Alkalinity movement down acid soil columns was faster when lime and plant residues were combined than when either was applied separately

Subsurface soil acidity is a serious constraint to crop production and is inherently difficult to correct through conventional application of lime. Thus, new approaches to ameliorate subsurface soil acidity are needed. A column leaching experiment was established to determine whether the plant residues, when combined with lime, could facilitate lime dissolution and alkalinity movement down soil columns (10 cm in diameter × 45 cm long) to ameliorate acid subsurface soil layers. Five plant residues from canola (Brassica nappus L.), field pea (Pisum sativum L.), lucerne (Medicago sativa L.), oats (Avena sativa L.) and vetch (Vicia villosa L.) (C/N ratios of 52, 13, 16, 53 and 12, respectively) were mixed with soil (18 g dry matter kg−1) in the top of soil columns (0–10 cm) either limed (target pH 7 in CaCl2) or non-limed and incubated for 3 months at 25°C. Soil columns were leached six times over the 3-month incubation period and destructively sampled at 1 and 3 months for chemical analysis. The soil amended with plant residues created favourable pH gradients and facilitated downward movement of alkalinity when lime was added. However, net increases in pH in the 10–12-cm layer after 3 months were only observed in two non-legume residue-amended treatments (canola and oat residues, 0.22–0.43 pH unit increase), but not in three legume residue-amended treatments (field pea, lucerne and vetch residues), although those treatments had the greatest pH increase in the short term (1 month). In conclusion, surface incorporation of lime in combination with plant residues accelerated the amelioration of subsurface acidity in the immediate zone (10–12 cm) below the amended layer. Canola and oat residues were superior in terms of increased soil pH over a longer term (3 months), possibly due to their higher C/N ratios. Highlights: Can plant residues, combined with lime, facilitate alkalinity movement down soil columns? Few studies have examined leaching, alkalinity movement and pH gradients using agricultural soils. Lucerne and vetch had the greatest pH increase (1 month) but canola and oat persisted (3 months). Alkalinity movement was faster when lime and plant residues were combined than when applied alone.