Why it is bad to add sand to soil?

Question

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?

Answer 1

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.