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 

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 

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...


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" <>

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 

Jim Shields

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