Influence of sample depth on extractable nutrient concentrations, pH and the phosphorus buffering index of pasture soils in south-eastern Australia

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.