16

Motivation: this comment from stormy:

And DO NOT ADD SAND to garden soil!!! ARRRGGHH. Again, when one takes clay soil, gravel, lime, gypsum, water and then ROTATE what does one get? Concrete. Sand is right in there. Does NOTHING for the soil. The ONLY WAY to improve ANY soil is the addition of DECOMPOSED ORGANIC MATTER. Period. Allow me to elucidate!! I love this question and it is so vital to understand.

I usually add some sand to my soil. I did a few granulometric analysises, which show that my soil has high percentages both of clay and of silt (so very low in sand). On a figure (which now I can't find anymore), I saw that more sand will help to keep more water in soil (there is, IIRC, a sweet point of around 30% of sand; much more sand will make water flow away).

Really I add sand only when planting trees, on sub-soil, and a little sand on seedlings, not to have mould.

So, is it bad to add sand? Why?

11
  • 1
    @RoryAlsop: Usually I add sand, but today I learnt in a comment (here in gardening.SE) that adding sand is alway bad. So I'm asking why people think it is bad. Additionally, because we should not ask generic (opinion based) questions, I specified my case, and I hope to see if my reason is good, or why I'm doing it wrong. – Giacomo Catenazzi Jan 22 '17 at 15:41
  • 1
    I hope it is sand is not too bad as my soil is 99 % sand for about 10 ft depth. The area is ancient seacoast dunes ; The surface 6" has significant organic matter , of course most tree roots are in the sand. – blacksmith37 Jan 24 '20 at 16:46
  • 1
    "when one takes clay soil, gravel, lime, gypsum, water and then ROTATE what does one get? Concrete." WRONG, WRONG, WRONG. The "active ingredient" in concrete is cement, and has nothing to do with mixing sand with clay. Yes, there is sand in concrete, but it is the cement that turns it in to rock-hard concrete, not the sand. – James Aug 10 '20 at 15:15
  • 1
    @James: is cement lime? I live very very near an old lime extraction (very pure calcium) + cement manufacturer. But I agree: to get cement, we need few more components and ovens. – Giacomo Catenazzi Aug 10 '20 at 15:57
  • 1
    I hope it is just an opinion of a garden "writer". I live in an area that was coastal sand dunes many millions of years ago. When I dig more than 6 " in my sandy soil I reach 95% sand. This is a portion of the Piney Woods, temperate jungle or rainforest. Apparently the trees don't know sand is bad. About anything grows here if you give it water .With one caveat , being sand with no limestone, the pH is less than 6 . – blacksmith37 Aug 10 '20 at 20:43
13

This reference says that you need to add prohibitive amounts of sand to remediate clay soils. And if you don't get it right, you get a soil like concrete

The problems occur when sand and clay are mixed in incorrect proportions. An ideal soil has 50% pore space (with the remainder consisting of minerals and organic matter). The pore spaces in a clay soil are all small, while those in a sandy soil are all large. When one mixes a sandy and a clay soil together, the large pore spaces of the sandy soil are filled with the smaller clay particles. This results in a heavier, denser soil with less total pore space than either the sandy or the clay soil alone. (A good analogy is the manufacture of concrete, which entails mixing sand with cement - a fine particle substance. The results are obvious.) A soil must consist of nearly 50% sand by total volume before it takes on the characteristics of a sandy soil. For most sites, it would be prohibitively expensive to remove half the existing soil and add an equal volume of sand and then till it to the necessary 18-24". Mineral amendments of large particle size, such as perlite, may provide some benefit but can also be costly depending on the size of the site. (Reducing this task to amending only the planting hole is a recipe for plant failure and perhaps will be addressed in a separate column.)

Source: Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University - The Myth of Soil Amendments Part II

If you're going to add organic matter to your clay soil, make sure it's evenly distributed and not just in one area.

The wrong thing to do is to dig planting holes and fill them with organic soil amendment since that creates pockets of soggy ground that fill up with water and rot roots. Also roots will think they are in a container and go around and around inside the amended plant hole and never get out into the surrounding soil.

Source: Never Add Clay to Sand or Sand to Clay

Planting pits are used in Africa in clay soils to catch precipitation but that's a different situation.

And rich planting pits are a different matter again, and are not done in clay soils.

2
  • EXCELLENT answer Graham!!! You put it all together and this should be saved for a grouping called; Gardening Myths Buried or something, grins!! This is a tough one to teach. People think sand equals great drainage and it is just not true. Getting rid of myths will always make gardening easier. THANK YOU!! – stormy Jan 25 '17 at 19:25
  • 1
    I added literally tons of sand to get my clayish garden up to snuff. Lots of organics too. The soil supports happy vegetables now, and no longer forms huge deep fissures if I let it go dry. I think percent-wise I got away with about 30%, but that's going to vary depending on how badly clayed up your garden spot is. This is not a chore for anyone not willing to put in a lot of backbreaking labor. A 1 ton pickup truck is also helpful. – Wayfaring Stranger Feb 18 '17 at 15:18

Not the answer you're looking for? Browse other questions tagged or ask your own question.