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Recent advances in fractal theory may be applicable to the characterization of soil structure. This study explored two such applications. Aggregates were assumed to be approximated by cubes of constant dry density. Under this condition, the fractal dimension, D, provides a measure of fragmentation. The value of D increases with increasing fragmentation. The influence of energy input (wet sieving) on the D of aggregates from a Conestogo silt loam soil (finesilty, mixed, mesic Aquic Eutrocrept) was investigated. Aggregates were obtained from five cropping treatments, ranging from 15 yr of continuous corn (Zea mays L.) to 15 yr of continuous bromegrass (Bromus inermis Leyss.). The value of D was estimated from cumulative size‐frequency distribution data plotted on a log‐log scale. Computed values of D ranged from 2.51 to 3.52. Energy input, cropping history, and their interaction all had a significant effect on D. Fragmentation increased with energy input and time under corn production. Relations between D and the breakdown of individual aggregates were investigated. A scale‐invariant breakdown model was developed and tested. The model permits calculation of apparent probabilities of failure, P, as a function of aggregate size, x. Stepwise multiple regression analysis selected the rate of change in P with x as the best predictor of D following energy input. Fractal theory offers potential for modeling aggregate breakdown, as well as characterizing the degree of fragmentation.

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Poultry production in Kentucky increased almost 200% between 1991 and 1995. Their waste is typically land applied, and fecal pathogen runoff and infiltration may cause nonpoint source groundwater pollution. We looked at the preferential flow of fecal coliforms through undisturbed soil blocks since fecal bacteria typically infiltrate the soil profile to contaminate groundwater. Poultry manure was uniformly distributed on top of sod-covered or tilled (upper 12.5 cm) soil blocks and the blocks were irrigated. Drainage was collected in 100 uniformly spaced cells beneath each block and analyzed for fecal coliform content and drainage volume. The spatial distribution of drainage and fecal eoliforms through the soil blocks was not uniform. Fecal coliforms appeared where most drainage flowed. Drainage water from each soil block consistently exceeded 200 000 fecal coliforms per 100 mL and was as great as 30 million fecal coliforms per 100 mL of leachate collected. Fecal coliforms leached as a pulse, but the breakthrough of fecal coliforms through tilled blocks was delayed with respect to the breakthrough of fecal coliforms through sod-covered blocks. Rainfall on a well-structured soil will cause the preferential movement of fecal bacteria, even with unsaturated flow conditions, and could contribute to fecal coliform concentrations in shallow groundwater that exceed standards for domestic discharge and primary contact water in Kentucky (200 fecal coliforms/100 mL).

Temporal fluctuations in soil structural stability within cropping treatments are often as large as differences between crops during the growing season. The relative importance of soil moisture, roots, and microbial biomass as factors contributing to this variation were investigated. Research was conducted on a fine‐silty, mixed, mesic Typic Eutrochrept intergrading to a fine‐silty, mixed, mesic Typic Haplaquept soil near Elora, ON. Six perennial forage treatments, established 2 yr previously, were compared with spring‐seeded conventional and zero‐till corn (Zea mays L.). The plow layer of each cropping treatment was sampled at monthly intervals. A combination of wet sieving and turbidimetry was used to provide a rapid assessment of wet aggregate stability (WAS) and dispersible clay (DC), respectively. On average, the forages had significantly less DC and greater WAS than the two corn treatments. Structural stability was found to decrease with increasing soil water content (r = 0.80 for DC and r = 0.74 for Was, both significant at P = 0.01). The corn soil experienced less extreme drying in the spring. Stepwise multiple‐regression analyses selected soil moisture and microbial biomass (in that order) as significant predictors of structural stability within the growing season. Using both parameters, it was possible to explain up to 85% of the temporal variation in DC and WAS on a by‐treatment basis.

The development of structure in soils is also important for controlling wind and/or water erosion processes (Lal The development of well-structured soils is a goal for achieving and Elliott, 1994;Skidmore et al., 1994). For example, sustainable and productive agricultural systems. Nevertheless, the maintenance of soil structure in continuous no-till (NT) soils has the presence of large stable soil fragments in seedbeds sometimes been thought to induce soil conditions that are detrimental promotes the occurrence of surface roughness, reducing to crop yields. The objectives of this research were to characterize soil erodibility by wind (Chepil, 1953) as well as water the effects of periodic tillage disruption in otherwise NT systems on erosion (Deizman et al., 1987). Well-structured soils soil properties and the yields of winter wheat (Triticum aestivum L.), have higher infiltration rates and can maintain their pore double-cropped soybean [Glycine max (L.) Merr.], and maize (Zea M. Díaz-Zorita,