Mass proportion of microaggregates and bulk density in a Brazilian clayey oxisol

The physical properties of Brazilian Oxisols are closely related to the development of their microstructure, which typically consists of stable microaggregates > 1 mm. There is no model available to predict changes in microstructure in Oxisols. The objective of this work was to relate the proportion of microaggregates to the bulk density (D-b) in the soil studied. Five sites of a typic Haplustox under native vegetation (two sites) and pasture (three sites) were sampled. Soil bulk density, sand, silt, and clay content and aggregate-size distribution were measured from the surface to 1.6 m deep in increments of 0.1 in. Thin sections were prepared from undisturbed samples collected in duplicate from 0- to 0.1-, 0.3- to 0.4-, 0.8- to 0.9-, and 1.5- to 1.6-m depths, and backscattered electron scanning images (BESI) were generated. Clay content ranged from 672 to 798 g kg(-1) and bulk density between 0.87 and 1.18 g cm(-3) among the 80 samples studied. Bulk density was poorly correlated with clay content (R-2 = 0.358) and at any depth was not significantly smaller under native vegetation than under pasture. Visual assessment of BESI revealed that soil material corresponded to either microaggregates (< 0.1 mm) in loose arrangement or to much larger aggregates. Quantification of BESI from the deepest sampling depth of all soils showed that 96.2 and 95.7% of microaggregates were < 0.8 mm with 73.2 and 95.7% between 0.1 and 0.5 mm under native vegetation and pasture, respectively. The mass proportion of microaggregates can be estimated using the < 0.84-mm soil material that is obtained by dry sieving (Phi < 0.84). Finally, our results showed that Phi < 0.84 varied with D-b. Linear regression coefficients were calculated for the relationship between Phi < 0.84 and the reciprocal of bulk density (1/D-b) [Phi < 0.84 = 1.97 (1/D-b) - 1.52, R-2 = 0.82], assuming no interaction between microaggregates and macroaggregates, the porosity of these two structural types was estimated as 0.71 and 0.51, respectively.