SUBSOIL acidity is a serious problem in many tropical and sub-tropical soils1–3. The high acidity, low calcium contents and often toxic levels of soluble and/or exchangeable aluminium severely impair plant-root development in these soils1–3. The relative immobility of surface-applied liming materials limits their ability to reduce subsoil acidity. Recently, the use of gypsum or phosphogyp-sum has been advocated as an alternative to lime3–7. On the other hand, it is well known that humic substances can mobilize and form complexes with metals in soils8–11. Here we report that a newly available, coal-derived calcium-fulvate is highly efficient as a carrier of calcium in the soil profile. Moreover, subsoil pH was considerably higher when calcium-fulvate was applied to the soil surface, than when gypsum, calcium-EDTA, Ca(OH)2 or CaCO3 were applied.
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van der Watt, H., Barnard, R., Cronje, I. et al. Amelioration of subsoil acidity by application of a coal-derived calcium fulvate to the soil surface. Nature 350, 146–148 (1991) doi:10.1038/350146a0
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