CONFIRMATION OF NITRATE LEACHING POTENTIAL OF SELECTED SOILS IN THE BURDEKIN DELTA REGION

ELECTROMAGNETIC induction instruments (EMI) were used to identify major variation in apparent electrical conductivity of earth (ECa) to depths of 1.5 and 6 m, along five transects in areas of the Burdekin delta that were known to have moderate to high levels of nitrate nitrogen (NO3-N) in irrigation waters drawn from the aquifer. The variation in ECa in the low electrolyte environment was clearly related to clay content of the 0–1.5 and 0–6 m profiles at 11 sites where soil samples were obtained by drilling. Properties of the surface 1.5 m of soil provided a reasonable reflection of deeper soil properties, justifying the use of the former for previous modelling of soil leaching fraction with the SALF model. There was generally good agreement between categories of ECa values and the leaching hazard zones derived from SALF and shown on the 1:50 000 soil map. Several apparent anomalies were identified between boundaries for leaching hazard on the soil map and those inferred from more detailed ECa data. These differences were attributed to the relatively large scale of the soil map, compared to close range ECa data and possible generalisation of soil properties across the various manifestations of comparable soil associations for the modelling. Eight of the 11 drill sites supported NO3-N loadings between 20 and 60 kg/ha in a 6 m profile, but two sites contained 198 and 320 kg/ha each. The 0–3 m zone of the latter two sites contained 89 and 131 kg N/ha and this is potentially accessible to roots of sugarcane. The high NO3-N profiles supported ‘bulges’ in nitrate between 2 and 3 m that were related to changes in soil properties. EMI instruments have a clear role in detailed identification of leaky soils so that more site specific nutrient and water management strategies can be implemented to reduce leaching of nitrate into the aquifer.