Agriculture Reference
In-Depth Information
sites on the humus to play a role in nutrient exchange.Ahigh-organic-matter soil
will have a low “effective” exchange capacity at low pH, because many of the
negative exchange sites will be filled with tightly boundAl and Fe.Adding base
cations, especially Calcium, will raise the pH and the Calcium++ ions will displace
theAl and Fe with “exchangeable” Ca.
OK, let's pull this information together. We have discovered that:
1)Alkaline soil nutrients, largely Calcium, Magnesium, Potassium, and Sodium,
are positively charged cations (+) and are held on negatively charged (-) sites on
clay and humus.
2) The amount of humus, and the amount and type of clay, determine how much
Cation Exchange Capacity a given soil has.
3) We have also discussed the ideal base saturation percentages of these
nutrients which according to the work of ProfessorAlbrecht, is approximately:
65% Ca (Calcium)
15% Mg (Magnesium)
4% K (Potassium),
1-3% Na (Sodium)
4) We have talked a little about the effect of those ratios on soil texture and pH
and why they are not hard and fast "rules".
The next step is to understand how the plant, and the soil life, gets those nutrients
from the exchange sites, the "exchange" part of the story.
Trading + for +
In the same way that acid rain can leach cations from the soil, plants and soil
microorganisms more or less "leach" the cation nutrients from their exchange
sites. These alkaline nutrients are only held on the surface with a weak, static
electrical charge, i.e. they are "adsorbed". They are constantly oscillating and
moving a bit, pulled and pushed this way and that by other charged particles (ions)
in the soil solution around them. What the plant roots and soil microorganisms do
is exude or give off Hydrogen ions, H+ ions, and if these H+ ions are in high
enough concentration in the soil solution that some of them surround the nutrient
cation and get closer to the negatively (-) charged exchange site than the nutrient
cation is, the H+ ions will fill the exchange site, neutralize the (- ) charge, and the
nutrient cation will be free of its static bond and can then be taken up by the plant
or microorganism.
The way this works specifically with plant roots and microbes is that they expire or
breathe out carbon dioxide into the soil. This carbon dioxide (CO
2
) combines with
water in the soil and forms carbonic acid (H
2
CO
3
); the H+ Hydrogen ions from the
