Abundance, casting activity, and cast quality of earthworms in an acid Ultisol under alley-cropping in the humid tropics

Abstract

The density of earthworms, their surface cast production and the composition of surface casts were studied on an acid Ultisol in S.E. Nigeria. Maize and cassava were grown as food crops, continuously or every other year in a maize/fallow rotation, in alley-cropping systems with Flemingia congesta or Dactyladenia barteri as hedgerow trees. Except in the unproductive Flemingia continuous cropping system, > 60 earthworms per square metre were recorded in spite of the low soil pH(H 2O) of 4.1, but biomass was low (approximately 2 g m −2). Surface casts consisted of tubular, up to 3 cm high casts of Pontoscolex corethrurus and some other large earthworm species and small granular casts of Eudrilidae earthworms. The standing mass of casts after 5 months of activity (May–September) ranged between 260 and 570 kg ha −1. Tubular cast production between September and December dropped from 97-18 g m −2 day −1. In spite of the lower biomass than reported in literature for less acid soils, and in light of the low nutrient status of the soil, the estimated amounts of C and nutrients returned to the surface in casts were considerable: 750 kg C, 51 kg N, 16 kg Ca, 2.5 kg Mg and 2 kg K per hectare per year on average.