Influence of Ryegrass on Physicochemical and Mineralogical Properties of a Fragipan Soil.

Soils containing fragipan horizons occupy roughly 12% of the land area in the USA.  These naturally occurring, impermeable horizons form as a result of weathering and cementation of silica and aluminum in association with iron.  Fragipan horizons can be found at shallow soil depths.  This limits row crop production due in large part to inhibited root growth and effects on plant available water.  There is a need to explore management strategies to degrade or remediate fragipan horizons. Field experiments at the University of Illinois noted that annual ryegrass as a cover crop impacted properties of a fragipan horizon which allowed the subsequent row crops (corn and soybeans) to have a greater rooting depth.  While the growth of ryegrass is known to weather pure clay minerals and soil clays, its effect on fragipan properties is unclear.  The objective of this study is to evaluate the potential of ryegrass to degrade fragipan horizons in greenhouse studies using intact soil cores by evaluating changes in chemical, mineralogical, and physical properties.  Six 1 ¾ inch diameter soil cores were removed from a field site in Western Kentucky from a Zanesville silt loam (Typic Fragiudalf) to a depth of 120 cm.  Three cores were seeded to annual ryegrass and the remaining three served as controls.  All cores were incubated in a greenhouse during ryegrass growth under controlled conditions.  The cores were sacrificed and the horizons determined using standard methods.  Water-soluble elements (Si, Al, and K) and pH were determined immediately.  Visual inspection of the cores showed that ryegrass roots made it down into the fragipan horizon (Btx1).  Soil pH values were 5.2 in the Ap and decreased to 4.4 in the Btx2 of the control columns.  The cores planted to ryegrass exhibited pH values of 5.6 in the Ap and slightly decreased to 5.2 in the Btx2.  Water-soluble Si values decreased 1.8-fold in the ryegrass treatments at the Ap horizon and these differences disappeared with depth when compared with control cores.  Potassium levels were also lower in ryegrass cores.  In contrast, solution Al was elevated in ryegrass treatments, particularly in the Ap and Btx2 horizons.  These patterns suggest that ryegrass might be altering the mineralogy of this soil and ongoing experiments using XRD and FTIR are evaluating this possibility.  In addition, physical properties of the fragipan clods will be evaluated.