NPK ratios?
J.E. Shields (Sun, 03 Feb 2008 07:23:11 PST)

Hi Gene and all,

My knowledge is even more antiquated than yours, I'm afraid. I graduated
from college in 1956 and have not dealt with inorganic chemistry much since
then. Many of the chemistry books I have on the shelf are almost that old,
and none of them deals with inorganic chemistry.

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.

As I understand it, phosphate can bind to the surface of the matrix of
silicate and aluminate, and I surmise this is probably by forming covalent
binds to them the same way they bind to each other. Phosphate would be too
large to diffuse into the interior of the polymeric matrix, while cations
do so readily. The negatively charged phosphate would not be attracted to
the negatively charge interior of the matrix, but with cations present
could bind to the surface.

My vague recollection is that ammonium ions (NH4+) are bound loosely, and
mainly by lignic acids in soils. It is also an ion exchange process.

I'd like to hear what an inorganic or soil chemist has to say on this
subject! We could see how far of the target I have wandered.

Jim Shields
in central Indiana (USA)

At 10:41 PM 2/2/2008 -0500, you wrote:

Please bear with me -- I'm trying to remember things from my college days,
and that information has had 30+ years to degrade. (Corrections are
appreciated!)
But, if I remember correctly, soil clay particles have a net negative
charge. Certain plant nutrients, such as calcium, potassium, magnesium,
and ammonium, have a positive charge. Thus, these materials would be held
in the soil, bound to clay particles. Nitrate and phosphate, on the other
hand, have a negative charge. If not taken up by plants, they would pass
through the soil, to contaminate groundwater, or cause algal population
explosions in nearby ponds, lakes, and streams. Perhaps this is why high
levels of phosphate are recommended for crops. Ammonium bound in the soil
may not be readily available to plants, but it could be converted to
nitrate by bacterial action, which plants could then use -- a "long term"
source of nitrogen.
I wonder if fertilizer rates recommended for plants grown in regular soil
have much relevance to those of us who grow/garden in "soilless" media.

Gene

Eugene Zielinski
Augusta, GA

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