Do Soil Health Practices Impact Subsurface “Legacy” Phosphorus Losses from Soils on the Delmarva Peninsula?

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Commodities

Practices

• Crop Production: conservation tillage, cover crops, drainage systems, no-till, nutrient management
• Education and Training: extension, on-farm/ranch research
• Natural Resources/Environment: soil stabilization
• Soil Management: soil analysis, soil chemistry, soil quality/health

Legacy stores of soil phosphorus (P) are common in agricultural soils on the Delmarva Peninsula due to historical applications of poultry litter at nitrogen (N)-based rates. Legacy P losses from Delmarva soils occur primarily through subsurface pathways (e.g., preferential flow and matrix flow) due to flat-topography and open ditch drainage networks. Subsurface P losses contribute to water quality degradation in sensitive waterbodies like the Chesapeake Bay. Farmers implement soil health practices (e.g., conservation tillage and cover crops) to increase soil organic matter, control erosion, and reduce N losses. These practices are also effective at reducing particulate P losses in runoff and mitigating P losses from newly applied manure. However, soil health practices may be ineffective at controlling, or even increase, subsurface leaching. This study investigates how soil health practices affect soil P distribution and hydrologic connectivity (i.e., P stratification and accumulation at depth), which impact subsurface legacy P losses on the Delmarva Peninsula. We will pair five agricultural fields with long-term soil health practices (conservation tillage and cover crops) with fields without soil health practices. Composite soil cores will be collected, divided by depth, and analyzed for water extractable P, Mehlich 3 P, operational soil P pools, pH, organic matter, and dry soil aggregate distribution. We will run the Delaware P Site Index and Maryland P Management Tool at each site to rate the risk for edge-of-field P losses. Our results will help better understand how soil health practices impact the risk of subsurface P loss on the Delmarva Peninsula.

The goal of this project is to understand how long-term (>5 years) implementation of soil health practices affect the distribution of soil P and hydrologic connectivity in artificially-drained soils on the Delmarva Peninsula. The specific objectives of this project are to:

Objective 1: Use the Delaware P Index and the Maryland P Management Tool versions 1 and 2 to determine the risk of edge-of-field P loss. We will run the Delaware P Site Index (PSI) and Maryland P Management Tool (PMT) on Delmarva fields with and without a long-term history of soil health practices to identify fields with a high overall risk of edge-of-field P loss. Ten fields that are identified as high risk will be used in this study. Fields with a long-term history of soil health practices (conservation tillage and/or cover crops) will be paired with fields with little to no history of soil health. Paired fields will have similar soil types and cropping rotations.

Objective 2: Compare the vertical distribution of P in soils with and without a long-term history (>5 year) of soil health practices. Accumulation of soil P near the soil surface can increase the risk of dissolved P losses in runoff. In contrast, accumulation of soil P at depth is suggestive of subsurface P losses in leachate. We will investigate soil P stratification within the topsoil and P accumulation at depth at each study field. Soil cores will be collected from several locations in the field and split into 15 depth increments. Soil samples will be analyzed for soil P using a variety of chemical extraction methods to evaluate differences in soil P dynamics through the soil profile in fields with and without a long-term history (>5 year) of soil health management practices.

Objective 3: Determine how soil health practices impact hydrologic connectivity. Hydrologic flow p due to the formation of preferential flow pathways (large connected poathways that connect soil P sources to drainage ditches result in a high risk of P loss. Without this hydrologic connectivity, there is a very low risk of P loss. Soil health management practices can affect hydrologic connectivityres). We will determine how soil health practices impact hydrologic connectivity by comparing the dry soil aggregates size distribution in soils with and without a history of soil health practices. Soils with a wider size distribution of soil aggregates suggest the formation of preferential flow pathways through which P can leach.