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a Department of Soil and Water Sciences Faculty of Agricultural, Food and Environmental Quality Sciences The Hebrew University of Jerusalem POB 12, Rehovot 76100 Israel
b Department of Soil, Water, and Climate College of Agricultural, Food, and Environmental Sciences University of Minnesota 439 Borlaug Hall 1991 Upper Buford Circle St. Paul, MN 55108
shenker{at}agri.huji.ac.il
Mineral stability plots are a handy tool that is becoming more widely used as computer geochemical models have spread among soil and environment scientists. These plots may indicate possible geochemical processes in various systems and suggest discrete mineral phases that control the activity of various soluble components in soil solutions. But, as is true for many other tools, they may lead to erroneous conclusions as well. Our purpose herein is to note a few pitfalls that should be avoided in using stability plots.
In their recent paper, Sharpley et al. (2004) aimed to determine the effect of continual long-term (10–25 yr) manure applications on the forms and solubilities of P in soils. Their sequential extraction data indicate that although acidic (4.8 
pH 6.8), all of the studied unmanured soils contained Ca-P phases (P extracted by 1 M HCl). Subsequent to long-term manure application the pH increased slightly in all soils to range from 5.7 to 7.6, and the sequential P extraction data suggest that most of the applied P was precipitated as additional Ca-P phases rather than being adsorbed on the surface of other solid particles or precipitating as Fe- or Al-P phases. The Ca-P accumulation is in line with the expectation that at equilibrium hydroxyapatite (HA) and fluorapatite will form instead of the Fe-P mineral strengite and the Al-P mineral variscite in soils with pH higher than approximately 5.5, and as pH increases even the more soluble Ca-P minerals, such as brushite, will be more stable than these Fe- and Al-P phases (Lindsay, 1979). However, oversaturation alone is merely a prerequisite for mineral formation; nonequilibrium is the common state in soils and kinetics of precipitation would be an important factor to dictate whether and to what extent various Ca-, Fe-, or Al-P minerals would accumulate in the soil. To examine which Ca-P mineral phases formed, Sharpley et al. (2004) used geochemical modeling based on extraction concentrations after equilibration for 16 h in 0.01 M CaCl2, using a 1:5 soil/solution ratio. Figure 6 in their paper shows a "double function" plot of 
vs. (log Ca2+ + 2pH) of the soil extracts, and by comparing their data to the stability plots of several Ca-P minerals they attempt to determine which Ca-P minerals are present in the soils studied. Thus, by showing that all the data points for the unmanured soils fall below the HA line, they propose that HA was the main mineral form of Ca-P in these soils. Similarly, based on soil extracts that were oversaturated with respect to HA and undersaturated with respect to dicalcium phosphate dihydrite (DCPD), they propose that tricalcium phosphate (TCP) and octacalcium phosphate (OCP) dominated the Ca-P forms in the manured soils.
We propose that neither statement is adequately established. Using the data presented in their paper, we calculate that for five of the unmanured soils, even if all the HCl (1 M)-extracted inorganic-P is assumed to have originated from Ca-P minerals and to have been dissolved completely by the extractant (1:5 in 0.01 M CaCl2) during the short extraction period (16 h), the solutions would have been undersaturated with respect to all Ca-P minerals, and it is not possible to say which form of Ca-P was actually dissolved. Thus, even if equilibrium were to be attained in 16 h, it is not possible to distinguish which Ca-P mineral is controlling P concentration under field moisture conditions, or which Ca-P mineral dominates the Ca-P phases of the soils. For the manured soils most of the data points plotted in Fig. 6 of Sharpley et al. (2004) were above the HA line and below the TCP line, but Ca and P could have been dissolved from small quantities of any of the Ca-P minerals for which solution is shown to be undersaturated, even if most of the Ca-P phase was in the form of HA. Thus, with a low soil/solution ratio metastable phases present in low concentrations may dissolve completely, even though these phases may control P solubility at field moisture conditions.
The short equilibration time adopted by Sharpley et al. (2004) would have further limited the utility of an equilibrium plot for identifying any Ca-P mineral as a dominant phase. Mackay et al. (1986) found that dissolution of finely ground (<180 µm) rock phosphate materials approached steady state in various moistened soils only after 30 d, and Pierzynski et al. (1990) found that soil suspensions (1:2 in deionized water) attained steady state relative to OCP or HA only after 42 to 105 d, while shorter extraction times resulted in undersaturation for these minerals.
Geochemical studies, based on solution composition, can provide important information, but must be used with caution in characterizing the mineralogical composition of soils. Toward that end, x-ray diffraction (XRD), electron induced x-ray emission spectroscopy, or other mineralogical tools, should be used. For example, Beauchemin et al. (2003), who similarly found by sequential extraction that Ca-P minerals were the dominant P form in the B horizon of an acidic loamy soil (pH 5.5) amended for >25 yr with animal manure, used x-ray absorption near-edge structure spectroscopy (XANES) to estimate that 11% of the total P content of this soil occurred as HA and 45% as OCP.
Determination of the phase controlling P solubility in field soils can be important in understanding plant P supply over the rather short periods involved in P uptake. Kinetic control by the least stable mineral, regardless of its amount, or adsorbed P, must be considered. Thus, relatively large amounts of HA in a soil may not be able to maintain a P concentration that satisfies plant demand, while an ample supply may result from even small amounts of meta-stable OCP or DCPD. Solution data and geochemical modeling can be used to ascertain which soil phase or phases control the activities of soluble constituents and can also suggest processes such as mineral formation or transformation that take place in a soil; however, conclusions should be based on firm data that indicate near equilibrium with a specific mineral over a wide range of conditions that result in its dissolution or precipitation. For example, Brennan and Lindsay (1998) used geochemical calculations to study transformations of iron hydroxides after altering redox conditions in suspensions of synthetic iron oxides and soil. Their experimental results were collected over a wide pe + pH range of 2 to 14, and since their data fell along distinct lines of Fe(OH)3 (amorphous) in the 14 to 11 pe + pH range, and of Fe3O4 (amorphous) in the pe + pH range of 11 to 4, they could differentiate the range within which each mineral controlled Fe activity. Hence they could suggest that in their experiment, ferric oxides were transformed on reduction to Fe3O4 (amorphous) rather than Fe3O4 (magnetite). Their conclusion was indeed supported by XRD examination of the solid phases.
In another study, by using geochemical modeling Shenker et al. (2005) showed that on soil reduction and release of adsorbed P from dissolution of ferric hydroxides, TCP and HA controlled P activities in two rewetted calcareous wetland soils but not in two other gypsum-rich peat soils from the same wetland. Their conclusion was based on an experimental data set that reflected a long equilibration period (120 d) with wide ranges in pe + pH values (2–12) and P concentrations (2–40 µM), such that TCP and HA were identified as having been formed in two of the soils because P solubility data fell along phase lines for these minerals.
No such tendency is apparent in the paper by Sharpley et al. (2004), nor indeed would be expected considering the short equilibration period and the low soil/solution ratios employed, which would have completely dissolved metastable DCPD, OCT, and TCP phases. Consequently, no conclusions can be reached as to which Ca-P minerals actually precipitated in the soils studied, or to what extent. The retained P could have precipitated as any of the Ca-P minerals when oversaturation occurred. The extent of such precipitation will be affected by kinetic considerations and various factors that control it. Thus, HA precipitation might be inhibited by the presence of soluble organic substances, even if solutions were oversaturated with respect to this mineral, as was shown by Inskeep and Silvertooth (1998). On the other hand, the undersaturation found by Sharpley et al. (2004) for extracted solutions might be related to dilution, if dissolution proceeded so slowly as to prevent re-establishment of a semi-equilibrium state within the 16-h extraction period.
NOTES
REFERENCES
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