Clay mineralogy and genesis of fragipan in soils from Southeast Brazil

• The fragipan genesis was dependent on parent material.
• Face to face adjustment of kaolinite favored the formation of fragipan horizons.
• The correlation between goethite and total porosity was positive and significant.
• The correlation between amorphous materials and total porosity was negative and significant.

Fragipan horizon has a hard consistency when dry but is brittle when moist. Such a horizon restricts root growth and water infiltration due to the low volume of macropores and discontinuous voids. In Rio de Janeiro state in Brazil, neighboring soils were developed from different materials (sediments and granite/gneiss) and were subject to the same environmental conditions; one manifested fragipan characteristics in the subsurface and the other did not. The main objective of this study was to characterize and quantify clay minerals and relate their properties to the genesis of fragipan horizons. The fragipan in the studied soils show high bulk density (1.67 g cm− 3) and low average macroporosity (0.03 cm cm− 3), total porosity (0.42 cm cm− 3) and hydraulic conductivity (1.43 cm h− 1). Sequential and interrelated causes favored the formation of fragipan horizons: 1) face to face adjustment of kaolinite (Ka) filling of larger spaces occurring between sand grains, favored by low goethite contents and the absence of hematite and gibbsite; and 2) mineral binding and cementation of sand, silt and clay fractions by amorphous materials. Higher amounts of goethite and lower amounts of amorphous materials in the clay fraction were associated with horizons with higher total porosity.