Studies of earthworm populations in orchards

Abstract

SUMMARY The amount of leaf litter buried by worms between leaf fall and spring was measured in two stages in five grass and two arable orchards near Wisbech, Cambs. A known number of weighed leaves was placed on the ground beneath wire cages at leaf fall and again several weeks later and those remaining at the end of each stage were counted and weighed. Unburied leaves lost about one‐quarter of their dry weight during each stage. Buried leaves were pulled into burrows by Lumbricus terrestris (Linn.), apparently the only species present that did so. Populations of L. terrestris beneath the cages were estimated from the number brought to the surface by dilute formalin. In grass orchards, the number and weight of leaves buried was closely correlated with the estimated fresh weight of L. terrestris. Some orchards had 3/4 to 1 ton of L. terrestris per acre and in these over 90% of the normal leaf fall (1/2 ton dry weight) was buried during winter. Earthworm activity could account for the observed amounts of leaf litter removed in winter and observed differences between orchards could be attributed to differences in their L. terrestris population. Apple leaves were buried more rapidly in arable than in grassed orchards, probably because they were the only ones and were easier to find and bury when there was no grass. Evidence to support this came from one grass orchard where the number of leaves buried from each cage was inversely proportional to the amount of surface vegetation. Twice as many leaves were buried in cages initially containing 200 leaves as in cages containing 100 leaves. The rate at which L. terrestris buried leaves depended on soil temperature, and burial probably stops at 2° C, which is about 1.6° C. below the lowest weekly average temperature reached in Wisbech soils in winter. The long‐term effect of leaf burial was shown by the contrasting soil profiles of a typical grass orchard and one where L. terrestris is rare, where the crumb structure is poor and where the accumulated remains of up to 15 years leaf fall now form a surface mat 1/2 ‐1 1/2 in. thick.