Journal of Environmental Quality 32:363-367 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
SHORT COMMUNICATION
Visualizing Bromide and Iodide Water Tracer in Soil Profiles by Spray Methods
Jianhang Lu and
Laosheng Wu*
Department of Environmental Sciences, Univ. of California, Riverside, CA 92521
* Corresponding author (laowu{at}mail.ucr.edu)
Received for publication January 14, 2002.
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ABSTRACT
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In this study we developed and tested a spray method to visualize bromide water tracer in soil profiles. The method is based on the transformation reaction of a white precipitate into a colored one (Prussian blue) in the presence of Br-. After application of water containing bromide (0.2–0.4% wt.), a soil profile is dug out from the irrigated area and sprayed with a Br- indication suspension containing ferric ion and silver ferrocyanide precipitate. About two hours later, the pattern of irrigation water movement in the soil profile appears due to the formation of Prussian blue complex. We describe the method and demonstrate its use in a field experiment to visualize water flow paths. Since this method might be subject to possible interference from Cl-, a newly designed method with iodide ion as a water tracer and its indication solution containing soluble starch and ferric ion is also presented and recommended for use in soils with high chloride background.
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INTRODUCTION
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TRACERS ARE OFTEN USED to study water and solute transport behavior in the environment. Conventional techniques for measuring tracer concentrations, such as sampling soil cores or soil solutions, have a rather poor spatial resolution and are likely to miss the narrow flow patterns entirely (Ghodrati and Jury, 1990). To observe on-site flow pathways under natural conditions, many techniques have been developed, such as dye tracing (Corey, 1968; Schwartz et al., 1999), fluorescence imaging (Aeby et al., 2001), ground penetrating radar (Vellidis et al., 1990; Freeland et al., 1997), nuclear magnetic resonance imaging (Liu et al., 1994; Posadas et al., 1996), time domain reflectometry (Vanclooster et al., 1995; Nissen et al., 1999), radio scanning (Brown et al., 1999), electrical resistivity tomography (Slater et al., 1997), and single photon emission computed tomography (Perret et al., 2000). Of all the methods employed, dye tracing is the most popular (Corey, 1968), but it can only mimic the movement of adsorptive organic matter, such as some pesticides and herbicides. For tracing water itself, dyes are unsuitable as they move slower than water (Flury and Fluehler, 1995; Perillo et al., 1998). Other techniques are inconvenient for field use due to the need for complicated devices. Moreover, these methods use fluorescent dyes, radioactive materials, high concentration salts, or other environment-unfriendly materials. For the most ideal and frequently used bromide tracer, which can faithfully represent the movement of water (Flury and Papritz 1993), no on-site observation method has been reported up to now.
Compared with the above-mentioned methods, spray technique is a convenient and economical way for observing preferential flow in a soil profile. The technique uses a color change reaction that is triggered by the tracer in applied water. After a soil profile is sprayed with an indication solution, the flow pattern is revealed by the color change map and can be easily photographed by regular film or digital camera. Tamm and Troedsson (1957) were probably the first to employ a spray method to observe water movement in soil. They used thiocyanate ion as a water tracer and triggered a red-color reaction by spraying a ferric chloride solution on the soil profile. Unfortunately, the method is not practical since the thiocyanate ion is highly toxic (Bhunia et al., 2000) and subjected to quick chemical and/or biological transformation (Bowman, 1984). Another spray method was designed by Van Ommen et al. (1988). They used iodide ion as water tracer, and applied powdered starch on the excavated horizontal soil profile, which was followed by spraying a bleaching-liquor (Cl2) to form iodide–starch blue complex. Recently, Wang et al. (2002) reported a simple technique using ammonium carbonate as a water tracer (a buffer solution to increase soil pH) and spray with a pH indicator. Though accompanied by disadvantages, such as toxicity of the tracer and chemicals used, high tracer concentration, and instability of formed color, these methods are able to show flow patterns under field conditions.
In this paper, we present a new spray method to visualize bromide ion on a soil profile, which will be of great significance because bromide is the most widely used soil water tracer and is often used as a reference for other tracers (Bowman, 1984). We also report a modified iodide-starch method that may serve as an alternative for the bromide-spray method for soils with high chloride background.
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Mechanism for Visualizing Bromide Tracer
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In devising a practical spray method, it is essential to find an appropriate color development reaction that can be initiated by the water tracer. The reaction should be sensitive and invulnerable to interferences from soil solution, and the formed color should be highly visible against the background of wet or dark soil. We tested several color development reactions for Br-, such as the formation of bromophenol blue (American Public Health Association, 1998) and the formation of Fe 
x red complex by the precipitate transformation of AgSCN to AgBr. However, all these methods failed because the formed color was not intense enough or because the necessary reagents could not be mixed in one indication solution. Finally, we found that the formation of Prussian blue complex derived by the precipitate transformation of Ag4Fe(CN)6 into AgBr was suitable to indicate Br- tracer in soil.
In solution, silver bromide is considerably less soluble than silver ferrocyanide. In the presence of Br- and excess Fe3+, the following precipitate transformation reaction can occur readily:
Prussian blue complex has high staining strength and its brilliant blue color is highly visible against the background of wet or dark soil. The equilibrium constant for this reaction is 1.03 x 109, based on the Ksp of AgBr (3.5 x 10-13) and Ag4Fe(CN)6 (1.55 x 10-39) (Sillen and Martell, 1964). Silver ferrocyanide is a pale-white fine precipitate and can be easily prepared by mixing the solution of AgNO3 and K4Fe(CN)6:
and Fe(NO3)3 is added to the formed suspension as a source of Fe3+ ion.
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Mechanism for Visualizing Iodide Tracer
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Iodide ion is considered to have similar transport properties as Br- because of their chemical similarity (Bowman, 1984). With the unique reaction between I2 and starch, I- ion is easy to visualize. In the original method developed by Van Ommen et al. (1988), the starch layer and bleaching-liquor mist must be carefully applied, since excess Cl2 will bleach the blue-purple color of starch–iodide complex. Moreover, when soil is relatively dry, for example, after an extensive period of soil water redistribution, the starch layer is difficult to wet with iodide solution in the soil. Hence, we suggested using Fe3+ as a mild oxidizer for I-, which will not bleach the formed starch–iodide complex color. In addition, we dissolved starch in the indication solution rather than applying starch powder directly to the soil profile to avoid the wetting process of starch layer by I- solution in soil. After these modifications, the concentration of iodide tracer solution was considerably lowered, from an original of 5 to 15 g L-1 down to 2 to 3 g L-1. Its indication solution consists of 50 g L-1 starch and 0.05 M Fe(NO3)3. The application method is exactly the same as the bromide method. After spraying, it takes about one hour for the color to develop. The color is dark blue-violet.
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Materials and Methods
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Preparation of Bromide Indication Suspension
To prepare 1.0 L bromide indication suspension, slowly pour a solution of 8.45 g K4Fe(CN)6 in 0.4 L deionized water into a solution of 13.59 g AgNO3 in 0.6 L deionized water accompanied by vigorous magnetic stirring. A fine white precipitate of Ag4Fe(CN)6 will form immediately. Keep stirring for 10 min and then allow the Ag4Fe(CN)6 precipitate to settle down. Pour out about 0.5 L of the supernatant. In another beaker, dissolve 24.24 g Fe(NO3)3·9H2O and 0.51 g AgNO3 in 0.5 L of deionized water. The small amount of AgNO3 is incorporated to suppress the excess pre-formation of Prussian blue complex in the final indication suspension. Finally, combine the silver ferrocyanide suspension and the Fe3+ solution by mixing them together. The white silver ferrocyanide suspension will change its color to a slightly grayish blue due to pre-formation of a small amount of Prussian blue complex. The slightly blue color indicates the depletion of Ag+ in the indication suspension, which guarantees the sensitivity of spray solution to Br-. It does not affect the visibility of Prussian blue complex against wet soil background. However, since the slightly blue color will slowly intensify during storage, we suggest storing the silver ferrocyanide suspension and Fe3+ solution separately (in brown glass bottles), keeping them in the dark, and mixing them no longer than 5 h before use. The color of Br- indication suspension can be adjusted by stirring and by drop addition of 0.10 M AgNO3 or 0.025 M K4Fe(CN)6 solution.
Preparation of Iodide Indication Solution
To prepare 1.0 L iodide indication solution, weigh 50 g water-soluble linear starch, wet it with 0.05 L deionized water, and then add to 0.8 L boiling deionized water. Keep stirring until the solution becomes clear. Cool the solution to room temperature, add 20.20 g Fe(NO3)3·9H2O, stir to dissolve, and bring the volume to 1.0 L. Store the solution in a brown glass bottle. It will be stable for at least 30 d.
Spray Technique
To trace water movement, apply water containing bromide or iodide tracer on the soil in the area of concern. After infiltration and redistribution processes cease, excavate a vertical or horizontal soil surface. Smooth the profile with a flat shovel, horizon by horizon from top to bottom. Be careful not to artificially cross-contaminate the surface. Add indication solution to a sprayer. A pressurized sprayer that can release a continuous and fine mist of indication solution is recommended. Spray the indication solution onto the surface as uniformly as possible by long slow strokes. For bromide tracer, shake the sprayer to get uniform suspension before spraying and keep spraying until a vague light-blue film of Ag4[Fe(CN)6] appears on the soil profile. The color change will occur gradually after spraying. Allow 2 to 3 h for color development in the bromide method and 1 to 2 h in the iodide method to get the high color contrast. In a windy, dry climate or under direct sunshine, cover the profile with a plastic sheet to minimize surface evaporation. Once formed, both of the colors are stable even as the soil becomes dry.
Field Application
This experiment was conducted in the Coachella Valley Field Station, University of California on 19 and 20 June 2001. The soil was a coarse loamy sand (mixed, hyperthermic Typic Torrifluvents). Initial moisture content was 1.3% (gravimetric percentage) at 0.2 m and 4.6% at 0.5 m depth. The experimental setup consisted of two nearby sites, which have the same size of 1.0 x 1.5 m. With a hand garden sprayer, we irrigated one plot with a KBr solution containing 30 g L-1 Br- and the other with a KI solution containing 20 g L-1 I-. The total application was 0.1 m, and the application rate was about 3 cm h-1. Temporary ponding appeared during application in both plots.
Vertical soil profiles were excavated across the plots one day after the infiltration. The profiles were shaved until the surface was smooth. The prepared bromide or iodide indication solution was then sprayed onto the smoothed profile surface. Precautions were taken to avoid cross-contamination on the profile surface during profile excavation and shaving. Color patterns of water flow paths gradually appeared on the wet soil background, and were photographed with a Hewlett-Packard (Palo Alto, CA) PhotoSmart 618 digital camera.
Column Experiment
Three soils were used: a dark silt loam (Whitman, WA), a dark-gray clay loam (Irvine, CA), and a silty clay (Imperial, CA). The first two soils have high organic matter content (39.2 and 31.6 g kg-1, respectively) and were selected to represent dark soils. The third soil has a very fine structure (clay and silt content more than 90%) and high salt content (electrical conductivity in saturated extract is 8.86 dS m-1). Approximately 1.3 kg of soil were packed in a plastic column (5.2-cm i.d.) to a bulk density of about 1.40 Mg m-3. The soil length was about 0.43 m. To achieve relatively uniform initial moisture content, 0.2 L of tap water was added to the top of each column, and the columns were left one week for water redistribution. After that, 0.05 L of 3 g L-1 bromide-containing water was applied to the top of each column (ponded application). Eight hours later, the columns were vertically cut in half at the middle. The soil surface was smoothed with a spatula and sprayed with bromide indication suspension for color development.
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Results and Discussion
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Field Results
The pictures of the same area on the soil profiles before and after spraying are shown in Fig. 1
. Before spraying, the water flow pattern was invisible both in the bromide site and iodide site, because the soil was originally wet (Fig. 1a,c). One to two hours after spraying, the flow pattern was clearly displayed by the formed Prussian blue color in the bromide site (Fig. 1b) and dark blue-violet color in the iodide site (Fig. 1d). Water penetrated about 0.5 m beneath the soil surface after a 24-hour redistribution and a fingered-flow pattern occurred in both plots. A detailed wetting front was clearly discernible, which indicates that these techniques are capable of capturing the flow heterogeneity under natural field conditions. Interestingly, the finger sizes of preferential flows were quite different in those two adjacent sites, possibly because the iodide site was much more layer-structured than the bromide site.
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Fig. 1. Wetting front patterns in a coarse loamy sand soil (24 h after the end of water application): (a) bromide tracer, before spray; (b) bromide tracer, 2 h after spray; (c) iodide tracer, before spray; (d) iodide tracer, 1 h after spray. The patterns are invisible on wet soil background in (a) and (c). After spraying, they are visualized by the Prussian blue color (with bromide as tracer) in (b) and dark blue-violet color (with iodide as tracer) in (d). The scale on the right-hand side is in 10-cm increments.
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When the tracer indication solution is evenly sprayed onto the soil profile, the color intensity of the flow-pattern map is related to the tracer concentration in the soil. Thus, based on the photographed color images, semiquantitative information on the bromide–iodide tracer distribution can be obtained, which represents the movement of water in the soil. Since the color only stained the surface of soil, the soil beneath the stained layer is essentially unaltered, thus by combination of multilayer profiles, the three-dimensional spatial distribution map of water in soils can also be achieved.
Applicability of the Bromide and Iodide Spray Method
A major concern regarding spray methods is the visibility of the formed color against the background of dark or wet soil. The iodide method works well in light-colored soils, but is not suitable for dark-colored soils because its dark blue-violet color is not distinguishable from the background of wet deep-color soils. However, the bromide method still works, even for deep-color soils because of the high visibility of the brilliant Prussian blue complex. In the columns packed with black or black-gray colored soils (the left two columns in Fig. 2)
, one can easily see every detail of the wetting fronts of applied water. Moreover, the light-blue film of Ag4[Fe(CN)6] sprayed on the soil surface somewhat masks the deep color of wet soils and increases the contrast of the blue color formed against the soil background.
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Fig. 2. The brilliant blue color from the bromide spray method is highly visible against the background of wet deep-color soils. Soils are a silt loam (left), a clay loam (middle), and a loam clay (right). The pictures show the wetting fronts 8 h after water application.
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Nevertheless, Cl- ion, which is common in soils and irrigation waters, may cause interference in the bromide method. The transformation reaction of silver ferrocyanide to silver chloride:
has an equilibrium constant of 0.075, based on the Ksp of AgCl (1.2 x 10-10) and Ag4Fe(CN)6 (1.55 x 10-39). In solution, the color change is discernible when the concentration of Cl- is higher than 0.2 g L-1. Since chloride in soil extracts may range from a few to several hundred ppm (Adriano and Doner, 1982), high Cl-–containing soils may cause a blank reaction. To test the possible interference, we sprayed Br- indication suspension onto 16 soil samples collected from different areas. The Cl- concentration in these soil samples ranged from 0.020 g L-1 to 0.228 g L-1 (measured in saturation extracts). However, no blank reaction was observed. The bromide method works well even on the silty clay soil (the right column in Fig. 2), which has the highest Cl- content. Possibly this is because the concentrations of Cl- in the soils are not high enough to cause interference or because of the slow diffusion of Cl- to the exposed soil surfaces. Moreover, it is easy to test the applicability of this method to high Cl-–containing soils by simply spraying the bromide indication solution to the target soil to see if color change will occur. In the circumstance of Cl- interference, the iodide method could be used as an alternative.
Sensitivity of the Methods and Application Rate of Tracers
The sensitivity of a spray method depends on the contrast between the formed blue color and the soil background. However, the sensitivity is difficult to define since it varies with different observers or the image recording devices and the image-processing technology. Moreover, in a practical field use, many factors such as soil background color, soil surface texture, and the initial soil water moisture can also affect the visibility of formed color. In other words, besides the chemical properties of the color reaction, the sensitivity of a spray method is also related to soil characteristics and its water regime.
In solution, a well-prepared Br- indication suspension has a good sensitivity to Br-. When mixed with the Br- indication solution at a volume ratio of 1:1, a Br- concentration of 0.01 g L-1 is high enough to be discernible with the naked eye. For homogenized soil samples pre-spiked with increasing amounts of Br-, the blue color is visible in most soil samples when Br- content is above 0.06 g kg-1, and it is highly visible when Br- concentration is above 0.16 g kg-1. For light-color soils, such as a Hanford fine sandy loam (Fresno, CA), the blue color occurs when the Br- content is above 0.02 g kg-1.
The lowest concentration of Br- water tracer in application water depends on the sensitivity of its indication suspension, soil characteristics, and initial water content. Generally speaking, to employ the bromide method, a Br- concentration above 0.16 g kg-1 (in the soil where the water flow goes) is expected to ensure the good visibility of formed blue color. Our laboratory column studies and field application experiences showed that for originally dry soils, since Br- does not adsorb to negatively charged soil minerals, and its concentration does not decrease as water advances, a Br- application rate as low as 1 to 2 g L-1 is high enough to display the color. For originally wet soils, appropriate Br- application rates are 2 to 3 g L-1 for clay and loamy soil, and 3 to 5 g L-1 for sandy soil. Higher tracer concentration is required in sandy soil because it holds less water than fine-textured soils, especially long after water redistribution. Moreover, if initial soil water content is high (more than 15% in fine-textured soil or 8% in sandy soil), a higher concentration of Br- tracer (e.g., 4–6 g L-1) may be applied to ensure good visibility of the water-flow pattern.
For iodide tracer, an application rate of 2 to 3 g L-1 in fine-textured soil and 3 to 4 g L-1 in coarse soil will give satisfactory results. Since I- ion has a low oxidation potential, its concentration may decrease rapidly under aerobic field conditions (Bowman, 1984). Higher iodide tracer concentration should be employed if the water-flow patterns will be observed several days later after water application.
Toxicity of the Chemicals Used
The reagent silver nitrate is poisonous, but becomes nontoxic after it is mixed with K4Fe(CN)6 to form the indication suspension. Ferrocyanide anion is kinetically inert and is considered to be of very low toxicity (Pearce, 1994; Shifrin et al., 1996). Prussian blue is widely used in the manufacture of inks, pigments, and drugs. Ferric ion is corrosive to metal or hand. Direct skin contact with the indication solutions should be avoided. Since the tracer indication solution only stains the surface of soil profile, its residue in the environment is small and easy to remove. Inasmuch as the stained soil may release some unreacted Ag+ or other chemicals to beneath soil and ground water, we suggest that the stained surface of the soil profile be retrieved and disposed of properly.
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Conclusions
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Bromide is the most widely used tracer in soil and hydrological studies because of its low background concentration and zero retardation in soils. Even an extensive soil sampling scheme cannot show all the features of the wetting front patterns as completely as the spray method. The methods presented in this study are promising due to their simplicity, wide applicability, very small residue in the environment, and low cost for large-scale field application. They provide a direct means to observe water-flow paths under natural conditions and help to gain comprehensive insights into water transport mechanisms in soil.
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ABSTRACT
INTRODUCTION
