Positive charge discovered across Western Australian wheatbelt soils challenges key soil and nitrogen management assumptions

Nitrogen management in Western Australia (WA) and in cropping areas elsewhere in Australia assumes that soil contains negligible or no positive charge and is therefore unable to retain nitrate against leaching. The amount of water needed to displace nitrate is thus assumed to be the drainable volume of water held by the soil (1 pore volume), and in sandy soils about 100 mm drainage is assumed to be required to displace nitrate by 1 m. The clay mineralogy of the highly weathered soils of the WA wheatbelt is dominated by kaolinite and iron and aluminium oxides. This mineralogy suggests likely occurrence of positive charge and anion exchange capacity (AEC), since these minerals can carry positive charge under normal acidic field situations. We measured AEC of soils sampled widely across the WA wheatbelt by independent leaching and batch equilibration methods of charge measurement. This showed widespread occurrence of positive charge and AEC in these soils. AEC ranged from 0 to 2.47 mmolc/kg and is linearly correlated with the potassium chloride or monocalcium phosphate extractable sulfate content of the soil. This correlation provides a rapid screening method to identify soils with positive charge. Application of ion-chromatographic theory showed that AEC has a large effect in delaying nitrate leaching by up to 12.5 pore volumes. The most highly charged soil (2.47 mmolc/kg) thus needed 12.5 times more water to displace nitrate than currently assumed. This potentially large delay in nitrate leaching affects the optimum amount and time of fertiliser application, rates of soil acidification attributed to nitrate leaching and the benefit of ameliorating subsoils to allow roots access to subsoil water and leached nitrate. It also calls into question the use of anions such as bromide to trace water flow and estimate recharge in these soils.