Many intensively managed soils contain phosphorus (P) concentrations greater than required for optimum production. Soils with P concentrations in excess of the agronomic optimum can have unnecessary losses of P that can adversely affect water bodies. Reducing excessive soil-P concentrations is important for the economic and environmental sustainability of intensive agriculture, such as the Australian dairy industry. However, little is known of decreases in extractable soil-P concentrations when P fertiliser applications are reduced or omitted from soils with P concentrations and properties representative of intensive pasture grazing systems. Decreases in extractable P (calcium chloride (CaCl2), Olsen and Colwell) were monitored for up to 4.5 years for six Australian grazed pasture soils (Red Ferrosol, Brown Kurosol, Grey Dermosol, Brown Dermosol, Podosol and Hydrosol) with contrasting textures and P-buffering indices (PBI). Sixteen treatments consisting of four initial extractable-P concentrations (Pinit) paired with four ongoing P fertiliser rates (Pfert) were established for each of the six soils, except on an extremely low-PBI Podosol, where a range of Pinit concentrations could not be established. The resultant decreases in P were larger with higher Pinit concentration and lower rate of ongoing Pfert, except in the extremely low PBI Podosol where decreases in initially high CaCl2-P concentrations were large irrespective of ongoing Pfert. There was a greater proportional decrease in the environmentally extractable P compared with agronomically extractable P, with mean decreases in CaCl2-P of 57%, Olsen-P of 25%, and Colwell-P of 12%. The Pinit concentrations, which were well above agronomic optimum, remained above this target. This study advances scientific knowledge of extractable soil-P concentrations when P fertiliser inputs are withheld or reduced from grazed pasture soils, and aids land and catchment managers in estimating likely changes over time.
Pasture soils of mainland Australia are routinely sampled to a depth of 100 mm, whereas pasture soils of Tasmania and New Zealand are sampled to a depth of 75 mm. Despite this difference, there are no published studies to allow accurate conversion between the 2 sampling depths. This study was undertaken to examine the effect of soil sample depth (75 and 100 mm) on soil nutrient analyses: Olsen phosphorus (P), Colwell P, Colwell potassium (K), KCl40 sulfur (S), pH (H2O and CaCl2), and P buffering index (PBI+ColP). Our findings suggest that on most soil types, Olsen P, Colwell P, and Colwell K soil test results could use a generic factor of 1.17 when converting between 75 and 100 mm samples. More detailed equations including direct or indirect measures of soil texture and P sorption capacity were also determined. We found there was no significant difference in P and K concentrations between the different depths in soils which had very low P sorption capacities, high sand contents, and were located in high rainfall zones. Additionally, soil sampling depth generally had no effect on extractable S concentrations, pH, or PBI+ColP, and therefore no conversion is recommended for these soil tests for any soil type.
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