Genesis of cohesive soil horizons from north-east Brazil: role of argilluviation and sorting of sand

Abstract

Cohesive horizons are widespread along the Brazilian coastal tablelands and, despite their importance to soil management under intensive cultivation, their genesis remains, in many aspects, controversial. Studies diverge and indicate the participation of different mechanisms in the formation of these soil horizons. Although clay illuviation has been considered as one of the processes determining the formation of this soil attribute, recent studies suggest a limited participation of this soil process. The aim of the present study was to evaluate the relative contribution of argilluviation and sorting of sand to the genesis of cohesive horizons in an Ultisol toposequence from a toposequence from north-east Brazil. Soil samples were collected from cohesive and non-cohesive horizons from four soil profiles. We determined bulk soil density (BD), penetration resistance (PR), particle size distribution and the percentage of different sand fractions. The sand fractionation data were treated statistically to obtain different grain size parameters, such as mean grain size, sorting and kurtosis. An additional micromorphological study was performed on cohesive and non-cohesive horizons. The lowest BD and PR were recorded in non-cohesive horizons (1.53 Mg m–3 and 0.74 MPa, respectively), whereas significantly higher values of BD (1.66–1.80 Mg m–3) and PR (2.61–7.71 MPa) were observed in cohesive horizons. Thin sections of cohesive soil horizons showed a significant reduction in void sizes and interconnecting pores compared with non-cohesive soil horizons. The higher values of BD and PR observed in the cohesive horizons are apparently related to this poorly interconnected void network. Textural pedofeatures observable with optical microscopy, which indicate argilluviation, were only found in one cohesive horizon. Sand fractionation showed a predominance of medium and fine sand fractions and indicated the dominance of poorly selected sand in cohesive horizons, probably causing the higher BD and PR values. However, the presence of textural pedofeatures (clay coatings) in one of the cohesive horizons studied may indicate a combined contribution of sand heterogeneity and argilluviation in the genesis of these horizons. Overall, the results of the present study showed that subtle variations in the distribution of basic soil components (i.e. particle size fractions) may determine the formation of cohesive horizons and thus must be taken into account in future studies aiming to predict their spatial distribution and to establish more efficient management strategies.