Agriculture Reference
In-Depth Information
Calcium and Magnesium are often long gone. The northern tier states, from
Washington in the West to Pennsylvania and New York in the East were largely
covered with glaciers asrecently as10,000 years ago, which brought them a fresh
supply of minerals, and clays of high exchange capacity are common.
Organic Matter and Humus
Regarding soil organic matter (SOM) and humus, obviously any area that gets
more rainfall tends to grow more vegetation, so the fraction of the soil that is made
up of decaying organic matter will usually increase with more rainfall. Breakdown
of organic matter is largely dependent on moisture, temperature, and availability
of oxygen.As any of these increase, the organic matter will break down faster.
Moisture and oxygen being equal, colder northern areas will tend to build up more
organic matter in the soil than hotter southern climates, with one extreme being
found in the tropics where organic matter breaks down and disappears very
quickly, and the other extreme being the vast, deep peat beds and "muck" soils of
some North temperate climates.As always, there are exceptions, such as the
everglades of Florida, where lack of oxygen combined with stagnant water have
formed the largest peat beds in the world; the area around Sacramento California
is another example: there were muck (peat) soils 100 feet deep when that river
delta was first farmed by European settlers.
Ordinary organic matter from the compost or manure pile, or the remains of last
years'crops, doesn't have much exchange capacity until it has been broken down
into humus, and from what we know, the formation of humus seems to require the
action of soil microorganisms, earthworms, fungi, and insects. When none of them
can do anything with organic matter as food anymore, it has become a very small
but very complex carbon structure (a colloid) that can hold and release many
times its weight in water and plant nutrients. The higher the humus level of the soil,
the greater the exchange capacity. One way to increase humus in your soil is by
adding organic matter and having healthy soil life to break it down or to add a soil
amendment such as lignite (also known as Leonardite), a type of soft coal that
contains large amounts of humus and humic acids. If the mineral balance of the
soil is optimal, especially with an adequate supply of Sulfur, any fresh organic
matter grown in or added to the soil will tend to form stable humus. Without
balanced minerals and adequate Sulfur, much of the organic matter will
decompose completely and be off-gassed as ammonia and CO2.
Variable Exchange Capacity
Humus can have an exchange capacity greater than even the highest CEC clays,
but it is a variable exchange capacity that correlateswith soil pH. In soils with a pH
below 6 there will be an excess of H+ ions in the soil/water solution and many of
the negative - exchange sites will be occupied by acidic cations such asAl+++
and Fe++. As soil pH increases due to added Ca, Mg, K, and Na, theseAl and
Fe ions will combine with negatively charged OH- ions in the soil-water solution,
forming insolubleAluminum and Iron oxides and freeing up the negatively charged
