Posted: 24/07/2012 at 23:04
There's a difference between sharp sand (which is crushed rock; In fact, it is fine grade grit) and natural sand, such as beach sand, which has rounded grains. Beach sand (which is what builders use) should generallly be avoided for horticultural purposes.
If you have clay soil, adding sharp sand (as well as coarser types of grit, too) will improve it. It is not simply a matter of improving drainage, but of improving the actual structure of the soil, (for example, so that it is better aereated, or to allow roots to penetrate more easly.) The surfaces of the grains also change certain kinds of ion exchange which enables plant roots to take in some nutrients more easily. The science of soil is extremely complex and very interesting.
In purely practical terms, and proven by my own experience, if you want to grow carrots and other root vegetables on clay soil, simply add sharp sand. It works.