Should Soil P Limit Compost Use for Impervious Disconnection?.

Impervious disconnection is an urban stormwater practice that seeks to modify soils to reduce runoff. One practice that is being evaluated as an impervious disconnection best management practice (BMP) is the use of compost as a soil amendment to decrease bulk density and increase hydraulic conductivity and soil water-holding capacity.

The Chesapeake Bay Program (CBP) Urban Stormwater Workgroup convened an Expert Panel to define and develop nutrient and sediment load reduction recommendations using disconnection of existing impervious area runoff from stormwater drainage systems as a new BMP to be adopted by the CBP for credit toward Watershed Implementation Plans.

Among the recommendations of the Expert Panel was the incorporation of compost having a phosphorus concentration less than 30 mg/g to a depth to 15 cm in a soil whose P concentration is <55 Mehlich 1 or <127 Mehlich 3 mg/kg soil.

I will present data to show the value of soil-incorporated compost as a runoff BMP from soils whose soil P concentrations are greater than 127 mg M3-P/kg soil. 

Five treatments, applied annually from 2000-2004 to a Piedmont silty clay loam, for the production of various horticulture and agronomic crops included an unfertilized control, fertilization according to soil test lab recommendations, and amended with poultry litter, poultry-litter-yard waste compost, and biosolids-wood chips compost at agronomic N rates.

At the end of the 5-year period, Mehlich 3-P was excessive (>127 mg P/kg) in the compost-amended soils. Soil water soluble P correlated with M3-P in all treatments except for the biosolids compost, whose high Fe content likely reduced P solubility.

Rainfall simulation at 5.0 cm /hr was performed to evaluate the effects of treatment on infiltration, runoff, and loss of P in the runoff. Simulated rainfall was applied for 30 minutes after runoff began, and runoff was collected for volume and chemical composition.

Runoff was decreased (as infiltration was increased) by the treatments in the order control > fertilizer > poultry litter > biosolids compost > poultry litter-yard waste compost.

The cause of the reduced runoff was apparently due to increased infiltration and water-holding capacity in the higher organic matter-amended soils. Runoff and bulk density were inversely correlated to soil total C, which were highest in the compost-amended soils.

The reduction in runoff in the compost treatments decreased total P loss to lowest in the poultry litter-yard waste compost-amended soil. Even the biosolids compost-amended soil had no greater loss of total P than the unfertilized control, the fertilized soil, or the poultry litter-amended soil. There were no differences among treatments in loss of total dissolved P or dissolved reactive P.

Compost can safely be used as a soil amendment to reduce runoff without concern of  P loss even when soil P concentrations are raised above soil P threshold levels. The benefits accrue largely from increased soil organic matter and associated soil physical properties, such as reduced bulk density.