Free is always good but it may not always be the best stuff. Unless you have huge raised beds, importing the soil you need to fill them should not be a big expense. And is worth the investment. Look for a bulk soil provider that offers a 'garden planting mix' (or sometimes called a 3- or 5-way mix). This is the ideal mix for raised beds, includes some sort of OM or compost and should run about $25-30 a cubic yard.

C) If you must, the compost/sand can work. But the compost will continue to breakdown and you will experience a lot of shrinkage. It may also be too rich, especially for herbs, which prefer a lean soil. And it will have very fast drainage that may require too frequent watering to keep the mix properly hydrated.

    Bookmark   June 23, 2011 at 9:50AM
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gargwarb

This is also a good point:
A) It's not all that easy to "make" loam. If you don't have access to regular, inground soils you can use, it's unlikely you can find the raw ingredients to "make" loam. You can't just go buy something labeled 'silt' or 'clay'.

This, I'm not completely on board with, considering how little we know about the native soil:
Look for a bulk soil provider that offers a 'garden planting mix' (or sometimes called a 3- or 5-way mix). This is the ideal mix for raised beds, includes some sort of OM

I say again, don't make a decision on the type of soil without taking into consideration the texture of the native soil and the proposed import soil. I see "garden planting mix" soils that are anywhere from 15% to 60% combined silt and clay depending on the source. A mis-match of textures can result in serious drainage problems down the road. Sand over clay is slightly better than clay over sand (to mention the extremes) but still not a good situation. As stated in my original post, everything can change with texture.

If you do place a sand on top of a sandy soil, that is far better than placing a heavy texture soil on top of a sandy soil. Of course it will drain quickly but so will the subsoil, resulting in relatively even soil moisture throughout, which is the best you can hope for.

Of course, if you have a heavy textured native and you put sand in the planter then water will move quickly through the sand and come to a crawl at the interface, resulting in too dry conditions in the top of the profile while soil at the interface is too wet.
Conversely, if you place a heavy textured soil on top of a sandy native soil, the water movement won't just slow down. It will stop completely due to the capillary action of the smaller poor spaces in the more finely textured import soil until a large enough head of water builds up to push the water into the subsoil, again, resulting in saturated soil conditions.

As far as the OM goes, I regularly see "planter mixes" anywhere from 85/15 blends up to 50/50. Be sure you know what you're getting. They can call anything they want "planter mix". Any more than an 80/20 can result in an unreasonable amount of settling. (which seemed an issue in regards to the sand/compost blend but somehow isn't an issue when it's a pre-mixed blend and quite possibly higher in organic than 20%?) Just pointing to the pile that says "Planter Soil" on the sign can lead to problems. Match the texture of the import to the native and don't use something too high in organic.

If you want to get into types of organic, that's a whole different discussion.

    Bookmark   June 23, 2011 at 11:32AM
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gardengal48

I guess I am more used to the very uniform quality of bulk soil blends offered by most suppliers in my area. And there is minimal concern about layering imported soil over existing soil, even with a clay based native soil. This is an extremely common practice that I encounter in my business on a daily basis -- either just as an additional layer to add to existing planting beds, creating berms or mounds or filling raised beds. Provided the depth is sufficient, there is no negative impact on drainage, whereas there very well could be if the imported soil was incorporated into smaller pockets of existing soil, like individual planting holes. In fact, layering a sufficient depth of faster draining imported soil is a recommended solution for those who must deal with heavy, clay soils.

My advice for anyone contemplating purchasing bulk soils - or compost, for that matter - is to visually inspect the product before purchase.

    Bookmark   June 23, 2011 at 11:58AM
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gargwarb

My advice for anyone contemplating purchasing bulk soils - or compost, for that matter - is to visually inspect the product before purchase.

Agreed.

In fact, layering a sufficient depth of faster draining imported soil is a recommended solution for those who must deal with heavy, clay soils.

Depending on the depth of the planter, which we don't know in this case. A couple of feet deep and you're planting annuals, absolutley! 8 inches deep and it can put you in a dilly of a pickle if the required head to push the water into the native subsoil is about 6 inches of saturation.

I guess I am more used to the very uniform quality of bulk soil blends offered by most suppliers in my area.

It's a very regional thing. In some areas I work, that's the case. In others, it's all over the map.

    Bookmark   June 23, 2011 at 1:55PM
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brownmola

Thanks guys, I found a place that I can pick up really cheap topsoil which I am going to mix with the free compost.

    Bookmark   June 23, 2011 at 10:39PM
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