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Published online 19 April 2006
Published in Soil Sci Soc Am J 70:960-966 (2006)
DOI: 10.2136/sssaj2005.0296
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Nutrient Management & Soil & Plant Analysis

Density Changes around Phosphorus Granules and Fluid Bands in a Calcareous Soil

Ganga M. Hettiarachchia,*, Enzo Lombib, Mike J. McLaughlina,b, David Chittleborougha and Peter Selfc

a Soil and Land Systems, School of Earth and Environmental Sciences, The Univ. of Adelaide, Waite Campus, Urrbrae, SA 5064, Australia
b CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia
c Adelaide Microscopy, The Univ. of Adelaide, Adelaide, SA 5005, Australia

* Corresponding author (ganga.hettiarachchi{at}adelaide.edu.au)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We employed x-ray computed microtomography (X-ray CT) to observe differences in moisture around fertilizer P granules (monoammonium phosphate, MAP) versus injection zones of fluid P fertilizer (technical grade monoammonium phosphate, TG MAP) in a calcareous soil over time. X-ray CT allows nondestructive visualization of small columns containing soils and fertilizers. We were able to visualize the increase in density around the highly hygroscopic fertilizer granule over time. It appeared that both water flow toward the granule and precipitation of P could be responsible for the development of about 1 mm thick high density zone immediately adjacent to the granule. The mass flow of water toward the granule may have slowed or restricted the diffusion of fertilizer P from the granule, thus increasing the chances for P fixation through precipitation reactions. Also, the granule became less dense with time indicating the progress of granule dissolution. In contrast, injection of fluid fertilizer (TG-MAP) in soil did not result in moisture changes over time as evidenced by a lack of X-ray CT detectable density differences in the soil column. These data support previous findings that, when P is supplied in granular form, P diffusion and isotopic lability in calcareous soils are reduced compared with equivalent liquid fertilizer formulations, probably due to precipitation reactions induced by osmotically induced flow of soil moisture into the fertilizer granule.

Abbreviations: BSE, backscattered electron • MAP, monoammonium phosphate • SEM, scanning electron microscopy • TG-MAP, technical grade monoammonium phosphate • X-ray CT, x-ray computed microtomography


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE MAJOR REASON that supply of phosphorus (P) to cultivated plants is a common limiting factor for crop production is the low availability of P to plant roots rather than low P content in soils. Although farmers have used large amounts of P fertilizers to improve crop growth over many decades, only a small fraction of the applied P as fertilizers are taken up by plants in the year of application (McLaughlin et al., 1988) while the residual effectiveness of P fertilizer for subsequent crops declines with time (Barrow, 1973). This is especially true for high P-fixing soils such as highly calcareous or strongly weathered acidic soils. Calcareous soils are widespread around the world, especially in areas with arid and Mediterranean climates. It is estimated that over 800 million hectares of calcareous soils are cultivated worldwide (FAO, 2000).

Recent research conducted by Holloway et al. (2001) showed that, in calcareous soils of southern Australia, liquid TG-MAP was 4 to 15 times more effective than granular MAP in increasing the grain yield of wheat. This was further confirmed with a pot trial where liquid and granular fertilizers were homogeneously mixed in the soils to minimize the placement differences evident under field application of these fertilizer types. Results suggested that both chemical and physical factors in combination cause fluid fertilizers to be more efficient than similar granular products. In previous investigations (Lombi et al., 2004, 2005) we showed that when granular products are used in calcareous soil, P solubility, diffusion, and lability are lower than when a fluid P source is used. However, the specific mechanisms responsible for the differential response observed are still poorly understood.

Several reactions occur when a highly water-soluble fertilizer granule is placed in soil, and these were recently reviewed by Hedley and McLaughlin (2005). The first reaction is wetting of the granule, which is predominantly by capillary flow of water from soil into the porous granule, and by water vapor transfer from the soil or atmosphere to the hygroscopic phosphate salt (Lawton and Vomocil, 1954; Williams, 1969). This movement of water occurs in a direction opposite to that of dissolved P diffusion and hence may slow or restrict the diffusion of P, thus increasing the chances for P fixation due to precipitation reactions. Benbi and Gilkes (1987) identified two reaction zones in soil adjacent to fertilizer granules: a P saturated zone immediately adjacent to the granule, where the P sorption capacity of the soil is exceeded and precipitates of P form with the metal ions and organic matter released from the soil due to low pH and high salt concentrations; and a P unsaturated zone, where the P sorption capacity of the soil is not exceeded. In highly calcareous soils a number of solid-phase Ca-phosphate forms precipitate in the zone adjacent to the granule (reviewed by Sample et al., 1980). Movement of dissolved Ca and other cations with mass flow of water toward and into the granule may enhance formation of P precipitates in calcareous soils. In the case of fluid fertilizers, our hypothesis is that while the fertilizer solution has a high ionic strength (high osmotic pressure), the osmotic effect of the fluid P addition will not be as antagonistic or detrimental to the rate of outward movement of P as it was for the hygroscopic/capillary/osmotic effects of the granule P due to the amount of water added with fluid P, and hence explain the high efficiency of fluid fertilizers observed in calcareous soils.

Previous observations of soil moisture movement toward fertilizers are based on destructive "bulk" methods and are limited to granular fertilizers (Lawton and Vomocil, 1954). Computed tomography measurements have been correlated with a variety of soil properties such as bulk density (Anderson et al., 1988), porosity (Phogat and Aylmore, 1989; Grevers et al., 1989), and water content (Phogat et al., 1991; Hopmans et al., 1992). Those studies have shown that low density areas are pores, air or water and higher density areas are solid particles. Moreover, Rogasik et al. (1999) suggested that dual energy scanning is needed to relate changes in soil water with respect to solid and air filled space. It rose from the fact that with the single energy-level scanning, it is only possible to calculate soil physical properties in samples at microscale level if soil volume is composed of only two phases. However, this can be overcome by scanning the soil samples in a 3-phase composition twice at a single energy-level, first the under present moisture level and second, after complete drying or saturating to a 2-phase composition, assuming that variation in water content do not cause structural changes (Hainsworth and Aylmore, 1983; Grose et al., 1996).

We used an X-ray CT technique to visualize over time the differences in apparent soil moisture distribution around fertilizer P granules compared with injection zones of fluid P fertilizer in a calcareous soil. Further, scanning electron microscope (SEM) was employed to visually assess the degree of weathering of P granules as a function of incubation time.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A gray calcareous sandy loam soil (Calcixerollic xerochrept; Soil Survey Staff, 1992), collected from upper Eyre Peninsula, South Australia was used in this experiment. This soil had a pH of 8.5, a clay content of 50 g kg–1, an organic C content of 12 g kg–1 and contained 870 g CaCO3 kg–1 (Martin and Reeve, 1955). Wheat crops grown on this soil showed increased response to fluid fertilizers compared with granular P fertilizers in terms of grain yield and grain P concentration (Holloway et al., 2001). The two treatments were 42 mg of a single MAP granule (2.8–3.1 mm in diameter containing 10% N, 22% P, and 0% K) and 75 µL of dissolved TG-MAP (commercial grade product containing 12% N, 26% P, and 0% K). A weight of 0.144 g of powdered TG-MAP was added to 300 µL of deionized water to create a liquid TGMAP of 37.4 mg of P in 300 µL (or 9.36 mg of P in 75 µL), which was the same quantity of P applied in a 42 mg MAP granule. Air-dried, sieved <2 mm, and moist soil (30% of its water holding capacity as measured using the procedure described by Jenkinson and Powlson, 1976) was packed halfway through a low-density polypropylene tube (1 cm in diameter, 3 cm height) to obtain a soil density of 1.2 g cm–3. The MAP granule was carefully placed in the middle of the tube and the tube carefully filled almost to the top with additional moist soil. For the fluid fertilizer treatment, moist soil was packed about two thirds way through a polypropylene tube (about 2 cm soil height) and 75 µL of liquid TG-MAP was injected using a needle through a piece of low density plastic peristaltic pump tubing driven into the soil (from the center of the surface to about 0.75 cm deep into the soil). Both the needle and pump tubing were driven into the soil together to ensure soil was not driven up into the center of the tube. The point of injection of TG-MAP was approximately the same as for the MAP granule placement. The tubes were allowed to incubate at room temperature for 15 min and 24 h before imaging. The tube was imaged using a SkyScan-1072 high-resolution desk-top micro-CT (SkyScan, Aartselaar, Belgium) system. The x-ray source (80 kV and 120 µA) used has an effective source size of between 3 and 4 µm. A 1-mm thick Al filter was placed in the path of the x-ray beam to reduce beam hardening artifacts by filtering the low energy x-rays. The two-dimensional shadow images were obtained by rotating the soil sample 0.68° stepwise through 185° on a rotating sample holder with an exposure time of 4.9 s per frame and eight frames per each rotation step. The spatial resolution of the projection images and tomograms was 14.88 µm. Data acquired were processed with the aid of reconstruction software supplied by the instrument manufacturer (Cone reconstruction, Skyscan). The reconstruction details can be found in Rosenfeld and Kak (1982).

A complementary incubation study with MAP granules was also conducted to observe the porosity, density, and nature of the residual granule remaining in soil after incubation using a SEM (Philips XL30 FEG-SEM, Philips Electron Optics, Eindhoven, The Netherlands). Moist soil was packed into plastic Petri dishes (8.7 cm in diameter, 1.1 cm high) and granules were placed in the middle and the Petri dishes were covered. Samples were incubated in the dark for 1 to 5 wk in a controlled environment (25°C/20°C day/night temperature, 16-h day period). Incubated granules were carefully extracted and adhering soil particles were carefully removed using a magnifying glass and tweezers. New and incubated granules were cross-sectioned, then oriented and mounted onto aluminum specimen holders using Araldite. The samples were subsequently placed in an oven at 105°C for 15 min to aid polymerization and then samples were coated with 30 nm of C by evaporation, to provide electrical conductivity and maximize the backscattered electron (BSE) signal. Another set of MAP granules incubated and handled as described previously for SEM analysis for 1 d to 5 wk, individually digested with aqua regia (1:3 v/v conc. HNO3 and conc. HCl) and elemental composition measured by ICP–AES.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The theory of CT has been widely covered in many publications (Aylmore, 1993; Anderson and Hopmans, 1994). Briefly, x-ray photons produced by bombarding accelerating electrons to a metal target in an x-ray tube are used by CT scanners. When x-rays pass through an object or media either the photo-electrical absorption or Compton scattering causes attenuation of the x-ray beam. The transmitted intensity through the object homogeneous in density and composition over the x-ray travel distance (X) follows Beer's law.

Formula
Where, I0 is the intensity of the incident x-ray beam and µ is the linear attenuation coefficient. The linear attenuation coefficient depends on the density of the material ({rho}) and the mass attenuation coefficient (µ*).

Formula
The mass attenuation coefficient depends on the atomic number of the material through which the x-ray passes and the photon energy of the x-ray beam, so that for heterogeneous materials (i.e., soil) the attenuation coefficient will vary across the beam path due to density differences of materials/particles in the sample, provided the packing density remains the same. In µ-CT techniques, a sample is scanned at various rotation steps intervals over 180° for a specified length of time and the transmitted beam (shadow image) at each step is collected by an x-ray camera. Computer software is used to reconstruct a cross-section (tomograph) of a sample from all scans in a given plane. Therefore, the CT image is essentially a map of attenuation coefficients (Anderson et al., 1988).

Data for the whole soil column (about 3 cm tall) comprised of 980 horizontal tomograms (slices), with each tomogram representing a horizontal slice thickness of 14.9 µm. The tomogram in Fig. 1a shows a horizontal single reconstructed slice representing a cross-section at the center of the MAP granule for the scan started 15 min after placing the granule in soil (T1). The horizontal slice shows the air/water-filled pores in dark blue to dark green (low density) while the soil matrix appears in dark to light green (medium density) and orange to magenta (high density). The MAP granule appears as light green to dark green color with occasional orange colored spots indicating that it had a density lower than that of the majority of soil particles. The tomogram in Fig. 1b shows the same horizontal slice at the center of the MAP granule for the scan started 24 h after placing the granule in soil (T2). The change of color of the MAP granule from light/dark green to dark green was indicative of a decrease in granule density over time as resulted due to the net effect of progressive absorption of moisture (increase in density) and dissolution of the MAP granule (decrease in density). This was further verified by subtracting the image in Fig. 1a from 1b. A two-dimensional differential horizontal tomogram across the center of the MAP granule is shown in Fig. 2a , which was obtained by subtracting T1 image data from T2. In this figure, a gray scale range was used to indicate increasing attenuation from dark to light gray. Dark regions correspond to increases in density and light regions correspond to decreases in density. If two tomograms are identical then attenuation values should be identical throughout. The attenuation distribution expressed as Hounsfield Units (HU) for the center transect through the granule (Fig. 2a) is shown in Fig. 2b. This clearly shows that density changes occurred in the granule as well as in the soil adjacent to the granule. It was evident from the Fig. 2 that the MAP granule became less dense with time while a region about 1-mm thick immediately adjacent to the granule became denser with time. Total elemental concentrations of incubated granules in soil for 24 h and 1 wk showed that nearly 85% of granule P dissolved within the initial 24-h period and no significant P loss in the granule afterward indicating that there should not be any visual observable changes between the structure of MAP granules incubated for 24 h or 1 wk. Comparison between the BSE signal of a cross-section of an original granule and the 1 wk incubated granule in soil is shown in Fig. 3 . The structure of the MAP granule markedly changed with incubation in soil becoming visually more porous compared with the compacted structure of the original MAP granule and this supports the observations made by the x-ray CT technique that the granule became less dense with time. No visual changes however, observed in the structure of the MAP granule between 1- and 5-wk incubation times.


Figure 1
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  		Fig. 1. Horizontal tomograms taken at two times across the center of a monoammonium phosphate (MAP) granule. (a) Tomogram immediately after granule placement (T1) (b) Tomogram 24 h after granule placement (T2). The color scheme employed ranges from dark blue to dark green for low attenuation values (low density) while dark to light green for medium attenuation values (medium density) and orange to magenta for high attenuation (high density) values.

 

Figure 2
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  		Fig. 2. (a) Resulting tomogram and (b) line graph plot with resulted attenuation values after subtracting T1 image data from T2 data shown in Fig. 1.

 

Figure 3
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  		Fig. 3. Backscattered electron micrographs of cross-sections of monoammonium phosphate (MAP) granules (a) original, and (b) incubated for 1 wk in soil.

 
Figure 4 shows horizontal tomograms at the point of fluid TG-MAP injection 15 min after (T1) and 24 h after (T2) fluid injection. The color scheme employed was similar to that described before for Fig. 1. It seems from the x-ray CT data that there were no time-dependent density changes at the point of fluid P injection or in the soil immediately adjacent to the injection point. This was confirmed by subtracting T1 image data from T2 data to show identical attenuation values throughout (Fig. 5 ).


Figure 4
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  		Fig. 4. Horizontal tomograms taken at two times across the point of fluid P injection. (a) Image immediately after injecting the fluid (T1). (b) Image 24 h after injecting fluid (T2). The color scheme employed ranges from dark blue to dark green for low attenuation values (low density) while dark to light green for medium attenuation values (medium density) and orange to magenta for high attenuation (high density) values.

 

Figure 5
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  		Fig. 5. (a) Resulted tomogram, and (b) line graph plot with resulted attenuation values after subtracting T1 image data from T2 data in Fig. 4.

 
When a dry granule of single superphosphate was placed in moist soil, Lawton and Vomocil (1954) observed a shell or layer of moist soil around the fertilizer granule which had a noticeably higher moisture content than the surrounding soil. Furthermore, they found that as the moisture content of the soil mass increased from 1.6 to 7.7%, moist soil shells were formed around the granules up to about 4.5% for a sandy soil. At higher moisture contents (for example, >7.7% for sandy soil) no moist shell could be separated from the soil mass, indicating the total water stress in the two zones is the same. Similarly, the time-dependent increase in soil density adjacent to MAP granules observed in this study could be indicative of water accumulation due to the osmotically induced flow of soil moisture toward the fertilizer granule. It is also possible that precipitation products in this zone may also contributed to the higher soil density observed as shown by mass balance calculations below. The granule of MAP weighs 42 mg and contains ~9.3 mg P. Assuming 85% of this mass of P (~8 mg) enters and is retained in a 1 mm-wide zone of soil adjacent to the granule (Fig. 2b), containing about 56 to 65 mg soil (assuming the diameter of the MAP granule as 2.8 to 3.1 mm), indicate that P precipitation reactions could also significantly increase the density of this zone.

Any flow of moisture toward the granule would inhibit P diffusion from the granule and hence would enhance precipitation reactions involving P (Khasawneh et al., 1974). The apparent absence (or at least non detectable with x-ray CT) of osmotically induced flow of soil moisture toward injection points of fluid fertilizers should facilitate the greater outward movement of P from the point of fertilizer placement with fluid forms compared with granules as reported by Lombi et al. (2004) and thereby reduces the chances of creating micro environments with supersaturated soil solutions with respect to various solid Ca-P species.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The results from this study provide some insight into the physical and chemical processes occurring when fertilizer granular- or fluid-P applied to calcareous soils. The current study supports the hypothesis that the reduced lability of granular P compared with equivalent liquid fertilizer formulation (Lombi et al., 2004, 2005) may be influenced by precipitation reactions induced by reduced P diffusion due to osmotically induced flow of soil moisture toward the fertilizer granule. X-ray CT is a useful non destructive technique that can be used to visualize the hypothesized water absorption and/or precipitation reactions in and around fertilizer granules or fluid bands on their addition to soil. New approaches to studying both chemical and physical changes in and around fertilizer granules or fluid bands in soils would be essential to solve important agronomic issues namely optimization of fertilizer application or designing efficient fertilizer formulations.


   ACKNOWLEDGMENTS
 
This study was supported by a linkage grant from the Australian Research Council (LP0454086), the South Australian Grains Industry Trust and CSBP Ltd., Western Australia. MJM and EL gratefully acknowledge the support of Grains Research and Development Corporation and the Fluid Fertilizer Foundation.

Received for publication September 7, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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