" wrote in
message oups.com...
I wonder if anyone has an opinion about the water retention of sand
when mixed with the general purpose compost you buy in a garden
centre. My experience is that, added at 25%, the mix actually retains
more water than the compost alone. Maybe this is partly due to the
sand breaking down the compost into fine and evenly distributed
particles or something.
Any thoughts welcome

It matters not what the actual % of water is in that mix. Of more importance
is the % of air that is in the root region. Sharp sand
/vermiculite/Perlite/wood chips will give a better aeration due to particle
size.
Your original calculations also need to compare water retention over a fixed
time and relate to w/w and take account of bulk density.