Metrics for Crop Nutrition Sustainability in Australia & New Zealand.

Sustainable, efficient and effective cropping systems are underpinned by, amongst other things, sound nutrient management. Balanced nutrition does infer the correct blend of nutrients, but it is becoming more important to balance nutrient removal and nutrient use. The most usual nutrient use efficiency metric is removal-to-use ratio, also called partial nutrient balance, removal efficiency, output/input ratio. Regional agricultural nutrient balances provide limited information to policy makers, the distribution of these values is as important as the measure of central tendency.

Using public data on agricultural production, regional fertilizer use and crop areas, removal-to-use for N and P were estimated at state levels for particular time periods. The estimation requires robust estimates of biological N fixation, nutrient concentrations in grains, fertilizer use and manure inputs by crop and the use of other materials (such as sugar mill byproducts) to create regional metrics.

In Australia, sugar cane nutrient management is indexed against N partial factor productivity, and the dairy and grains industries are developing removal to use values as part of production sytem benchmarking. New Zealand used a well-developed nutrient balance model (Overseer®) for estimating N leaching limits to preserve stream water quality. 

Neither a high nor a low NUE is an implicit target, but raising low values, which usually indicate inefficient use of added nitrogen, and lowering very high values, which usually indicates mining of soil nitrogen, will require appropriate interventions at the farm level, so that the farmer engagement is important in achieving progress to efficient nutrient use.