The importance of soil carbon and nitrogen for amelioration of acid sulphate soils

Abstract

When exposed to air and adequate moisture, soils containing sulphides (sulphidic soils with pH > 4) become oxidized and generate sulphuric acid to form 'sulphuric soils' (pH < 4). Treatment of this acidity is undertaken by addition of lime. In this study, we investigated the effectiveness of adding plant organic matter, and simple carbon and nitrogen compounds, as alternatives to lime to sulphuric and sulphidic soils. In sulphuric soils under aerobic conditions, organic matter increased pH, the extent depending on the nitrogen content. Lucerne hay, which had the largest nitrogen content, increased the pH from 3.7 to 8.0, while pea straw and wheat straw effected smaller changes, in proportion to their respective nitrogen contents. Lucerne hay also caused the greatest reductions in soil redox potential and sulphate content, consistent with the action of sulphate-reducing bacteria. Similarly, incorporation of organic matter under aerobic conditions effectively prevented sulphidic soil acidification and reduced the redox potential and sulphate content. The individual effects of carbon and nitrogen compounds were then examined and compared to plant organic material. Glucose was ineffective at both small and large concentrations, while molasses increased the pH slightly to 4.6 and acetate to 5.9. None of these carbon compounds was as effective as complex organic matter. Nitrogen added alone as nitrate or ammonia had little or no effect on pH, whereas organic nitrogen in the form of urea caused the pH to rise to 6.3 and reduced the redox to less than 0 mV but had no significant effect on sulphate content.