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Fluid Journal : Fluid Journal 2002-2004
Early Spring 2004 Fluid Journal 2 Isotopic dilution. Using an isotopic dilution technique it was also found that relatively more P was isotopically exchangeable when applied as TGMAP than as MAP. Therefore, it appears that P not only diffuses farther from the injection point but also remains more available (isotopically exchangeable) when applied as a fluid than as a granular. A reason for this may be related to the fact that after an initial rapid dissolution of the granules, a significant percentage of P (10 to 20%) does not appear to diffuse out of the granules within 12 weeks. This was evidenced using EDXMA of granules exposed for different periods of time to a gray and red calcareous soil maintained at 60 percent of the water holding capacity. Dot maps representing the distribution of P, Ca, and Al in a granule exposed for 5 weeks in a gray calcareous soil are shown in Figure 2. It is clear that all these elements are present in large concentrations in the exposed granule. At the moment, it is not yet clear what is the proportion of Ca and Al that originates from the soil and from the fertilizer granule itself. The crystalline form of P in the granules after incubation was examined using X-ray diffraction. Several crystalline phases were identified, including the poorly soluble crandallite (CaAl3(PO4)2(OH)5.(H2O)). It is likely that crandallite is a poor source of P for crop production in calcareous soils, even in the long term. Additional data from the isotopic dilution studies indicated that when P was supplied at different rates as granular MAP, a large portion of P was rapidly fixed. By contrast, application of APP was less pronounced and mobilization of native P may have occurred. However, due to the complexity of the system and of the methodologies used, further experimentation is required in order to ascertain whether, and to what extent, mobilization of native P occurs. Conclusions Differences in P-bioavailability measured in the plant uptake experiment are likely to be due to a slower rate of dissolution of MAP in highly calcareous soils, or to rapid immobilization of the dissolved P. Data in Figure 1 certainly seem to indicate that P from fluid fertilizers diffuses through the soil better than when P is supplied in granular form. The application of liquid fertilizers may facilitate more homogeneous distribution of soluble orthophosphate (or polyphosphate) ions in the soil, avoiding precipitation of solid phase calcium phosphates (or calcium polyphosphates) in the zone of fertilization. However, preliminary results obtained measuring diffusion and lability of various sources of P in petri dishes containing calcareous soils indicate that both dissolution and diffusion are key factors in controlling Figure 1. Percentage of P from fertilizers at different distances from the point of application of granular MAP and liquid MAP (TGMAP). Figure 2. Scanning electro micrograph of a MAP granule incubated to 5 weeks in a gray calcareous soil and the corresponding EDXMA dot maps of P, Ca, and Al.
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