Agriculture Reference
In-Depth Information
long-term consequences of land use practices. These include, for example, predictions of agronomic
crop performance, the fate and whereabouts of agricultural and industrial chemicals in the vadose
zone, and rheologic behavior of materials for engineering interpretation and use of soils. The irony
is that soil characterization data are primarily used for soil survey and classiÝcation, and are rarely
used as inputs for prediction models. Ideally, soil characterization data should be the same as input
data for dynamic, process-based simulation models. If this were the case, it would result in greater
demand and use of soil surveys and soil characterization data. But soil science is far from being
ready for use as identical parameters for classifying soils and for use as input in prediction models.
Soil texture and clay content are still the most commonly used predictors of soil behavior, but too
often fail when used alone for this purpose. We know, for example, that a very Ýne Vertisol and very
Ýne Oxisol will behave and perform differently. For this reason, the family category of Soil Taxonomy
(Soil Survey Staff, 1999) speciÝes particle size as well as mineralogy for grouping soils into perfor-
mance classes. But there are instances in which samples with nearly identical texture and mineralogy
produce unexpectedly different results in the Ýeld and when analyzed in the laboratory for accessory
properties, such as absorption isotherms, potentiometric titration curves, and zero points of charge.
The failure of particle size and mineralogy to predict soil behavior stems from the fact that
both are surrogates of two other more fundamental soil properties, namely, speciÝc surface and
surface charge density. Although particle size substitutes for speciÝc surface, we expect a very Ýne
kaolinitic sample to have a lower surface area than a very Ýne montmorillonitic sample. Mineralogy
also provides additional information about the sampleÔs surface charge characteristics. This implies
that if speciÝc surface and quantitative mineralogy were used as differentiating criteria in place of
particle size and qualitative mineralogy, more consistent results would follow. I believe there is
still some unÝnished business in soil science that needs to be resolved before better prediction will
be possible. This unÝnished business is the quantitative analysis and characterization of noncrys-
talline or amorphous materials in soils.
QUANTIFYING AMORPHOUS MATERIALS
Amorphous materials are generally thought to be conÝned to Andisols and related soils formed
from volcanic rock. Well-known minerals with short-range-order, such as allophane and imogolite,
are often grouped with, and treated as, amorphous materials. Amorphous materials have properties
normally associated with high speciÝc surface, but do not lend themselves to particle size analysis
or mineralogical analysis. Allophane and imogolite exceed smectites in speciÝc surface, and posses
high water retention capacity. But unlike smectitic soils, allophanic soils are endowed with very
high saturated hydraulic conductives. The surface charge on amorphous materials, including allo-
phane and imogolite, are pH dependent, and the net surface charge can be net negative, net zero,
or net positive, depending on pH and the silica-sequioxide ratio of the material. Characterization
data for a range of materials of this kind can be found in SCS, USDA (1976).
Amorphous materials would not be a cause for concern if they were conÝned to Andisols.
Tenma (1965) applied a method proposed by Hashimoto and Jackson (1965) to measure amorphous
silica and alumina content of Andisols, Oxisols, and Ultisols of Hawaii. HashimotoÔs and JacksonÔs
(1958) method involves boiling samples in 0.5 NaOH for 2.5 min, and was designed to dissolve
amorphous silica and alumina without affecting crystalline minerals. Tenma (1965) performed the
dissolution analysis on the clay fraction separated from deferrated samples previously treated with
H
to remove organic carbon. His results conÝrmed the high amorphous silica and alumina levels
in Andisols, but showed unexpectedly high levels in the Oxisols and Ultisols. The amorphous silica
and alumina content in the clay fraction ranged from 3.9% to 11.6% and 3.2% to 12.7%, respec-
tively, in the Oxisols and 1.3% to 7.4% and zero to 6.7% in the Ultisols. The results of TenmaÔs
(1965) MasterÔs degree thesis were not published because of the uncertainty of the dissolution
procedure to discriminate between amorphous materials and crystalline minerals. But signs of large
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