Seems to me like a job for "Mr. Wizard", that is, an experiment. How
about mixing up two batches that represent your ideals of tilth. Put
one in a 5 gallon drywall bucket with holes punched in the bottom.
Add a measured amount of water to saturate the soil. Place bucket
over something to catch the water that passes through. Measure water
and repeat for the other mixture(s). Make your choice.

It is worth recalling that many clay soils offer better retention of
minerals and other ions in the soil as they tend to bind to the
surface of the clay particles. Our local black clay has the
consistency of heavy bearing grease went wet and concrete when dry.
Mixed with enough (a lot, believe me) of organic matter, it is pretty
good gardening soil. What passes for "loam" hereabouts (Central
Texas) is really just a mixture of red clay and river silt. It is
about as fertile as growing in a pot of marbles in my experience.

On 13 Jun 2005 13:44:09 -0700, "simy1" wrote:

In my new garden I have now replaced the soil of about half the beds
with either manure, composted wood chips, or soil from my other garden
which was itself once 100% compost. The original soil was extreme sand.
Right now it is sandy loam and it will improve further.

I am mostly interested in water retention, because I like these $100
water bills less and less. With the other beds, when I change the soil,
should I go 100% compost or 50% compost-50% clay (which I can pick up
from my neighbor). In other words, is 100% compost always the best for
water, or given that the organic content is high anyway, go with some
clay as well? Obviously, no matter what I do, some sand will always be
in there as the earthworms churn and mix the various components.

In my other garden, by the way, I have seen that when a lump of clay
comes in (mixed with the manure) it does disappear in the soil in a
matter of 1-2 years, so adding clay in a loamy soil can be done.