The leaching of N fertilisers has led to the formation of nitrate (NO₃) accumulations in deep subsoils (>5 m depth) of the Johnstone River catchment. This paper outlines the chemical mechanism by which these NO₃ accumulations are formed and maintained. This was achieved via a series of column experiments designed to investigate NO₃ leaching in relation to the soil charge chemistry and the competition of anions for exchange sites. The presence of variable charge minerals has led to the formation positive surface charge within these profiles. An increase in the soil solution ionic strength accompanying the fertiliser leaching front acts to increase the positive (and negative) charge density, thus providing adsorption sites for NO₃. A decrease in the soil solution ionic strength occurs after the fertiliser pulse moves past a point in the profile, due to dilution with incoming rainwater. Nitrate is then released from the exchange back into the soil solution, thus buffering the decrease in the soil solution ionic strength. Since NO₃ was adsorbed throughout the profile in this experiment it does not effectively explain the situation occurring in the field. Previous observations of the sulfate (SO₄) profile distribution indicated that large SO₄ accumulations in the upper profile may influence the NO₃ distribution through competition for adsorption sites. A subsequent experiment investigating the effect of SO₄ additions on NO₃ leaching showed that NO₃ adsorption was minimal in the upper profile. Adsorption of NO₃ did occur, though only in the region of the profile where SO₄ occupancy was low, i.e. in the lower profile. Therefore, the formation of the NO₃ accumulations is dependent on the variable charge mineralogy, the variation of charge density with soil solution ionic strength, and the effects of SO₄ competition for adsorption sites.