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  again vermiculite
From: jhlawrence <jhl at kuentos.guam.net> on 1997.04.21 at 15:08:13(658)
a bit more about the mineral vermiculite

from Brady's "The Nature and Properties of Soil"]
> This abundance of nutrient cations being held in the substrate
> is used by plants to their advantage; it forms a storehouse of
> nutrients, preventing them from being leached into the water
> column. [OR used by the algaes! It is my belief that some aquatic plants
just can't absorb nutrients in solution as well as they can by cation
exchange by their microscopic rootlets. Brady explains that they do this
by releasing H+ ions which dissolve (or bind with the negative nutrient
ions) so that they can be absorbed. - sp][the ability of soil materials
to hold nutrient cations is measured by CEC - sp]
>
> Before briefly discussing what the various components are,
> here are two comments. First, humus, vermiculite and the
> smectites are far superior in CEC to other common soil components.
> Second, laterite (a tropical clay subjected to extensive
> weathering over geologic times scales which is often sold as a
> soil additive for plant tanks) is composed mostly of Fe and Al
> oxides and kaolinite-like clays, which will contribute very
> poorly to the CEC of the substrate.
>
> * Layer silicate clays form small colloids in the soil with a
> layered structure. They have a large external (and frequently
> internal, between-the-layers) surface area. VERMICULITE and
> the SMECTITES are called expanding clays since water and cations
> are allowed to move between the layers, forcing them apart. This
> creates an internal surface area which exceeds the external and
> gives them their large pH-independent CEC's. ILLITE, CHLORITE,
> and KAOLINITE do not expand in this way, resulting in lower
> CEC's. Approximately half of their CEC is pH-dependent (goes to
> zero at low pH). The mechanisms causing the CEC's of the layer
> silicates is described by Brady.
>
> * Fe and Al OXIDES often occur in temperate regions mixed with
> layer silicates, and sometimes dominate in the weathered soils
> of the tropics (e.g. laterites). At high pH they carry a small
> negative charge (less than kaolinite), and at low pH they become
> positively charged and can counteract the CEC of layer silicates.
> The most common are gibbsite (Al2O2.3H2O) and goethite
> (Fe2O3.H2O). [Thus these are poor at holding nutrient cations
> for plants but at low pH they may hold anions]

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