Sustainability of nutrient management in grain production systems of south-west Australia

Abstract

Balancing nutrient inputs and exports is essential to maintaining soil fertility in rainfed crop and pasture farming systems. Soil nutrient balances of land used for crop and pasture production in the south-west of Western Australia were assessed through survey data comprising biophysical measurements and farm management records (2010–15) across 184 fields spanning 14 Mha. Key findings were that nitrogen (N) inputs via fertiliser or biological N2 fixation in 60% of fields, and potassium (K) inputs in 90% of fields, were inadequate to balance exports despite increases in fertiliser usage and adjustments to fertiliser inputs based on rotations. Phosphorus (P) and sulfur (S) balances were positive in most fields, with only 5% returning losses >5 kg P or 7 kg S/ha. Within each of the three agroecological zones of the survey, fields that had two legume crops (or pastures) in 5 years (i.e. 40% legumes) maintained a positive N balance. At the mean legume inclusion rate observed of 20% a positive partial N budget was still observed for the Northern Agricultural Region (NAR) of 2.8 kg N/ha.year, whereas balances were negative within the Central Agricultural Region (CAR) by 7.0 kg N/ha.year, and the Southern Agricultural Region (SAR) by 15.5 kg N/ha.year. Hence, N budgets in the CAR and SAR were negative by the amount of N removed in ~0.5 t wheat grain, and continuation of current practices in CAR and SAR fields will lead to declining soil fertility. Maintenance of N in the NAR was achieved by using amounts of fertiliser N similar to other regions while harvesting less grain. The ratio of fertiliser N to legume-fixed N added to the soil in the NAR was twice that of the other regions. Across all regions, the ratio of fertiliser N to legume-fixed N added to the soil averaged ~4.0:1, a major change from earlier estimates in this region of 1:20 under ley farming systems. The low contribution of legume N was due to the decline in legume inclusion rate (now 20%), the low legume content in pastures, particularly in the NAR, and improved harvest index of lupin (Lupinus angustifolius), the most frequently grown grain legume species. Further quantifications of the effects of changing farming systems on nutrient balances are required to assess the balances more accurately, thereby ensuring that soil fertility is maintained, especially because systems have altered towards more intensive cropping with reduced legume production.

Evaluating the Impact of Crop Layout Changes on N and P Nutrient Balance: A Case Study in the West Liaohe River Basin, China

“…The soil surface nutrient balance is mainly calculated by the difference between nutrient input and output in agricultural systems [18][19][20]. In agricultural systems worldwide, farm gate, soil surface, and soil systems are the dominant methods of budgeting nutrients [21,22].…”

Links between soilborne pathogens, plant parasitic nematodes, farm management and biophysical constraints in a southern Australian rainfed cropping system

“…Several factors driven by climate and soil differences between regions may be involved, i.e. we found greater rainfall and more fertile soils in the SAR meant plants produced more above ground biomass (Harries et al 2021), and presumably below ground biomass although this was not measured, which may enable plants to cope with higher background pathogens and nematode pests DNA levels. Also, soils vary in suppressiveness of soilborne disease and nematode pest symptoms.…”

“…Likewise, root disease incidence increased as temperature decreased, but also as rain decreased. Hence, in environmental conditions with greater water stress, the higher latitude NAR and CAR regions (Harries et al 2021), there was more root damage despite less soil DNA compared to the SAR. This is consistent with findings of Poole et al (2015) who analysed a sub-set of the Focus Paddock data to report that temperature and rainfall parameters explained most of the variation in root health but this was not always strongly correlated to soil DNA levels.…”

Cereal and oil seed crops response to organic nitrogen when grown in rotation with annual aerial-seeded pasture legumes

“…In our study, the pasture legume species produced over 5 t/ha of dry matter (Legume v. Fallow study), which provided approximately 50 kg/ha of N to the following crops. A wheat crop yielding three tonnes of grain, is predicted to require around 70 kg of N, depending on soil N levels (Harries et al , 2020). Based on our results it is feasible that a productive pasture that is dominant in annual pasture legumes and grown in the low to medium rainfall farming region would be able to meet the N requirements of a subsequent cereal or canola crop under average growing conditions.…”