Making better fertiliser decisions for cropping systems in Australia (BFDC): Knowledge gaps and lessons learnt

Soil testing remains a most valuable tool for assessing the fertiliser requirement of crops. The relationship between soil tests (generally taken from surface soil) and relative yield (RY) response to fertiliser is subject to the influence of environment (e.g. water, temperature) and management (e.g. cultivation, sowing date). As such, the degree of precision is often low when the soil test calibration is based on a wide range of independent experiments on many soil types over many years by many different operators. Hence, the 90% RY target used in soil test interpretation is best described by a critical range (critical concentration and confidence interval) for a given soil test rather than a single critical value. The present Better Fertiliser Decisions for Crops (BFDC) National Database, and the BFDC Interrogator that interacts with the database, provide a great advance over traditional formats and experiment-specific critical values because it allows the use of filters to refine the critical range for specific agronomic conditions. However, as searches become more specific (region, soil type) the quantity of data available to estimate a critical range becomes more vulnerable to data paucity, to outliers, and to clusters of localised experiments. Hence, appropriate training of the users of this database will ensure that the strengths and limitations of the BFDC National Database and BFDC Interrogator are properly understood. Additionally, the lack of standardised metadata for sites within the database makes it generally impossible to isolate the effects on critical values of the specific management or environmental factors listed earlier, which are therefore best determined by specific studies. Finally, the database is dominated (60%) by responses of wheat to nitrogen and phosphorus, meaning that relatively few studies are available for responses by pulses (other than narrow leaf lupins) or oilseeds (other than canola), especially for potassium and sulfur. Moreover, limited data are available for current cropping systems and varieties. However, the identification of these gaps can now be used to focus future research on the crops, nutrients, soils, regions, and management practices where data are lacking. The value of metadata and the need for standardised protocols for nutrition experiments were key lessons