THE INFLUENCE OF ORGANIC-MATTER IN REDUCING THE DESTABILIZATION OF SOIL BY SIMULATED TILLAGE

Different agricultural practices can result in a decline in soil organ ic carbon (SOC) and a consequent reduction in soil structural stabilit y. Experiments were conducted on soils with a range of SOC values, to quantify the destabilizing effects of increased tillage intensity. Dif ferent tillage intensity was simulated with the use of a falling weigh t, where specific energy levels, similar to those experienced during t illage, were reproduced. The level of destabilization was assessed by the quantity of mechanically dispersed clay (using a turbidimetric tec hnique) and the quantity of water-stable aggregates (WSA) > 0.25 mm re maining after being shaken in water. The quantity of clay dispersed in creased with increasing water content, in the absence of any mechanica l pretreatment, the rate of increase rising sharply with declining SOC . Following simulated tillage, and at water contents above the plastic Limit, clay dispersion increased in proportion to the energy of disru ption, and also increased with decreasing SOC levels. Below the plasti c limit all the soils were relatively insensitive to mechanical disrup tion. A simple empirical model was derived to link clay dispersion to SOC, water content and energy of disruption. The proportion of WSA dec lined sharply with decreasing SOC, and to a lesser extent following ti llage. The quantity of WSA following simulated intensive tillage (300 J kg(-1)) of grassland (SOC, 2.8-3.2 g (100 g)(-1)) was greater than t hat present, prior to tillage from fallow, arable and arable/ley rotat ion treatments (SOC 1.1-2.5 g (100 g)(-1)). Aggregate tensile strength was found to be relatively insensitive to differences in SOC. However , variations of strength within treatments, an indicator of soil friab ility, increased in proportion with SOC. A turbidity index was derived in which the turbidity of natural and remoulded aggregates was compar ed. Variation of this index with increasing mechanical energy is used as an indicator of the sensitivity of soils to damage during tillage. A visual representation is constructed to Link the sensitivity of soil s to damage during tillage with both SOC and water potential. These ex periments illustrate that management practices, which lead to a long t erm reduction in SOC, are responsible for an increase: in aggregate st rength and reduction in stability plus an increase in sensitivity of s oils to structural decline following subsequent tillage. (C) 1997 Else vier Science B.V.