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Phosphorus Distribution in Loessial Fragiudalfs as Affected by Relief

Department of Agronomy, Mississippi State Univ., Mississippi State, MS 39762

* Corresponding author.

ABSTRACT

Topographic effects on P form and distribution were determined in three loessial toposequences. Loring soils (fine-silty, mixed, thermic Typic Fragiudalfs) were sampled in the top- and midslope positions. Slopes ranged from 1 to 2% in the topslope and 9.5 to 13% in the midslope pedons. Phosphorus distribution reflected changes in topographic position and the influence of fragipan horizons (Btx) in the pedons. Nonoccluded, occluded, organic, and total P decreased while Ca-P increased as slope increased. Nonoccluded exceeded occluded P in the topslope Bt horizons, while occluded P dominated in the Btx horizons. Occluded P dominated midslope Bt and Btx horizons. Results suggest a loss of P and a reduction in weathering intensity as slope increased. Significant correlations were found between nonoccluded and occluded P and clay, dithionite- and oxalate-extractable Fe, and total Fe while Ca-P and organic P were correlated with pH and base saturation in the topslope pedons. In the midslope pedons, significant correlations were found between occluded P and clay and dithionite-extractable Fe while Ca-P and organic P were correlated with pH, base saturation, and organic matter. Maximum clay and total P occurred in similar Bt horizons in all topslope pedons. Changes in P distribution occurred into and below the Btx horizons, suggesting these changes occurred prior to and during fragipan formation.

Contribution from the Mississippi Agriculture and Forestry Exp. Stn. Paper no. J-7603.

Received for publication November 28, 1990.