Soil Aggregate Longevity as Determined by the Incorporation of Ceramic Spheres
1992
Staricka, J. A. | Allmaras, R. R. | Nelson, W. W. | Larson, W. E.
Soil aggregates influence the biological environment of the tilled layer but their rate of formation and breakdown is unknown. We estimated the life expectancy of aggregates in the field from the rate at which inert tracers were incorporated into soil aggregates of a Ves loam (a fine-loamy, mixed, mesic Udic Haplustoll). Ceramic spheres (1-3-mm diam.) were surface applied before primary tillage in the fall. Ten-kilogram soil samples taken after tillage were air dired and rotary sieved into six aggregate-diameter classes. The initial tillage incorporated <5% of the tracers into soil aggregates. Subsequent tillage incorporated more tracers into the aggregates so that, after 3 yr, 33% were incorporated with moldboard and 38% with chisel tillage. Moldboard tillage incorporated tracers into aggregates at a slower rate than chisel tillage, which is partially caused by deeper burial and thus less secondary tillage influences in moldboard tillage. Incorporation was faster into larger aggregates, supporting the hierarchical model of soil aggregation in which bond strength decreases as aggregate diameter increases. Aggregate diameter influenced incorporation rate more than tillage. The fraction of aggregates containing tracers was greater in the >40 mm than in the 12- to 40-mm class. Initially, nearly all tracers associated with aggregates were on the aggregate surface but with time this proportion decreased, the decrease being faster in the >40-mm class. Finally, these findings indicate that tillage-incorporated materials (fertilizers and pesticides) remain in interaggregate spaces during the first year after application, where they are most susceptible to movement and plant uptake.
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