Agronomic performance of sorghum (Sorghum bicolor (L.) Moench) and fertilizer use efficiency as affected by controlled and non-controlled traffic of farm machinery

Compaction adversely affects the physical properties of soils and the ability of crops to efficiently use water (rainfall, irrigation) and nutrients, and therefore reduces the amount of fertilizer recovered in grain. This study was conducted to investigate the effect of traffic compaction on sorghum response to nitrogen (N) fertilization. Soil conditions (density) representative of controlled (CTF) and non-controlled traffic (non-CTF) farming systems were achieved by removing compaction through subsoiling to a depth of approximately 300 mm and by performing six passes of a medium-sized tractor, respectively. The soil type used in the study was a Red Ferrosol (69% clay, 11% silt, and 20% sand), which is commonly used in Australia for grain production. Sorghum was grown during the 2015-2016 season and fertilizer was applied at rates between 0 (control) and 300 kg ha-1 N at regular increments of 100 kg ha-1 N using urea (46% N), urea-ammonium nitrate (UAN, solution, 32% N) and ENTEC® urea (46% N). Grain yield was approximately 40% higher in the traffic treatment representative of CTF compared with that of the non-CTF, and consistent with differences (P<0.05) in all measurements of crop yield components (total aboveground biomass, harvest index, and thousand-grain weight). Fertilizer type had no effect on grain yield, which confirmed that traffic compaction was the main factor affecting crop performance and N recovery in grain and biomass. The optimum N application rates were 145 kg ha-1 N for CTF and 100 kg ha-1 N non-CTF, which corresponded with grain yields of 3430 and 1795 kg ha-1, and agronomic efficiencies of 24 and 17 kg kg-1, respectively. Given current price ratios (nitrogen-to-grain) and fertilizer type used, gross margin penalties of up to AUD75 per ha may be incurred in non-CTF systems compared with CTF when zero-tillage is practised, and about double when shallow tillage is practised. This study also showed that, regardless of the N formulation used, N use efficiency cannot be significantly increased if the mechanization system does not allow for avoidance of traffic compaction. Therefore, the benefits of enhanced efficiency fertilizers may not be fully realized if soil compaction is not appropriately managed. Improved soil structural conditions are a pre-requisite for increased fertilizer use efficiency.