N. E. Smeck

Phosphorus dynamics in soils and landscapes

Abstract Transformations of P along both biological and pedologic pathways are reviewed and integrated in order to develop a comprehensive appreciation of P dynamics in soil ecosystems. The biological P cycle is driven by the essential role P plays in energy transport in biological systems. Mineralization of organic P is influenced by a process referred to as biochemical mineralization wherein hydrolysis of P is repressed by an ample supply of labile P and induced by a limited supply. Pedological P transformations are driven by leaching and increasing acidity which result in the weathering of primary minerals and the formation of secondary minerals. Although the biological cycle is more dynamic than the sluggish pedologic pathway, long-term evolution of P forms in soil ecosystems is controlled by the latter which continually moves P into a sink consisting primarily of occluded P. Occurrence of soluble P as an intermediate during P transformations accommodates translocation of P within soils and landscapes. Because both P content and forms and classes of Soil Taxonomy are related to stage of soil development, a relationship is proposed between taxonomic classes and P forms in soils.

Micromorphology of seasonally wet soils on the Illinoian Till Plain, U.S.A.

Abstract Two seasonally wet soils from southwestern Ohio in the north-central region of the U.S.A. were studied in order to relate micromorphology to pedogenic processes associated with fluctuating oxidation-reduction conditions. The soils formed in approximately 70 cm of Late Winconsinian loess overlying a composite paleosol developed in Early Wisconsinian loess and Illinoian till. Alternating oxidation-reduction conditions in the upper 150 cm were accompanied by abundant albic neoskeletans, Fe-Mn nodules, and partially decomposed, grainy argillans. Conditions were most reduced along saturated macropores between ped faces in about the upper 80 cm where thick albic neoskeletans occurred. Horizons from 50–80 cm were extensively degraded and were characterized by many Fe-Mn nodules and gray, eluvial zones (albic neoskeletans) surrounding islands of brown sesquioxidic material having inundulic plasmic fabrics. Argillic horizon degradation is one of the most important genetic processes in these soils. Embedded grain ferriargillans enriched in goethite were abundant in the lower paleosols from approximately 150–250 cm. Neogoethans and neomangans were evident at a depth of approximately 2 m which is the lowest point to which the water table falls in these soils each year.

Weathering of soil clays with dilute sulfuric acid as influenced by sorbed humic substances

Humic substances extracted from an Alfisol, Mollisol, and Histosol were sorbed to the clay (< 2 μm) suite extracted from an Alfisol Bt horizon. The clays with sorbed humic substances as well as clays with indigenous organic matter and with organic matter oxidized with H2O2 were weathered by incubation in 0.05M H2SO4 for 1320 h and in 0.0025M H2SO4 for 1176 h. To evaluate weathering, the quantities of Mg, K, Al, Si, and Fe released and clay dissolved were monitored. In both weathering studies, Mg was preferentially released by the clays. Because Mg occurs mainly in the octahedral sheet of clays, the high release of Mg suggests preferential dissolution of the octahedral sheets. In the 0.05M H2SO4 dissolution study, no differences were found with respect to K, Al, Si, and Fe release from the different clays, but slightly less Mg was released from clays with sorbed humic substances. At the completion of the incubation in 0.05M H2SO4, 21 to 24% of the clays had dissolved. Using a less concentrated acid (0.0025M H2SO4), notable differences were apparent in the degree of weathering among the clays. Approximately, 3 times more Mg was released from clays with organic matter oxidized than from clays with indigenous or sorbed humic substances. Similar quantities of Mg were dissolved from clays with indigenous or sorbed humic substances. K release was similar from all clays. Using 0.0025M H2SO4, a larger portion (5.8%) of the clay with organic matter oxidized was dissolved than of clays with indigenous (2.6%) or sorbed humic substances (< 1%). Data for Mg release and clay dissolution in 0.0025M H2SO4 both suggest that the weathering of clays was reduced by indigenous organic matter or sorbed humic substances. No weathering differences could be detected between clays with sorbed humic versus fulvic acids or between sources (Mollisol, Alfisol, or Histosol) of humic substances.

Spatial distribution of lepidocrocite in a soil hydrosequence

Abstract Three terrace soils comprising a hydrosequence were examined to determine how the spatial distribution of lepidocrocite was related to depth and duration of saturation. Vertical relief was 1.0 m with well drained, moderately well drained, and somewhat poorly drained pedons spaced ~60 m apart. All soils contained brittle, slowly-permeable subsoil horizons and were acidic with <35% base saturation throughout the upper sola. The well drained soil (Fragic Hapludult) had no morphological indicators of wetness within a depth of 180 cm, and water was perched above a brittle horizon at 82 cm for a total of only 41 days during the 3.4 year observation period. Nevertheless, trace amounts of lepidocrocite were detected in the subsoil. The moderately well drained soil (Typic Fragiudult) was saturated at a depth of 180 cm for 6% of the time, and water was perched on top of a fragipan at 74 cm for 13% of the time. Lepidocrocite was most abundant in this pedon and reached maximum concentrations below the fragipan in the capillary fringe of the regional water table (150–183 cm). The somewhat poorly drained member of the hydrosequence (Aeric Fragiaquult) was saturated at a depth of 180 cm for 96% of the observation period and also contained perched water above a fragipan for >90% of the time. Lepidocrocite occurred throughout this pedon but was most concentrated in fragipan horizons (86–135 cm) between the perched and regional zones of saturation. These horizons were saturated from 22 to 48% of the observation period. The results of this study suggest that lepidocrocite formation was favoured in horizons that were saturated for 5–50% of the time when soil temperatures exceeded 5ºC.

Etched thin sections for coupled optical and electron microscopy and micro-analysis☆

Abstract Coupled use of optical transmission microscopy and SEM—EDXRA in the study of soil thin sections initially proved difficult because of problems in differentiation of mineral grains and impregnating medium with the secondary electron detector of the SEM. It was also difficult to locate the same small micromorphological features with both techniques, especially when the feature was hidden below the surface of the thin section. A procedure was developed that included selective dissolution (etching) of the impregnating medium and is presented here. After etching, many micromorphological features could be more easily recognized and those immediately below the surface were partially exposed. Application of the procedure proved useful in analyzing bridging plasma too small to identify by optical transmission microscopy that connected silt grains in fragipan horizons.