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VIII. Recent and Future Developments

VIII. Recent and Future Developments

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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.



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.


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

instrumentation available.

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



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

I. Introduction

A. General

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

IV. Conclusions




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,

Voi. 49

Copyright 0 1993 by Academic Press, Inc. AU rights of reproduction in my form reserved.




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

(Mishra, 1980).

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

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