I have typical Seattle clay soil and I want to amend it before planting. I’m re-grading a large (~2000 sq. foot) area and have already added copious amounts of mulch, compost, overturned sod, etc. I’m planning on planting a cover crop of clover this summer, but before doing so thought it’d be a good idea to till in some sand. I was thinking about a level inch over the whole yard before tilling. Is there a particular chemical composition to use or avoid? How about grain size?
You may find this information from Colorado State University Extension on soil amendments useful. Excerpt: Don’t add sand to clay soil–this creates a soil structure similar to concrete.
Professor Linda Chalker-Scott of Washington State University also debunks
the idea of adding sand to improve clay soil.
For good advice on amending the soil, see these links:
Building Fertile Soil and Choosing and Using Cover Crops for the Garden and Orchard from the University of California, Santa Cruz Agroecology Program.
Growing Healthy Soil from Seattle Public Utilities.
It sounds to me as if you have already taken great steps toward improving
your soil, and adding sand not only will not be necessary, but would not
be a good idea.