Physical Property Remediation of Soils for Stormwater Control.

The development process usually involves grading  at least a portion of the landscape, removing existing vegetation and soil to create level areas for roads and buildings.  This process leads to exposure of subsoils with poor native properties for supporting vegetation and these soils become compacted from construction activities.  Infiltration rates in these soils can approach zero and vegetation is difficult to establish, resulting in rapid runoff similar to impervious areas.   We have been investigating methods to remediate these soils to both improve vegetation growth and to increase their infiltration capacity.  Tillage, with and without amendments such as compost, gypsum, and water-absorbing polyacrylamide, was tested on compacted subsoils for grass establishment, rooting depth, and infiltration capacity.  On four sites around North Carolina, the topsoil was removed and the area was compacted using a vibratory roller.  Tillage was applied using a backhoe to break up the soil followed by a rotary tiller to a depth of up to 25-30 cm.  Each location had a somewhat different mix of treatments, including different depths (12-15 or 25-30 cm) of tillage, the inclusion of compost as an amendment, and the addition of traffic in the form of a riding mower.   More than two years after tillage, bulk densities (BD) were significantly lower (1.2 vs. 1.5 g/cm³) in tilled areas in a Piedmont sandy clay subsoil.  Infiltration rates remained over 4X higher (29.5 cm h^-1) in the tilled areas, with 10-20X greater root biomass below 15 cm.  In a second experiment set up adjacent to that one, the addition of compost significantly reduced BD compared to tillage alone but did not improve infiltration except when traffic was allowed on the plots.  At a second location with a sandy clay loam in the mountains,  infiltration rates dropped by 50% over two years in tilled plots, but remained at more than 2X that of compacted plots (24 vs 9 cm h^-1).  At a third location with a sandy soil, infiltration rates were more than 5X in tilled versus compacted soils (24-28 vs 5 cm h^-1) almost two years after treatment, and the addition of compost had no effect.   Infiltration was about 25% lower after two years compared to initial measurements.    Overall, tillage to 15-30 cm appeared to be a good strategy for remediating soils compacted during the development process, and compost may provide some resiliency in maintaining the initial benefits.