Agriculture Reference
In-Depth Information
quantities of amorphous materials in non-Andisols would not go away. Electron microscope exam-
ination of clays consistently showed what appeared to be coatings on all particles (Jones and Uehara,
1973). The coatings were in fact ubiquitous and the problem was not in Ýnding them, but in locating
uncoated mineral surfaces. Uehara and Jones (1974) published electron micrographs of Ýnely
ground quartz, albite, and obsidian, all showing coatings as thick as 400 A
°
,
and speculated on the
role such coatings might play in crust cementation. In the same article they showed coatings on
gibbsite and goethite particles from a highly weathered Oxisol, and guessed that the coating was
mainly alumina. The electron micrographs seem to show the coatings occupying a large fraction
of the soil volume. The reviewers of the paper wondered if the coatings, which we assumed were
amorphous, were artifacts produced during sample preparation.
In Soil Taxonomy (Soil Survey Staff, 1999), acid oxalate extractable Al and Fe are used as
criteria for andic soil properties. This method is used to selectively extract organically complexed
Fe and Al, noncrystalline hydrous oxides of Fe and Al, allophane, and amorphous aluminosilicate
(Wada, 1989). What is needed, however, is not simply a criterion for andic properties, but a method
to quantify and characterize noncrystalline components of soils.
The most recent work by Jones et al. (2000) overcomes many of the uncertainties connected
with earlier effort to study amorphous soil materials. The method depends on spiking a sample
with a known amount of crystalline mineral, and comparing the measured with the added amount.
If the sample is free of amorphous materials, the measured and the added amount of spike would
be the same. If not, the measured amount would exceed the added value, because the x-rays upon
which this method depends cannot detect the amorphous fraction, and in the normalization process,
the ÑmissingÒ fraction is allocated to the spike.
What is remarkable about the Jones et al. (2000) study is the high amorphous mass fractions
measured in the clay fraction of non-Andisols. In eight samples collected from 0Ï15 cm depth of
Ýve Oxisols, two Mollisols, and one Ultisol, they measured mass fraction of amorphous materials
between 29% and 40%. These amorphous contents would be even higher if they were expressed
in terms of volume fraction, owing to higher water content and lower density of amorphous
materials. The average loss on ignition of the crystalline phases for ten samples, including two
Andisols, was 30%, whereas the water and other volatiles associated with the amorphous fraction
was 70% by weight, indicating that the latter has a low speciÝc gravity.
The above raises new questions about amorphous materials in soils. Are the samples studied
by Jones et al. unique because of their volcanic origin? Would the clay fraction of soils developed
from nonvolcanic parent rock also contain signiÝcant amounts of amorphous materials?
We know from experience that quartz, when crushed to near clay-size particles, produces large
quantities of amorphous silica (Uehara and Jones, 1994), and that amorphous silica can be seen
by electron microscopy to be associated with montmorillonite (Uehara and Jones, 1994). But do
soils formed from glass-free parent material produce amorphous coatings? For answers to this
question, samples were obtained from the International Institute of Tropical Agriculture Ýeld station
in Ibadan, Nigeria. The samples were taken from the plow layer of the upper (Oxic Paleustalf) and
middle (Typic Plinthusalf) section of a toposequence in alley cropping plots, established by Dr.
B.T. Kang many years earlier. Clay fractions from the two soil samples were analyzed by R.C.
Jones using the method described by Jones et al. (2000). The amorphous mass fraction for the
AlÝsols was 19.1% for the upper and 15.7% for the middle member of the toposequence. These
numbers are sufÝciently high to make me believe that amorphous materials are more common in
soil clays than is currently believed.
IMPLICATIONS
If amorphous materials occur as coating on mineral surfaces, the chemical properties of the
soil should be dominated by the chemistry of coating. In electron micrographs (see Figure 8.1),
