Total Potential Acidity Research Articles

ACTUAL AND POTENTIAL PYRITIC ACIDITY OF NSW CANELANDS

A peroxide oxidation combined acidity and sulfate method (POCAS test) for confirming actual and potential acidity of acid sulfate soils (ASS) has national endorsement in Australia. Analytical outputs include total actual acidity (TAA) and sulfate-S (% SCl). Outputs following treatment of the soil with hydrogen peroxide include total potential acidity (TPA) and total potential sulfate-S (% Sox). All can be expressed as kg H2SO4 tonne−1. This paper reports POCAS test results from an interactive, 1996–1999 survey of low-lying areas of 446 NSW cane farms (soil profile depths of ≤0.25, 0.25–0.5, 0.5–0.8, 0.8–1.2, 1.2–1.5 m). The survey covered the Condong, Broadwater and Harwood mill regions. The grand median level of TAA to 1.5 m at Condong was 2.96 kg H2SO4 tonne−1, almost three-times higher than the grand median for Broadwater (1.04 kg H2SO4 tonne−1) and over six-times the grand median for Harwood (0.46 kg H2SO4 tonne−1). Corresponding grand median levels of TPA were highest at Condong (9.24 kg H2SO4 tonne−1). This was around 2.1 and 3.9 times higher than at Broadwater and Harwood. The difference between TAA and corresponding TPA values, expressed as a percentage of TPA, increased with increasing profile depth, particularly beyond 0.8 m. At 0.25–0.5 m, the percentage differences were 45, 56 and 60 for Condong, Broadwater and Harwood, respectively. Corresponding percentages at 1.2–1.5 m were 98, 96 and 98, indicating very little oxidation of pyrite had occurred at depth. The data for SCl and Sox confirmed the presence of a considerable potential ASS hazard below 0.8 m relative to soil-profile segments nearer the surface. Soil texture class (fine, medium, coarse) had inconsistent effects on the percentage of pyrite oxidized. The findings support management practices that minimize the quantity of pyritic drain spoil reaching the surface during drain construction and maintenance, while any pyritic material mobilized during these operations should be neutralized to prevent strong acid release to the environment.

Assessment of peroxide oxidation for acid sulfate soil analysis. 2. Acidity determination

Total sulfidic acidity (TSA) and total potential acidity (TPA) are derived from peroxide oxidation of acid sulfate soil materials (ASS), and are measures of the sulfidic acidity and the net acidity (net acidity = sulfidic acidity + actual acidity – acid neutralising capacity), respectively. The TSA and TPA of 4 ASS materials were determined using a variety of peroxide oxidation procedures and compared with the sulfidic acidity and net acidity derived from the use of an acid–base accounting model. TSA and TPA values both varied greatly with each peroxide oxidation method used, and both measures were found to substantially underestimate (i.e. by 23–85%) both sulfidic acidity (as determined from the chromium reducible sulfur content) and net acidity (as determined by acid–base accounting). A major cause of this underestimation of acidity was the retention of acidity through the precipitation of jarosite during peroxide oxidation. Substantial clay mineral dissolution appears to have occurred during peroxide oxidation of the ASS materials, as indicated by increased soluble aluminium. Such dissolution may contribute to the underestimation of both sulfidic and net acidity for the ASS materials using peroxide oxidation methods. The loss of acidity to the atmosphere was identified as a possible additional interference. This study shows the peroxide oxidation methods examined here are subject to substantial interferences, which caused large underestimations of acidity, and consequently, are unable to reliably provide accurate measurements of sulfidic and net acidity in ASS materials. pyritic sulfur, total potential acidity, total sulfidic acidity, net acidity, jarosite, acid budget, acid neutralising capacity.

Spatial Variability of Soil Actual and Potential Acidity in the Mangrove Agroecosystem of West Africa

AbstractSpatial variability of soil acidity in coastal lowlands results from a complex interaction of climate, coastal morphology, river hydrology, vegetation, landform, and tidal flooding. This study was conducted to determine whether the causal factors of soil acidification can be related to soil total actual acidity (TAA) and total potential acidity (TPA) in 12 sites selected along four river basins in West Africa. A hierarchical framework was designed corresponding to the scale at which each factor has the greatest influence on acidification. In the dry season of 1991, soil samples to be analyzed for TAA and TPA were taken from three strips within each river basin, perpendicular to the river at different distances from the mouth, following a 40 by 20 m grid, at five soil depths. The contribution of the different causal factors of acidity spatial variability was analyzed with a nested analysis of variance (ANOVA) and related to the hierarchical framework. Geostatistics were used to study spatial variability at the most detailed scale. We defined main ecoregions identified between watersheds at the macroscale, subenvironments identified with distance from the river mouth within watersheds, and zones identified with positions within toposequences. Practical implications for water management in acid sulfate were developed. At the macroscale, a broad subdivision of the study area into two ecoregions was possible, at the mesoscale, site position along rivers significantly predicted a river‐dependent longitudinal variability of soil acidity, whereas at the microscale, kriged maps showed different patterns of soil acidity.