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VIII. Recent and Future Developments
L. A. G. AYLMORE
Figure 23. Three-dimensional image reconstructions obtained by subtractive imaging.
(A) Three highdensity layers in soil core. Note curvature at edges caused by entry of coring
tube into profile. (B) Lupine root in soil column.
CAT STUDIES OF WATER MOVEMENT
uating root material can be clearly defined in a soil column. The distortion
of the soil layers by the passage of the corer is clearly shown in Fig. 23A.
IX. SUMMARY AND CONCLUSIONS
Application of computer-assisted tomography to X- and pray attenuation measurements has provided an exciting new method for nondestructive imaging within a solid matrix, with considerable potential for studying
soil behavior and soil/plant /water relations in space and time. However,
the information provided is currently limited by the capabilities of the
Commercially available medical CT scanners have proved useful for
visual studies of soil structure, the advancement and stability of wetting
fronts, and the structural changes following wetting and drying. However,
the usefulness of these systems and of single-source y CAT scanning systems in studying soil systems is invariably restricted by their inability to
distinguish between changes in water content and bulk density in swelling
and shrinking soils and by the associated physical relocation of soil elements that can occur. Thus their quantitative applications have been
limited to the measurement of water drawdowns in proximity to plant
roots in nonswelling soils and statistical assessments of macroporosity
distributions before and after complete wetting and drying cycles. Though
fast in operation, the quantitative usefulness of X-ray scanners is limited
by the polychromatic nature of the beam and the process known as “beam
hardening.” Furthermore, the proprietary nature of these commercial systems usually makes software modification or extensions impossible.
In view of their substantially lower cost and superior quantitative characteristics, pray tomographic systems are likely to prove ultimately the
most useful for soil and plant studies. Simultaneous measurement of the
spatial distributions of water content and bulk density in soils that exhibit
swelling and dispersion has been shown to be feasible using CAT applied to
pray attenuation. However, the relatively
dual-source (13’Cs and 169Yb)
low photon emission from y sources and the propagation of statistical
errors necessitate large counting times to provide acceptable accuracy and
restrict the use of present y systems to the study of steady-state or only
slowly changing systems. Realization of the full potential of this technique
will require substantial improvements in scanning geometry and counting
electronics to improve the speed and precision of measurements. Incorporation of fan beam geometry together with improved multiple-beam detection systems (MacCuaig et al., 1986) will reduce scanning times by an
L. A. G. AYLMORE
order of magnitude, but will inevitably increase overall instrument costs.
Improved dimensional resolution will enhance structural definition in soil
systems. However, current pixel dimensions of the order of 0.5 to 1 mm
are quite adequate to allow meaningful resolution of many of the controversies associated with water extraction by plant roots. Reduction in scanning times to allow more rapid monitoring of changes in soil water content
would seem a priority for soil and plant studies. Improved image and data
analysis software allowing two- and three-dimensional visualization and
quantitative analysis of scan data will also greatly enhance these activities.
Much of my work in this area was funded by the Australian Research Grants Committee
whose support is gratefully acknowledged. I am grateful to my colleagues in the Soil Physics
Section of the Department of Soil Science and Plant Nutrition, The University of Western
Australia, in particular Mr. R. D. Schuller, Dr. M. A. Hamza, and Dr. V. K. Phogat, for their
helpful comments on the manuscript.
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Isabel0 S. Alcordo and Jack E. Rechcigl
Institute of Food and Agricultural Sciences,
Agricdtural Research and Education Center,
University of Florida,
Ona, Florida 33865
B. World Production and Utilization of Phosphogypsum
C. Physical and Chemical Properties of Phosphogypsum
11. Uses of Phosphogypsum in Agriculture
A. Source of S and Ca for Crops
B. Ameliorant for Aluminum Toxicity and Subsoil Acidity
C. Ameliorant for Sodic Soils
D. Ameliorant for Nonsodic Dispersive Soils, Subsoil Hardpans, and HardSetting Clay Soils
E. Bulk Carrier for Micronutrients and Low-Analysis Fertilizers
111. Environmental Considerations
A. Effects on Surficial Ground Water
B. Effects on Soils
C. Effects on Crop Tissues
D. Effects on Ambient Atmosphere
Gypsum (CaSO,.xH,O) is available for agricultural use either as mined
gypsum or as a chemical by-product. Gypsum by-products are produced
during phosphoric, hydrofluoric, and citric acid manufacture and as a
Adwnrts in Ag~~nmny,
Copyright 0 1993 by Academic Press, Inc. AU rights of reproduction in my form reserved.
I. S. ALCORDO AND J. E. RECHCIGL
result of pollution control systems processes, such as in the neutralization
of waste sulfuric acid and in flue-gas desulfurization. Phosphogypsum is
the term used for the gypsum by-product of wet-acid production of phosphoric acid from rock phosphate. It is essentially hydrated CaSO, with
small proportions of P, F, Si, Fe, Al, several minor elements, heavy metals,
and radionuclides as impurities. Rock phosphate deposits are found
throughout the world, and on these deposits the phosphoric acid industries
are built. Countries with no natural phosphate deposits import the rock to
produce phosphoric acid for their industry and agriculture. Therefore, the
production of by-product phosphogypsum is more widely distributed
around the world than are the natural deposits of rock phosphate. Thus,
among the gypsum by-products, only phosphogypsum is of worldwide
importance in quantity and distribution.
The three basic conventional processes used in wet-acid manufacture of
phosphoric acid are the dihydrate, the hemihydrate, and the hemidihydrate
processes. For each megagram (Mg) of P produced, the hemihydrate process yields about 9.8 Mg of dry phosphogypsum, whereas the dihydrate
and hemidihydrate processes yield about 1 1.2 Mg (Kouloheris, 1980).
Worldwide production of phosphoric acid, estimated at 1 1 million Mg of P
annually (Lin et al., 1990), also results in the production of approximately
125 million Mg of phosphogypsum. With only about 4% of the world's
phosphogypsum production being used in agriculture and in gypsum
board and cement industries, about 120 million Mg of phosphogypsum
accumulates annually; most of this excess is piled in stacks, and some is
stored in abandoned quarries or, in certain countries, dumped into waterways.
Australia produces 940,000 Mg of phosphogypsum annually, of which
200,000 Mg is used as soil conditioners or fertilizers. The rest is stockpiled
on land and in abandoned quarries. The stockpile in 1990 had reached 8
million Mg. Australia discontinued the use of phosphogypsum for making
plaster products in 1983 (Beretka, 1990).
India produces about 2.8 million Mg of phosphogypsum annually, and it
is used primarily as a soil amendment or conditioner for sodic soils
Since 1970 phosphogypsum production in Japan has stabilized at 2.5 to
3.0 million Mg annually, almost all of which is used in the cement, gypsum
board, and plaster industries. The amount of phosphogypsum being used
as fertilizer ranges from 25,000 to 48,000 Mg annually. As a result, Japan
has no stockpile of phosphogypsum (Miyamoto, 1980). Full utilization of