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Using Earthworms to Incorporate Lime into Subsoil to Ameliorate Acidity

Acid soils threaten agriculture in Australia as well as in many parts of the world by reducing plant productivity. Surface-applied lime (CaCO3) moves into the root zone only slowly but the process can be considerably hastened in the presence of earthworms. In a laboratory experiment, the abilities of three earthworm species in incorporating lime into the subsoil and the mechanisms involved were investigated. Aporrectodea longa was the most effective in moving surface-applied lime, increasing subsoil pHCa by 1.1 units (from 4.0 to 5.1) in the 7.5–10 cm layer after only 6 weeks. Aporrectodea trapezoides and a native anecid, Anisochaetae sp., were less effective than A. longa, but still increased pHCa by 0.7 unit in the 7.5–10 cm layer. The extent of lime incorporation by the different earthworms was unrelated to burrow size and abundance, but significantly related to the increase in water stable aggregates (>250 μm) found in the earthworm-inoculated soils. The water stable aggregates, largely in the forms of earthworm casts and burrow wall material (drilosphere) had significantly higher pH (>6) than the bulk soil (4.0). The results suggest that significant amounts of surface-applied lime can be carried down the soil profile by the earthworms either via ingestion and subsequent deposition as casts at depth or by directly attaching to the body surface.