SOIL NITRATE RETENTION

Soils containing positively charged surfaces will delay nitrate leaching, and current leaching models based on an assumption that WA soils only contain negative charge could result in unreliable nitrogen recommendations. This is likely to have an adverse impact on the grower's fertiliser use and profits. Both positive and negative charge occur in variable amounts on iron and aluminium oxide and kaolinite surfaces, depending on ambient soil solution pH and electrolyte concentration, and the amount of specifically bound anions such as phosphate held on their surfaces (Bolland et al. 1976; Bowden et al. 1977; Qafoku et al. 2004; Donn and Menzies 2005a, 2005b). These minerals are said to have variable charged surfaces, with positive charge density measured as anion exchange capacity (AEC) increasing at lower pH values and at higher electrolyte concentration. Positive charge retains the opposite nitrate charge electrostatically. This is referred to as nitrate retention or nitrate adsorption. Nitrate adsorption delays nitrate leaching. For example, field measurement on an acidic sandy kaolinitic soil with a small amount of goethite showed that nitrate leaching was delayed beyond that expected for a given amount of water. Nitrate leaching required 2.5 pore volumes (1 pore volume was in this case the drainable volume of water held by the soil at field capacity) of water for displacement (Wong et al. 1987). AEC increased from 0.2 to 0.8 [mmol.sub.c]/kg down this soil profile and only small AEC resulted in large delays (Wong et al. 1990a).

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