# Phosphorus Distribution in Soils from Australian Dairy and Beef Rearing Pastoral Systems
## 4 Discussion  
By removing fertiliser variables, such as timing and rate, some clear differences can be seen between soil types. These differences are most clearly demonstrated when comparing sites W1 and W2 (grey clay loam Dermosols) to sites W3 and W4 (red clay loam Ferrosols). Although separate, each of these sites were managed as a single site utilising similar farm management practices, with each receiving identical recent P fertiliser additions (24.8 kg\(\cdot\) P\(\cdot\) ha\({}^{-1}\), 365 d). Although separated by less than 2 km, these pairs of sites showed some clear differences that were attributed predominantly to soil type and this will be discussed in more detail later in this section. Total soil P concentrations were much lower for sites W1 and W2 (1838 and 1941 mg P kg\({}^{-1}\)) than sites W3 and W4 (3009 and 2680 mg P kg\({}^{-1}\)). Based on site averages, these higher total P concentrations, which were 51% greater for the Ferrosol sites, were not reflected in the results for the agronomic soil testing, with the Dermosol sites (W1 and W2) giving higher Colwell P values than the Ferrosol sites (W3 and W4), and similar Olsen P values (Table 2).  
Figure 4: Selected scatter plots demonstrating correlation between: (**a**) monoester P and orthophosphate; (**b**) diester P and pyrophosphate; (**c**) diester P and Morphophosphate; (**d**) Olsen P and Colwell P; (**e**) CCEP and WEP.  
Despite slight management disparity, there were discrepancies when comparing the P composition within the soils from Maffra sites M2 and M5 using NMR analysis. The red Sodosol (M2) gave 64.3% of total P in the form of orthophosphate (inorganic), whereas it was 49.8% for the yellow Sodosol (M5). Another feature of the yellow Sodosol is the relatively high proportion of pyrophosphate (8.5%) compared to all other soils. This was 44% higher than the next highest pyrophosphate percentage (site M4, 5.9%) and more than doubled the highest proportion achieved in soils from the Warragul area (ranging between 1.1% and 3.1%). As with any alkaline organic P extraction procedure there is always the possibility that hydrolysis may have influenced these results. Pyrophosph is unstable in aqueous solution and some may have been hydrolys during the extraction into inorganic phosphate [23; 48]. To some degree, the concentration of monoester phosphate is likely to be overestimated through the degradation of diester phosphate during the extraction process [48; 49]. This possibility needs to be taken in consideration when viewing these results.  
The lowest soil orthophosphate levels also corresponded with the lowest fertiliser inputs for the Maffra soils (site M3, 46.5% and site M4 43.9%). Sites M3 and M4, which are of a similar soil type, had not received P-based fertilisers for at least two years prior to soil sampling. Not surprisingly, these soils had the lowest total P concentrations (1447 mg P kg\({}^{-1}\) and 1470 mg P kg\({}^{-1}\) for M3 and M4, respectively), with the exception of site W7. Again these low fertiliser rates resulted in a high proportion of monoester P (44.6% and 46.4% for M3 and M4, respectively). This view is well supported by the scatter plot of monoester P and orthophosphate levels shown in Figure 4a. It is clearly obvious that monoester P level was highest when orthophosphate level in the soil was lowest.  
To better understand the cause of these variations and differences, it was necessary to further discuss the influence of important parameters, such as soil type and management practices, including fertilisation, moisture, pH and slope/elevation variations.