EFFECTS OF ROCK FRAGMENTS ON PHYSICAL DEGRADATION OF CULTIVATED SOILSBY RAINFALL

To understand better the role of rock fragments in soil and water cons ervation processes, the effects of rock fragments in maintaining a fav ourable soil structure and thus also in preventing physical degradatio n of tilled soils was studied. Laboratory experiments were conducted t o investigate the effects of rock fragment content, rock fragment size , initial soil moisture content of the fine earth and surface rock fra gment cover on soil subsidence by rainfall (i.e. change in bulk densit y by one or more cycles of wetting and drying). A total of 15 rainfall simulations (cumulative rainfall, 192.5 mm; mean intensity, 70 mm h(- 1)) were carried out. Before and after each rainfall application the s urface elevation of a 19-cm thick plough layer was measured with a las er microrelief meter. In all experiments, the bulk density of the fine earth increased with applied rainfall volume to reach a maximum value at about 200 mm of cumulative rainfall. From the experimental results it was concluded that the subsidence rate decreased sharply for soils containing more than 0.50 kg kg(-1) rock fragments, irrespective of r ock fragment size. Fine earth bulk densities were negatively related t o rock fragment content beyond a threshold value of 0.30 kg kg(-1) for small rock fragments (1.7-2.7 cm) and 0.50 kg kg(-1) for large rock f ragments (7.7 cm). Initial soil moisture content influenced subsidence only in the initial stage of the experiments, when some swelling occu rred in the dry soils, Surface rock fragment cover had no significant effect on subsidence of the plough layer. Therefore, subsidence of the plough layer in these experiments appears to be mainly due to changin g soil strength upon drainage rather than the result of direct transfe r of kinetic energy from falling drops, The relative increase in poros ity of the fine earth as well as the absolute increase in macroporosit y with rock fragment content will cause deeper penetration of rainfall into the soil, resulting in water conservation. Therefore, crushing o f large rock fragments into smaller ones is to be preferred over remov al of rock fragments from the plough layer.