Nitrogenous fertilizers and earthworm populations in agricultural soils

The influence of various inorganic and organic fertilizers was assessed in three long-term “classical” experiments and two short-term field experiments, one on grass and one on wheat. The long-term experiments included Broadbalk which had grown continuous wheat since 1843, Barnfield, continuous root crops since 1843 and Park Grass, continuous grass since 1836. Annual fertilizer treatments were farmyard manure (48 and 96 kg N ha−1), various forms of inorganic nitrogen (48, 96, 144 and 192 kg N ha−1), liquid and solid sewage sludge and sewage cake in a wide range of doses. In the three arable experiments, all species of earthworms were more numerous in plots treated with organic fertilizers than in untreated plots. There was a strong positive correlation (r = 0.9825) between amounts of inorganic N applied and populations of earthworms. Plots receiving both inorganic and organic N had the largest populations of earthworms. The effects of both inorganic and organic N were much less on earthworm populations in grassland than on those in arable crops, even in the long-term experiments, and there was some evidence of adverse effects when an excessive amount of liquid sludge was applied in a single dose. Effects of organic fertilizers were greater on populations of Lumbricus terrestris than on those of Allolobophora longa, A. caliginosa or A. chorotica.