Crop yield responses to surface and subsoil applications of poultry litter and inorganic fertiliser in south-eastern Australia

In the high-rainfall zone of south-eastern Australia, deep incorporation of organic matter has previously been reported to increase crop yields by improving access to subsoil water and nutrients, resulting from the amelioration of subsoil constraints. However, previous experiments did not separate the yield response resulting from nutrients contained in the amendment from yield response due to amelioration of subsoil constraints. In order to separate these effects, eight field experiments were conducted on a range of soil types across the medium- and high-rainfall zones of south-eastern Australia between 2014 and 2016. Grain yield and quality responses of a range of annual crops (canola, wheat, barley and lentil) to surface and deep placement of poultry litter and inorganic fertilisers with matched nutrition were assessed. Over 15 site × year combinations, there was no consistent, significant positive interaction between amendment and incorporation treatments necessary to demonstrate that deep placement of amendment (i.e. subsoil manuring) had advantages over surface application of the same amendment. Differences in crop yield in these experiments are attributed to nutrients (particularly nitrogen) supplied by the amendment, and not to the amelioration of subsoil constraints. Future research, including analysis of subsoil physicochemical properties and plant nutrient concentrations after treatment, is necessary to confirm the role of nitrogen and other nutrients in the crop response to subsoil manuring.