⎛ ∑ wi ln xi GMD = exp⎜ ⎜ ∑w i ⎝
⎞ ⎟ ⎟ ⎠
(4)
where wi is the weight of the aggregates of each size class g and lnxi the natural logarithm of the mean diameter of size classes. In a review of many field trials of conservation tillage on light textured soils in southern Australia, Chan et al. (2002), observed that although storage of soil organic carbon is greater in sites that are direct drilled, compared to conventional cultivation, the differences are only significant in higher rainfall areas (>500 mm), and that soil quality is likely to remain fragile in lower rainfall zones. Increases in soil carbon content are usually limited to the topmost soil layer. Organic matter increases soil aggregation due to organic carbon produced by decomposition. Additions of organic matter lead to increase in the macropores (Garnier et al., 2004). In agriculture, the soil is disturbed periodically under tillage; it is fractured into clods and fragments as well as having its natural aggregates separated. It is pulverized to a range of aggregate sizes that varies with the purpose of the tillage operation (Marshall and Holmes., 1979). Hewitt and Dexter (1980) reported that aeration and porosity in conventionally tilled plots were 30% higher than in minimally tilled soils. Mean void size was 17% higher, while mean aggregate size was 30% larger in minimally tilled than conventionally tilled plots. Ghuman and Sur, (2001) reported a significantly greater MWD of soil aggregates in the minimally tilled as compared to conventionally tilled treatments. Similarly, geometric mean diameter was significantly higher in the minimally tilled than the conventionally tilled treatment. Angers et al. (1993) positively correlated the improvement in soil structure stability with microbial biomass and water soluble carbohydrates, both of which also influence infiltration. Further, aggregation improvement in the minimally tilled treatment was also associated with the significant increase in organic carbon content and protection of the surface layer by crop residue mulch against the action of falling raindrops (Lal, 1989). Their results indicated that under conservation tillage treatments the aggregation of the surface layers improved (Ghuman and Sur, 2001). 18