NPK ratios? - now clay minerals
Dr Paul Chapman (Sun, 03 Feb 2008 12:37:23 PST)

Dear All,

It's not often that I organise myself to respond to emails on a reasonable
timescale to make sense to those following a thread, but I've been at home
for the last 2 weeks (very unusual with my job), and am up to date on
receipts.

Jim may be a biochemist, but he has a good memory... my Ph. D. (1986) was on
the breakdown of clay mineral structures as rocks weather into soils. Clay
structures are exactly what Jim describes, and do exactly what he says. The
only question is - how much? This depends on the surface area of the clay,
NOT the surface area just on the outside of the clay particles, but the
surface area of the "regular cavities", as Jim describes them, within the
particles.

Kaolins don't have many of these empty layers, so have a low surface area,
and low cation exchange capacity. Illites and chlorites, which are the most
common clay minerals in temperate northern hemisphere soils, except where
something special like volcanic activity has gone on, have moderate surface
areas. Montmorillonites and bentonites, which are the clay minerals used in
drilling muds and cat litter, have large surface areas, and very large
cation exchange capacity. Typical surface areas (sorry about the metric
units) vary from 10-20 square metres per gram for kaolintes, through 50-100
for illites, up to 600-700 square metres per gram (my guess 200,000 square
feet per ounce?) for montmorillonites.

If you happen to garden on monmorillonite clays, don't waste your money on
potassium fertiliser - you will have spent many thousands of dollars before
the exchange capacity of these clays is taken up, and there is some free
potassium for your plants. However, it is, of course, released as the clay
minerals break down over thousands of years...

Paul

Dr Paul Chapman, Wallington, Surrey, UK
South London commuter belt suburbia - zone 9a, where today I have got the
first flowerings of Crocus angustifolius, paschei and candidus, all from
seed exchange seeds and, I'm delighted to say, all true to name - many
thanks to the donors, whoever they were

----- Original Message -----
From: "J.E. Shields" <jshields@indy.net>

Well, I changed it back and forth a couple of times. I settled on "absorb"
because the kaolins can do an ion exchange, switching Ca++ for K+ in the
internal matrix of the clay. On the other hand, ion exchange phenomena are
probably best thought of as "adsorptions" as well, so (this morning) I tend
to agree with you. I have heard about the "different" clays of the North
American West, but I've not had any personal experience gardening in them.

I think you have it pretty much correct. Clay is an inorganic polymer,
mainly silicate and aluminate forming the extensive covalent polymeric
matrix. This matrix is anionic (i.e., negatively charged) and does
reversibly bind cations (i.e., positively charged ions). There are regular
cavities throughout the polymer, and these are where the cations are
located.

Jim Shields