Let me say right up front here that I believe that 40 percent clay and
60 percent organic matter is a GREAT soil. I'm not totally anti clay
soil at all. In fact, I submit that clay plus organic matter is highly
superior to sand plus organic matter. (Just to set the record
straight) Just ask Dave Amorde about my opinion on this...g

Cass commented:

It isn't really clay that locks up nutrient - or more accurately,
inhibits their availability for absorption. It's soil pH. It is true
that unless soil pH is balanced, nutrient become unavailable
chemically.

You are correct that it is pH, but that is only part of the story. The
clay actually does play a role.

Here is more than you wanted to know, but the actual role of clay
minerals in the process:

Clay minerals are products of the weathering of rock minerals such as
feldspars and other silicate minerals. They play an important role
both physically and chemically in soils and sediments. Physically,
because of their small particle size and flatness, (which leads to
easy compaction and paucity of drainage) and chemically because of
their ion absorptive abilities.

Clay minerals are aluminosilicates in that they most often contain
aluminium, silicon and oxygen atoms. Most clay minerals occur in platy
sheets which leads them to have large surface areas. In all clay
minerals except kaolinite a process known as ISOMORPHOUS
SUBSTITUTION takes place.

This process occurs when a tetravalent Silicon atom is replaced by a
trivalent Aluminum ion and when a divalent Magnesium atom replaces
Aliminum in the mineral lattice. The net result of the substitution is
to create a negative charge on the clay surface. (Negative charge
equals high pH).

Clay minerals are well known for their absorptive qualities in
relation to cations (positively charged ions) such as Iron, Magnesium
or Aluminum. When in the presence of liquid solution (when the soil
is wet and the iron molecules dissolved) the iron ions can attach
themselves into the actual mineral lattice of clay minerals.

The clay structure actually being the very place of chemical 'lock
up'.

Clay minerals are not the only culprit in Iron depletion of course.
The presence of alkaline carbonates such as calcite and other 'salts'
can react with the free iron as well.

Increasing the 'acidity' of the soil by adding things like organic
matter can neutralize the effect of the clay's chemical 'needs'.

I thought the pH of clay soils varies widely regionally. In the SE,
they say clay is acidic. In the west, they say clay is alkaline. In
fact, people with clay soils in the SE add lime to keep it from being
too acidic.

There are certain clays that are 'acidic'. These are chemically
weathered clays that have been changed by being in oxidation
environments for long periods of time. This is especially true in
humid tropical or subtropical areas (that are highly oxidizing
environments).

The majority of clays however are alkaline.

.... But if you live in flat clay, you need to create
drainage, and raised beds will do it for you. Replacing clay with top
soil will only create a pot and can actually create a drain into your
rose hole.

Don't forget that no one is talking about 100 percent potting clay
here. All of the soils we're talking about will drain to some degree.
Your above statement only makes sense if there is no drainage in the
soil at all.

What roses have you killed lately, anyway? I never hear about yer roses
dying. Fess up. ;~)

Ain't killed any for the last 2 years...... That's my story and I'm
stickin' to it....... heh heh



Bob Bauer
(The only reason I know anything about clay is because I used to be a
Geologist.)