Comparative Efficacy of Alkaline Slag, Biomass Ash, and Biochar Application for the Amelioration of Different Acidic Soils

This study aimed at comparing the multi-functions of by-product amendments for ameliorating acidic soils, namely, liming effect, aluminum immobilization and fertility improvement. An incubation experiment was conducted after mixing acidic soils with alkaline slag (AS), biomass ash (BA) and biochar (BC) at similar amounts of alkaline substances input, based on their Acid Neutralizing Capacity (ANC). Soil pH, exchangeable acidity, aluminum (Al) species, soluble and exchangeable basic cations were determined. Among the three amendments, BA application induced the highest soil pH increment in all three acidic soils, although the same amounts of ANC input. BA increased soil pH by 0.28–1.18 units more than AS and BC at 50 mmol kg−1 ANC input. The active alkalis, such as Ca(OH)2, contributed to the liming efficacy of BA. The decrease of soil active Al after amendments application was consistent with the increment of soil pH. The potential active Al pool and amorphous Al content in BC treatments were lower than that in AS and BA treatments, indicating that the stability of immobilized Al by BC was higher than BA and AS. Additionally, BC comprehensively increased the available K+, Ca2+, and Mg2+ in acidic soils. The ratio between soluble and exchangeable basic cations in acidic soils was also favorably optimized by BC. BA, BC and AS showed multi-functions of ameliorating acidic soils. BA presented the greatest liming efficacy due to its highest active alkalinity. BC played an important role in adjusting the supply and retention of basic cations in acidic soils.