Physical crust formation and steady-state infiltration rate in soils dominated by primary minerals in some South African ecotopes

Abstract

Physical crust formation is affected by the inherent soil properties such as texture and mineralogy. However, inconsistencies exist in soil mineralogy effects of soil crusting. In addition, investigations of soils dominated by primary minerals such as quartz, especially in South Africa, are scant despite the abundance of such soils. Effects of texture and aggregate size on crust formation and steady-state infiltration rate (SSIR) in soils dominated by primary minerals were investigated. Soil aggregate sizes of <2, 2–3 and 3–5 mm were exposed to ∼60 mm h⁻¹ simulated rainfall. Aggregate size significantly affected crust strength (P < 0.05). Physical crusts ∼0.2 to ∼0.8 mm thick with a surface layer of loose grains formed over a thin plasmic layer. Crusts with strengths between 0.25 and 3.42 × 10⁻⁴ kg m⁻² developed on the <2 mm aggregates compared with <2.23 × 10⁻⁴ kg m⁻² in the >2 mm aggregates. The reverse occurred in the Alice Jozini ecotope, which had a relatively low clay content of 120 g kg⁻¹. The SSIR was between 1.24 and 3.60 mm h⁻¹ in ecotopes dominated by primary minerals and clay content between 12% and 26%. In the Amatola Jozini ecotope, which was dominated by kaolinite and higher clay content, SSIR was 15.23 mm h⁻¹. Consequently, physical crust formation decreased but SSIR increased with increase in aggregate size if there was a concomitant increase in the clay fraction, decrease in primary minerals and presence of kaolinite.

Keywords: aggregate stability, quartz, soil mineralogy, soil structure, soil texture