"soil" is composed of different fractions. My students do soil
fractionation in one of their labs. The sand and silt drop down out of
water in 30 minutes or so. The clay, when sitting undisturbed, takes up to
48 hours to settle out so it wont settle out of moving water. Clay can be
precipitated out of water with aluminum sulfate.
http://govdocs.aquake.org/cgi/reprin...1/12010320.pdf
Al2(SO4)3 + 6H20 = 2Al(OH)3 + 6H+ + 3SO42-
however, it is acidifying so dont use it unless your water is pretty hard
already. Ingrid