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?