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,c
a USDA-ARS Pasture Systems and Watershed Management Research Unit Building 3702 Curtin Road University Park, PA 16802-3702
b AgResearch Limited Invermay Agricultural Research Centre Private Bag 50034 Mosgiel New Zealand
c USDA-ARS Pasture Systems and Watershed Management Research Unit Building 3702 Curtin Road University Park, PA 16802-3702
Andrew.Sharpley{at}ars.usda.gov
We thank Drs. Moshe Shenker and Paul Bloom for their letter and for giving us the opportunity to focus the reader on the objectives of our paper; namely to identify P forms and solubilities in manured soils, to explain their behavior and potential for P loss in runoff.
The true nature of Ca-associated P remains an elusive subject. Even so, the main point of our manuscript was to show that acid-based soil test P extractants, such as Mehlich-3, can overestimate the amount of P that is potentially released to water from soils that have received large amounts of manure over several years. This point holds true regardless of inferred speciation of Ca-P compounds.
Shenker and Bloom repeatedly make two points, (i) that ion solubility products should be combined with other data to support conclusions, and (ii) that the equilibration period was insufficient to determine a "true controlling phase."
By reading the literature (e.g., Pierzynski et al., 1990), a common theme is evident: studies using solubility equilibrium data should include supporting data, and double-function plots, such as Fig. 6 in our paper, can only infer solubility. We support this view and only use the double-function plot to show differences between manured and unmanured soils. At no stage do we conclude by these plots alone, that different or distinct Ca-P minerals are present. Moreover, we only infer that increasingly soluble Ca-P minerals have formed with the application of manure for up to 25 yr in some cases. These Ca-P forms are likely to represent amorphous phases forming via heterogeneous precipitation and as such are not "pure." The use of sequential fractionation data supports increased formation of HCl-P, commonly thought to represent apatite. Shenker and Bloom also make the point that if HCl-P was used as the sole indicator of Ca-P extracted into the equilibration solution, then they would be undersaturated vis-a-vis Ca-P minerals. We would like to point out that Ca-P is also found in bicarbonate- and resin-extractable P fractions (Tiessen and Moir, 1993).
The second point Shenker and Bloom make is that the equilibration period is insufficient to indicate the "true controlling phase." They also point out that kinetics are important. We completely agree that kinetics are important and pose the following question. Considering the aim of our research presented in Sharpley et al. (2004) was to show the forms and solubility of P in soil receiving manure with respect to P release and transport in runoff, what time frame should be considered? In essence, the equilibration of a soil with a solution at any one time effectively gives an indication of the phase dissolved at that time. As we used the same time for both manured and unmanured soils, the comparison between these soils is valid. We noted in the paper, "as more P and Ca are introduced into the system, P is increasingly precipitated into more soluble P forms." We would also like to refer readers to previous studies of ours that used double-function plots in combination with solid-state nuclear magnetic resonance spectroscopic techniques to show that increasingly soluble Ca-P phases do occur with increasing P and pH (McDowell et al., 2003).
Finally, we ask the readers to consider the following point. McDowell et al. (2001) showed using 33P, that P in runoff was best related to P released from soil to solution in up to 24 h and that the pool of P released beyond this and up to 90 d was a poor indicator of P in runoff. If in fact we used the equilibration time proposed by Shenker et al. (2005) of 120 d, how well would this relate to P in runoff?
We support the view of Shenker and Bloom that any methodology, whether it be analytical, field, or modeling must be used under the conditions for which it was developed. Our research covers the long-term field equilibration of soil and manure (10–25 yr) and the rapid release (1 min to several hours) of soil P to runoff water during a rainfall-induced flow event. Inasmuch, we believe the equilibration times we used relate to reaction times that exist during the release of soil P to rainfall/runoff water. The limitations of using P solubility or double-function plots to infer mineral stability were clearly defined by Sharpley et al. (2004). Inferences from them were made only in support of chemical extraction data (both individual and sequential). We believe the overall conclusion of our research, that the acid extractability (Mehlich-3) of soil P increased to a greater extent than water extractability in heavily manured soils, is still valid, and that these findings have important implications to environmental soil P test procedures and recommendations.
We appreciate the opportunity to have this dialog with other researchers and that it has clarified the appropriateness of using various methods to elucidate the extractability and solubility of soil P in terms of its estimation with regard to ever expanding agronomic and environmental contexts.
NOTES
* Richard.Mcdowell{at}agresearch.co.nz 
Peter.Kleinman{at}ars.usda.gov
REFERENCES
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