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United States Department of Agriculture

Agricultural Research Service

Title: Quantifying Spatial Variability in Soil Phosphorus at a Field Scale: Implications to Runoff and Soil Sampling

Authors
item Penn, Chad - OKLAHOMA STATE UNIV
item Bryant, Ray
item Needelman, Brian - UNIV OF MARYLAND
item Kleinman, Peter

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 2, 2007
Publication Date: October 1, 2007
Citation: Penn, C., Bryant, R.B., Needelman, B., Kleinman, P.J. 2007. Spatial distribution of soil phosphorus across selected New York dairy farm pastures and hay fields. Soil Science. 172:797-810.

Interpretive Summary: The advent of phosphorus-based management in agriculture has resulted in a re-evaluation of traditional strategies to sample soil phosphorus. Traditional soil sampling strategies have often relied upon composite or bulk soil sampling, whereby multiple soils from a single field are combined into one sample. However, it is unclear whether such sampling methods provide sufficient resolution for modern water quality protection initiatives. This study quantified spatial variability in soil phosphorus under several different management conditions and investigated how such variability can affect patterns of phosphorus runoff in agricultural fields. Findings indicate that traditional soil sampling strategies may result in inaccurate estimation of P losses in runoff.

Technical Abstract: Despite general concern that traditional soil sampling strategies may not provide sufficient insight into environmentally-important variation in soil test phosphorus (STP), there is no consensus that changes to these strategies, such as spatially-explicit or discrete soil sampling within a field, represent enough of an improvement as to justify the added cost. This study, conducted on four fields located on two dairy farms in Delaware County, New York, characterizes the spatial variability of STP, calcium, magnesium, aluminum, and iron within two pastures, an alfalfa field and a hay field at a 10 meter scale; interprets nutrient distribution maps to explain STP distribution relative to landscape and cultural practices; and investigates the potential impact of such variability on phosphorus (P) concentrations in runoff. Spatial dependence analyses indicate real patterns to the P distribution in all four fields, but the patterns are strongest in the alfalfa and hay fields. High P concentrations in areas near the gate and road, where manure applications would be most easy and accessible, reflect the history of field management. Differences in soil properties associated with different soil types appear to partially explain P and iron distributions in the hay field. In contrast to the two fields, P distribution within the two pastures indicates the presence of “hot spots” at which Mehlich-3 P is elevated at several random points, reflecting manure deposits by grazing animals. A rainfall simulation study showed soils located in the down slope position have more influence on dissolved runoff P concentrations compared to the soils located on the upper slope due to P sorption/desorption interactions between runoff crossing the soils in the down slope position. Consequently, the use of a composite sampling strategy, or an average P value for an entire field that masks STP patterns, may result in an inaccurate estimation of P losses to surface waters.

Last Modified: 4/20/2014
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