Re: Which soil is less permeable, sand or clay?

Sand is much more permeable than clay.  That means that both water and air 
can move faster through sandy soils than clay soils. In fact, some types of 
clays are used as waterproof liners on the bottom of ponds or beneath 
landfills to prevent water loss.  There are three reasons for this:
1. Clay particles are smaller than sand so they can be packed more tightly 
together. The spaces through which water can flow are thus smaller in clay 
so it is simply a matter of squeezing water through tinier holes.
2.  There is more surface area on all the particles in a clay soil than on 
all the particles in a sandy soil.  Water likes to stick to many surfaces 
and thus clay is stickier than sand.  The water is held more tightly to the 
surface.
3. Finally, some types of clay actually swell up when they are wet, 
clogging the already tiny pores.

As to the humus, yes one would expect different humus in a desert than in a 
 tropical forest.  Two reasons:  different types of organic matter to begin 
with and different rates and evironment for organic matter decay