Amendments to increase aggregation in United States southeastern Coastal Plain soils

Many U.S. southeastern Coastal Plain soils have a cemented subsurface hard layer that restricts root growth and decreases productivity. Soil properties are improved by tillage but might. also be improved by amending the soil. Wheat (Triticum aestivum L.) residue and polyacrylamide (PAM) were used to amend a Norfolk soil mix of 90% E horizon (the hard layer) and 10% Ap horizon (to assure microbial presence). Our hypothesis was that incorporation of wheat residue and/or PAM would improve physical properties, making the soil more amenable to root growth. Treatments contained 450 g of soil mix, 6.44 g kg(-1) ground wheat stubble, and 0, 30, or 120 mg kg(-1) of PAM (an anionic, linear formulation of size 12 MDa and 35% charge density). Duplicate sets of replicated treatments were incubated at 10% gravimetric water content for 30 and 60 days. Treatments were leached with 1.3 pore volumes of water. After leaching and equilibration to stable water contents, soil strengths were measured with a 3-mm-diameter flat-tipped bench-top penetrometer. At 30 days, the treatments were not significantly different; but at 60 days, treatments differed. Polyacrylamide decreased bulk density when added at the higher rate of 120 mg kg(-1) to the soil. The higher PAM rate also decreased the amount of water that was added to maintain treatments at 10% water content. Wheat residue amendments decreased penetration resistances and increased aggregation. Wheat residue and PAM amendments improved soil physical properties, especially when treatments were allowed to incubate for 60 days.