NGN - Carryover of nitrogen after crop failure - Western Region case studies

Growers that were hit by frost and heat stress in 2021, after applying robust nitrogen rates during the season when conditions looked promising, face the additional financial burden of increased nitrogen fertiliser costs in 2022. The increased costs are being driven by strong international demand for N fertiliser and transport costs. The requirement by growers for knowledge is twofold. More immediately, growers would like to know if they can reduce the 2022 nitrogen rates to lessen the financial burden after 2021. In the longer term, growers would like to better understand the risk of applying robust fertiliser rates when conditions are good i.e., less risky if the N will then be available for next years crop, if there is a failure in the year it is applied.

In essence, it is complex and can be highly variable. Soil N can accumulate in soil where mineral N is not utilised by the previous crop as shown in research conducted on N carryover from legume crops (Herridge et al., 1995). This carryover is reduced by factors such as weed N-uptake, leaching (greater in sandy soils), erosion and gaseous losses (Peoples and Swan, 2018) which are driven by microbial activity. Microbial activity is dependent on rainfall and soil temperature which when favourable for microbes, increases release of mineral N to more mobile forms making it more vulnerable to loss (McDonald and Hooper, 2013).

This investment aims to use grower case studies to locally explore and provide more information as to the conditions under which more N will be retained in the northern and eastern wheatbelt of WA. It is hoped that this information will give growers more confidence in making N fertiliser decisions after a crop failure the previous year.