Effects of agriculture on earthworm populations
It is now well established that grassland usually contains more earthworms than arable land (Tables 5, 7 and 10). This could be due to mechanical damage during cultivation, to the loss of the insulating layer of vegetation, or to a decreased supply of food as the organic matter content gradually decreases. Many workers have considered that these differences are largely due to mechanical damage during cultivation. When old grassland is ploughed, the number of earthworms in it steadily decreases with time after ploughing (Graff, 1953). In one such study (Evans and Guild, 1948), five years after grass was ploughed, the earthworm population had declined by 70%, although the population was unchanged by the first ploughing of the sward, so it is unlikely that mechanical damage was a primary cause of the decreased numbers of worms. Hopp and Hopkins (1946) also reported that cultivation of arable land in late spring did not decrease earthworm numbers. Indeed, it would be surprising if mechanical damage by ploughing was very important, because the plough merely turns the soil over, and probably has little effect on worms with deep burrows. Preparation of seed beds by rotary cultivation, harrowing, disking or rolling can be expected to damage more earthworms, but the regenerative powers of earthworms are so great that only a few would be killed outright. Edwards and Lofty (1971) investigated the effects of maximal and minimal cultivation of grass plots, on earthworm populations. They compared plots ploughed and cultivated in spring with others that were unploughed. The more the soil was cultivated during the first two seasons, the greater was the number and weight of earthworms in the soil (Table 20).
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