Brendan Scott

Phosphorus (P) fertilizers are commonly applied to agricultural soils for winter cereal production, and they represent a significant proportion of the variable costs of crop production. Many areas now have high plant-­‐ available soil P levels and regular application of P fertilizers is no longer required to achieve profitable yields (Sims and Vadas 2005). However, in all cases, soil testing can provide key information for improved decision making about P management. Recently in Australia, a national database (the Better Fertilizer Decisions for Crops (BFDC) National Database) of nutrient response trials for grain crops has been assembled (Watmuff et al. 2013). The treatment series entered were subject to rigorous checks to ensure they were valid for re-­‐analysis in the database. The database represents a valuable resource with which to assess response of winter cereals to P across a wide range of cropping eras (modern vs earlier periods), soil types, seasonal conditions, sampling depths, soil test methods and species. The objective of this study was to examine factors affecting critical soil P test concentrations and confidence intervals for wheat and barley grown in Australian soils by interrogating validated data held in the BFDC National Database. Most emphasis was placed on the Colwell extractant for soil P (Colwell 1963) since it was the most widely used test and the one for which the largest and most comprehensive dataset existed.