Agriculture Reference
In-Depth Information
Figure 8.1
Halloysite particle from an Oxisol coated with a substance that appears to be amorphous. (EM by
R.C. Jones.)
the amorphous fraction appears as a coat of paint on particles. The coatings on adjoining particles
seem to coalesce when they come in contact, and appear to serve as a binding or aggregating agent.
The picture changes when the volume fraction of amorphous materials exceeds 25%.
In Andisols, particles are few and far between. There is no meaning to particle size distribution
or even to speciÝc surface. Amorphous materials behave more like liquids than solids when
moist, but turn into elastic solids when dried. Uehara and JonesÔs (1974) speculation about the
role of amorphous silica on crust formation in arid regions was based on this model. Crusts that
form in irrigation furrows lose their strength when rewetted, and Uehara and Jones (1974)
speculated that high silica amorphous materials tended to soften and harden with wetting and
drying.
On the other extreme, we have low silica, high iron, and aluminum materials, such as in plithite,
that dry irreversibly. If we return to the earlier example of the very Ýne Oxisol and Vertisol, we
can visualize not only particle size and mineralogical differences, but differences in soil structure
and the resistance of structural units to the slaking action of water. What causes the self-mulching
behavior of Vertisols, and why do soil aggregates of Oxisols so often resist slaking in water? It is
easy to explain these characteristics in terms of the swelling or nonswelling properties of clay
materials, but what cements the particles in a dry, self-mulching aggregate of a Vertisol, or
water-resistant aggregate of an Oxisol? Are they the clay minerals themselves that act as cementing
agents, or does a ubiquitous amorphous fraction that varies in composition and reaction with water
play some role in soil cementation? Many more questions come to mind when one views surface
coating on soil particles, as shown in Figures 8.2 to 8.5.
