A global synthesis of fragipan silicon concentrations and relationships with soil physicochemical properties as a function of soil parent materials

Fragipans are dense, brittle subsurface horizons that restrict root growth and limit water-holding capacity. A common factor in these soils is a binding agent, potentially either phyllosilicate clay bridging, amorphous aluminosilicate/silica, or silica sorbed to Fe oxides. Others have suggested that no binding agent is present within these soil horizons and that the arrangement of individual particles is solely responsible for these horizons. In this article, a secondary data synthesis was utilized to demonstrate that silicon (Si) accumulation is a key pedogenic process in loess-derived fragipan horizons, but not in all fragipans. The average dithionite-extractable Si (Si d ) content of non-fragipan (n = 483) and fragipan (n = 175) horizons was significantly different, with average values of 0.88 ± 0.54 (μ±1σ) and 1.29 ± 0.97 g Si kg soil -1 , respectively. When considering differences within parent materials, loess-derived fragipans exhibited significantly greater Si d than non-fragipan loess-derived horizons. With depth, a consistent increase in Si d values was found in the zone of the loess-derived profiles that coincided with fragipan horizons. Silica molar ratios of dithionite-extractable oxides (e.g., Si d /Al d and Si d /Fe d ) also generally followed these same results. Additionally, a significant relationship between Si d and exchangeable Mg was present across all parent materials. This relationship had been previously observed but was discounted as an important feature of fragipan horizons. The results of this synthesis suggested otherwise and pointed to a shared source of both exchangeable Mg and Si, potentially due to the greater presence of smectitic clays in these horizons or Mg aiding electrostatic bridging between reactive Si surfaces, among several other hypotheses. Furthermore, Si d , Si d /Al d , and Si d /Fe d were related to clay content and the ratio of silt-to-clay content, which is likely because the silt and clay fractions are the source of the Si, Al, and Fe released in these extractions. This data synthesis suggested that Si accumulation may be common in loess-derived fragipans, but not in all fragipans, and that observing Si accumulation in fragipans is dependent on parent material.