The influence of surface incorporated lime and gypsiferous by-products on surface and subsurface soil acidity. II. Root growth and agronomic implications

Abstract

Lucerne (Medicago sativa. L) root elongation in acid soils amended by gypsiferous coal combustion by-products was investigated in a glasshouse study. Lime, fluidised bed boiler ash (FBA), and flue gas desulfurisation gypsum (FGDG) were mixed into the surface 50 mm of either an Allophanic (the Patua sand loam) or an Ultic (the Kaawa clay loam) soil column, at rates containing calcium equivalent to 5000 kg/ha of CaCO₃. Lucerne was grown on each column after it was leached with 400 mm of water. Whereas the lime treatment had no effect on root elongation in the acidic subsurface of the Patua soil, the FBA and FGDG treatments significantly improved lucerne root penetration into the subsurface soil (P < 0·05). This was due to the ‘self-liming effect’ induced by sulfate adsorption. Regression analysis indicated that the molar ratio of labile monomeric aluminium and calcium in soil solution (Al : Ca) was a good indicator of the degree of root growth into subsurface soil layers (R²= 0·94). In contrast, topsoil incorporated amendments did not influence root penetration into the acidic subsurface of the Kaawa soil, which is dominated by permanently charged clay minerals.

The ‘self-liming effect’ caused by gypsum application is not a sustainable practice. Lime should be applied to neutralise the topsoil acidity, when gypsum is used as subsurface soil acidity ameliorant. FBA, which contains both lime and gypsum, can meet these requirements.