# Phosphorus Distribution in Soils from Australian Dairy and Beef Rearing Pastoral Systems
## 4 Discussion
### Influence of Soil Type  
The use of NMR spectroscopy for investigating the P complexes of each soil type revealed distinct differences in signal strength distribution. Typically, diester phosphate and pyrophosphate were similar irrespective of soil type, but those of monoester phosphate and orthophosphate were different. This view is well supported by the scatter plot between diester P and pyrophosphate concentrations. Figure 4b shows that, with the exception of two outliers, reasonably good correlation was found between the concentrations of these two P species. Also, the scatter plot in Figure 4c reveals that diester P level is highest where fertilise input is lowest, as reflected by low orthophosphate level. This indicates that during low fertilizer input, a relatively large proportion of P is stored in soil in the organic diester P form. The average proportion of orthophosphate was 66.8% for the grey Dermosol (sites W1, W2, W6) and 72.2% for the red Ferrosol sites (sites W3, W4, W5). The monoester phosphate region on average accounted for 30.0% and 23.9% of signal intensity for Dermosol and Ferrosol soils, respectively. With the longer exposure (16 h) of the extracted P from the soil to the NaOH-EDTA solution, it is possible that the hydrolysis of labile P forms can lead to production of artefacts which can influence the observed difference of the P forms to some extent. However, the differences in soil mineralogy which reflects soil formation processes will have stronger influence on the observed trends of the P forms. For example, by definition, Ferrosols have B2 horizons which are high in iron (Fe) content [39]. This may also be reflected in the surface soil horizon (A) as Ferrosols lack strong texture contrast between the A and B horizons [39]. Soils high in Fe sorb P strongly due to their variable charged oxides and, therefore, Fe is an important factor in soil P sorption [50; 51]. Hence, the presence of higher Fe concentrations may account, in part, for the higher orthophosphate proportion (Table 4), much higher total P concentrations (Table 2), and relatively low concentrations of other soil P measurements (Table 2) in the Ferrosol sites with the same fertiliser application rates (that is, Dermosol sites W1 and W2 compared to Ferrosol sites W3 and W4). The Ferrosol sites (W3 and W4) had average concentrations of 432 mg P kg\({}^{-1}\) and 107 mg P kg\({}^{-1}\) for Colwell P and Olsen P, respectively. The comparative concentrations for the Dermosol sites (W1 and W2) were 701 mg P kg\({}^{-1}\) for Colwell P and 87 mg P kg\({}^{-1}\) for Olsen P. The higher Olsen P concentrations in soil from the Ferrosol sites indicate that the soil had more plant available P (as indicated by Olsen P values) than those from the Dermosol sites. On the other hand, the much higher Colwell P value obtained for the Dermosol sites indicate that the soil contains more structurally bound phosphate reserve than soil from the Ferrosol sites. Furthermore, the lower Colwell P obtained for soil from all of the Ferrosol sites (including site W5) is consistent with the lower total P extracted (73.3%-77.3%) with the NaOH-EDTA solution (Table 3) compared with those of the Dermosol sites, including site W6 (85.9%-90.5%). The significant difference also supports the view that the Dermosols have higher structurally bound (reserve) P. In general, the higher Colwell P obtained for sites in the Warragul area (110-730 mg P kg\({}^{-1}\)) than those in Maffra area (89-443 mg P kg\({}^{-1}\)) indicates higher structurally bound (reserve) P in soil within the Warragul area. In spite of the differences in the Olsen P and Colwell P levels in the different soil samples, Figure 4d shows that there is a reasonably good correlation between the Olsen P and Colwell P levels obtained for other soil samples when the W1 and W2 Dermosol sites are removed.