THE IMPACT OF 20 YEARS ABSENCE OF EARTHWORMS AND 3 LEVELS OF N-FERTILIZER ON A GRASSLAND SOIL ENVIRONMENT

Replicated plots of a sward sown to perennial ryegrass (Lolium perenne) growing at Hurley on a soil of the Griston series were given 188, 376 or 752 kg N fertilizer ha-1 year-1 for a period of 20 years. Half of each plot at the lowest and highest application rate of N fertilizer was treated frequently with the pesticide phorate, which kept those halves almost free of earthworms and other invertebrates. Measurements of several soil parameters were made during and at the end of the 20-year period. The pesticide treatment, i.e. absence of earthworms, dramatically increased soil bulk density, shear strength, penetrability and depth of leaf litter, but greatly reduced soil organic matter content, initial infiltration rate, pH and soil moisture content. Increasing the level of nitrogen fertilizer increased soil shear strength and penetrability, but had little or no significant effect on the other parameters measured. The soil profile was examined in detail to a depth of 1.5 m at the junction of a low-N-level pesticide-treated sub-plot and an untreated sub-plot. The main changes in the soil profile attributable to the pesticide treatment were confined to the topsoil. The leaf litter layer (F horizon) on the treated plot was pronounced and some 2-3 cm deep. The topsoil on treated plots had a more developed coarse angular blocky structure and was more brittle. There was no visible earthworm activity in the treated plot.