Agriculture Reference
In-Depth Information
The Technical Definition:
pH is a logarithmic measure of hydrogen ion
concentration, originally defined by Danish biochemist Søren Peter Lauritz
Sørensen in 1909.
pH = -log[H+]
where log is a base-10 logarithm and [H+] is the concentration of hydrogen ions in
moles per liter of solution.According to the Compact Oxford English Dictionary, the
"p" stands for the German word for "power", potenz, so pH is an abbreviation for
"power of hydrogen". In Sørensen 's original paper, pH is written as P
H
. The modern
notation "pH" was first adopted in 1920 by W. M. Clark for typographical
convenience.
How pH works in the soil
Whether a soil is acid or alkaline depends largely on rainfall and temperature.All
rain and snow is acidic, below pH 7, if only because it has CO
2
gas dissolved in it.
CO
2
gas dissolved in water, H
2
0, forms carbonic acid, H
2
CO
3
, which quickly
dissociates to HCO
3
- and H+. If the soil is alkaline, i.e. it has an excess of OH-
ions, these will be neutralized by the free H+ ions from the carbonic acid in the
rainfall.As the rain continues to fall on an alkaline soil, more and more OH- ions
will combine with H+ to form stable H
2
0 and the pH (the ratio of OH- to H+ ions in
solution) will drop. When there are no more OH- ions readily available for the H+
to combine with, the H+ ions from precipitation will begin to be in excess and will
be drawn to and start reacting with carbonate rocks or any other alkaline material
in the soil.
The reaction with Calcium carbonate limestone CaCO
3
in the soil goes like this:
CaCO
3
(solid) + CO
2
(gas) + 2H
2
O(liquid) --> Ca++ (aqueous) + 2 HCO
3
-
(aqueous). (Aqueous = dissolved in water.)
The newly-soluble Ca++ can be attracted to and held on negative exchange sites
on clay or humus, or it may be washed away to a lower soil horizon, depending on
whether there are available negative exchange sites to adsorb it, and on rainfall
and evaporation. In soils that have more rainfall than evaporation, eventually most
of the alkaline rocks will have their base cations extracted into the soil solution by
reacting with carbonic acid (or other acids in precipitation such as sulfuric acid
H2SO4). If the soil has abundant negative exchange sites available, many of the
soluble basecations will be held on those sites, keeping them from leaching away,
and where they remain readily available as nutrients to plants and soil organisms.
If the soil has a low CEC there will be little charge to hold the soluble cations and
they will quickly be washed away and lost.
At the point where most of the alkaline rocks have had their base cations
extracted, or where the amount of alkaline elements available balances the acidity
of the precipitation, the soil reaches equilibrium of + to - charges and a pH of 7.
As long as precipitation exceeds evaporation the acidification and leaching
process continues. The excess of H+ ions in the soil will increase and they will
