Agriculture Reference
In-Depth Information
too high, the soil may lose its beneficial granulation and structure and the
excessive Calcium will interfere with the availability of other nutrients. If you get
them just right for your particular soil, you can drive over the garden and not have
a problem with soil compaction.
Because Calcium tends to loosen soil and Magnesium tightens it, in a heavy clay
soil you may want 70% or even 80% Calcium and 10% Magnesium; in a loose
sandy soil 60% Ca and 20% Mg might be better because it will tighten up the soil
and improve water retention. If together they add to 80%, with about 4%
Potassium and 1-3% Sodium, that leaves 12-15% of the exchange capacity free
for other elements, and an interesting thing happens. 4% or 5% of that CEC will
be filled with other bases such as Copper and Zinc, Iron and Manganese, and the
remainder will be occupied by exchangeable Hydrogen , H+. The pH of the soil
will automatically stabilize at around 6.4 , which is the "perfect soil pH" not only for
organic/biological agriculture, but is also the ideal pH of sap in a healthyplant, and
the pH of saliva and urine in a healthy human.
So we are looking at two new things so far:
1) The Cation Exchange Capacity, and
2) The proportion of those cations in relation to each other: the percent of base
saturation (% base saturation) and their effect on pH.
We are also looking at two old familiar things, clay and soil organic matter, and
these last two need a bit more clarification.
How Clay and Humus Form
Clay particles are really tiny. They are so small that they can't even be seen in
most microscopes. They are so small that when mixed in water they may take
days, weeks, or months to settle out, or they may never settle out and just remain
suspended in the water.Aparticle that remains suspended in water like this,
suspended but not dissolved, is known as a colloid. Organic matter, as it breaks
down, also forms smaller and smaller particles, until it breaksdown as far as it can
go and still be organic matter.At that stage it is called humus , and humus is also
a colloid; when mixed into water humus will not readily settle out or float to the top.
Colloids, because they are so small, have a very large surface area per unit
volume or by weight. Some clays, such as montmorillonite and vermiculite, have a
surface area as high as 800 square meters per gram, over 200,000 square feet
(almost five acres) per ounce! The surface area of fully developed humus is about
the same or even higher. Other clays have a much lower surface area; some clays
actually have a very low exchange capacity, while humus always has a high
exchange capacity.
