Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2002
Publication Date: 3/10/2003
Citation: Shaw, J.N., Reeves, D.W., Truman, C.C. 2003. Clay mineralogy and dispersibility of soil and sediment derived from rhodic paleudults. Soil Science. 168:209-217.
Interpretive Summary: Long-term abuse of soils in the Tennessee Valley Region as a result of poor management, i.e., conventional tillage cotton monoculture, resulted in extensive soil degradation in the region. However, as a result of recent ARS research in cooperation with the Alabama Agricultural Experiment Station and the Alabama Cooperative Extension Service, and technology transfer efforts by USDA-NRCS, growers are now rapidly adopting conservation tillage practices. The surface soil layers in the region have appreciable amounts of silt and clay. These fine particles can disperse under rainfall, reducing soil tilth and degrading soil physical properties. We evaluated the clay dispersibility of these soils cropped to cotton in: 1) a no-till system without a cover crop, 2) a no-till system with a rye cover crop, 3) a no-till system with a rye cover crop and fall paratilling, and 4) a conventional tillage system. Surprisingly, we found that increased soil carbon under conservation tillage systems increased water dispersible clay quantities. We determined that chemicals called iron oxides were more important to reducing clay dispersibility and improving physical properties in the surface of this silt loam soil than soil carbon or organic matter. This information can be used by scientists and NRCS specialists to refine conservation systems for the region that further increase infiltration and improve cotton yields, while improving soil quality.
Technical Abstract: Surface horizons of Rhodic Paleudults in the Tennessee Valley, Alabama (USA), contain appreciable clay (< 2 µm) composed of quartz, kaolinite, hydroxy-interlayered vermiculite (HIV) and iron oxides. Similar clay mineralogical suites have been shown to be dispersive, which can degrade soil physical properties. Furthermore, studies have suggested mineralogical enrichment in dispersed colloids versus in situ soil, which can effect transport of sediment attached pollutants. The historical utilization of conventional tillage practices without cover crops has degraded soil quality in this region, however, reduced tillage systems are becoming common. We evaluated clay dispersibility and mineralogical partitioning for fine, kaolinitic, thermic Rhodic Paleudults cropped to cotton (Gossypium hirsutum L.) in: 1) a no-till system without a cover crop (NT), 2) a no-till system with a rye (Secale cereale L.) cover crop (NTC), 3) a no-till system with a rye cover crop and fall paratilling (NTCP), and 4) a conventional tillage system (CT). Water dispersible clay (WDC), extractable Fe, and soil organic carbon (SOC) were evaluated for surface (0-1 cm) samples. Particle size distribution (PSD) and clay mineralogy of in situ soil, runoff sediment, and WDC were also evaluated. Data indicated Fe oxides play a more significant role in clay aggregation than soil organic matter (SOM) in these high Fe systems. Higher clay amounts were recovered with dithionite extractable Fe removed (Fed) compared to SOM removal (33.2±3.5 % versus 24.8±4.5 %). The WDC quantities were related to SOC (r2=0.65), and negatively correlated with Fed (%) and water stable aggregates (%). Particle size of runoff sediment and mineralogy of runoff sediment and WDC was similar to in situ soil, suggesting models depicting erosion and sediment attached pollutant transport can be developed using in situ soil as a surrogate for sediment characterization in these soils.