>> glacier slurry which has a LOT of small rocks tightly compressed together. Once I break up the soil for planting, I leave those rocks in the bed because I honestly believe they help with the drainage in my clay soil.
I agree with you, but also with Al. If the rest of your mix includes clay or silt in a form that is not tightly bound into small peds or clods, heavy watering CAN wash the clayey or silt into the crevices between rocks, gravel and grit. That would plug the soil up tightly also.
Probably you have amended your clay with enough organic matter that it "holds together" instead of washing out freely and turning the bottom of each bed into clay pudding or concrete, and leaving the upper bed as sandy gravel.
P.S. Another theory of mine is that medium sand has SOME value in loosening clay soil IF there is also almost enoough organic matter and grit or fibers present. Without sand, the clay is too sticky to be broken up finely no matter how you work with tiller, garden fork or broad fork.
When fine sand is present, the clay is more firable, I guess at the boundaries between clay and sand particle. Now you can "fluff it up" - break it up into small clods, or create small channels and voids which MIGHT stay open if other things are OK.
Pure clay is very fluid and sticky. A clod of wet clay can deform and flow. It will flow into gaps between it and the next clod, then stick, so that an open matrix squeezes the air voids OUT and what is left all sticks together in one big blob of glue.
But if the clay has enough organic fibers (like bark or peat), plus grit (like tiny bark nuggets, crush rock and coarse sand), then each clayey clod may have enoguh STIFFNESS that it won't deform and merge with every other clod surrounding it.
I realized that when I had "fluffed up" some clay I had amended with a LOT of grit, to create drainage for bulbs. When I firmed down the mix I had just fluffed up, instead of silently compressing back into clay pudding, it made a gritty crunching sound afetr compressing a little, and then resisted being firmed, while it still had some open structure.
That "crunching" was probably from grit touching grit, and resisting compression directly. But I thought that some of it was from small clay balls with enough sand and grit and bark fibers mixed inside them to
resist deformation. Then their surfaces, with more fine and medium sand grains, crunched against other little clay balls.
But sand and grit won't help clay much
by themselves. They need at least a bare minimum of organic matter mixed with the clay, plus artful mixing, fluffing and firming while not-quite-dry to prevent the classic formula of "clay plus sand plus grit equals concrete". And somehow the organic matter prevents watering from elluviating * the clay and silt particles into the bottom of pot, forming clay pudding / concrete.
That's my theory of clay plus sand plus grit plus ORGANIC MATTER, anyway.
*
http://wiki.answers.com/Q/What...
