Agriculture Reference
In-Depth Information
the surface has a defect in it, which is the correct size to allow either or
both ammonium as NH
+
or potassium as K
+
to fit between the layers.
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Also, the unsatisfied bonds resulting from isomorphous substitution are close
to the surface and thus the charge is closer to that of the cations found on the
surface, resulting in a relatively strong attraction for both cations and water.
These two phenomena result in trapping of both water and cations between
crystals.
Water and cations trapped between layers are held so strongly that they do
not exchange with water and cations in the surrounding environment and thus
are not biologically available. For instance, ammonia trapped between the clay
layers is not a source of nitrogen for plants and will not be oxidized by bac-
teria to nitrite and nitrate and thus will not be a source of nitrate or nitrite
pollution.
Soil containing large amounts of fine grained mica clay can result in sur-
prising analytical results. An analytical procedure that breaks apart the fine
mica, such as strong-acid digestion, will show that the soil contains a large
amount of ammonia or potassium, which can cause concern if it is not known
whether the NH
+
and K
+
are unavailable and stable under normal environ-
mental conditions.
The Smectite Clays.
The smectite-type clays are distinctive in that they
expand and cause significant destruction to synthetic (human-made) struc-
tures. In this type of 2 : 1 clay iomorphous substitution occurs in the aluminum
sheet. If there is substitution of a lower-oxidation-state metal such as magne-
sium, there will be an unsatisfied pair of bonding electrons in the interior of
the crystal and there will be no noticeable change in the surface. Because the
charge is in the interior of the crystal, its attraction for cations is diminished
by distance. Thus smectite crystals are not held strongly together by cations
and are able to incorporate more water and ions between layers when the
environment is wet and less when it is dry.
The amount of swelling depends on the cations and the number of waters
(i.e., water molecules) of hydration that they have. Cations with higher
numbers of hydration waters cause a higher degree of swelling. Lithium thus
causes more swelling than does sodium, and so on. The greater the swelling,
the easier it is for water and other ions to move in and out between the clay
layers, making them both environmentally and biologically available.
The amount of swelling and contraction in smectites is quite dramatic. Typ-
ically soils containing large amounts of this type of clay will develop cracks
30 cm wide at the surface and >100 cm deep, and these cracks will allow surface
material to fall into them during dry periods. This characteristic is so unique
that these types of soils are given their own name. They are called
Vertisols
,
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Although it seems strange, NH
+
and K
+
ions are approximately the same size.
