Soil Phosphorus-Proton Coadsorption Studies And The Implications For Fertiliser Recommendations

Despite extensive research, the behaviour of phosphorus (P) in soils, a crucial element in plant production, is not yet fully understood. This study focussed on one of the outstanding issues, the co-adsorption of protons with phosphate in soils. Previous work on the co-adsorption of protons with P (as H2PO4-) on goethite observed an accumulation of solution acidity from a sub-stoichiometric H+:P sorption ratio. If these results also occur in soils, this could lead to accelerated acidification and present a significant challenge for sustainable agriculture. This highlights the need for congruent H+:P co-adsorption analyses to be undertaken for soils. This study investigated whether similar results occurred with the adsorption of orthophosphate on six Australian soil samples of varying phosphorus binding capacities and mineralogies. Considerable effort was invested in the development of a widely applicable laboratory method to quantify the soil H+:P sorption coefficient as part of the study. The method functioned by separately measuring the molar quantities of protons and phosphorus lost from a soil suspension when spiked with a range of initial P concentrations (as KH2PO4). Using this procedure it was found that H2PO4- sorption followed an apparent 2:1 ratio (2 protons adsorbed for each unit of P) for the majority of soil samples (average deviation from 2:1 = ± 5.8%). This the first time such measurements have been completed on soils. No significant pH change was measured in the soil suspensions post P addition and equilibration. Past P adsorption data on the same soils (supplied by NSW DPI) yielded similar co-adsorption results. Trials of the developed method on goethite were able to replicate a highly linear P-dependent pH decrease (r2 = 0.996) and a H+:P co-adsorption coefficient significantly less than 2:1 (P < 0.01), consistent with previous literature. To clarify whether ion exchange contributes to the observed H+:P sorption ratio, supernatants were analysed for major cations (Na+, Ca2+, Mg2+) and SO42-, but the results were inconclusive, predominantly due to a small sample size. The implications of H+:P sorption ratio for long term pH stability of soils with P fertilisation are immense and an area where future research on the topic is required. Clarification is also needed on whether the sorption ratio of commercial P fertilisers will mimic laboratory KH2PO4 behaviour, and if the same proton ratio holds true under field conditions.