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

Lucerne (Medicago sativa. L) root elongation in acid soils amended by gypsi ferous 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 containi ng calcium equivalent to 5000 kg/ha of CaCO3. Lucerne was grown on each col umn after it was leached with 400 mm of water. Whereas the lime treatment h ad no erect 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 effe ct' induced by sulfate adsorption. Regression analysis indicated that the m olar 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 (R-2 = 0.94). In contrast, topsoil incorporated amendments did not influence root penetration into the acidic subsurface of the Kaawa soil, wh ich is dominated by permanently charged clay minerals. The `self-liming ere ct' caused by gypsum application is not a sustainable practice. Lime should be applied to neutralise the topsoil acidity, when gypsum is used as subsu rface soil acidity ameliorant. FBA, which contains both lime and gypsum, ca n meet these requirements.