Managing Phosphorus Saturated Soils

Introduction

Five of the eight eastern panhandle counties in West Virginia’s portion of the Chesapeake Bay watershed include 98% of the state’s poultry industry. In 2018, reported broiler production was 83 million birds and turkey production was 3 million birds in the Potomac Headwaters region, resulting in over 100,000 tons of poultry litter being produced (NASS 2020 Annual Statistical Bulletin). During the last 50 years, most of the poultry litter produced was applied within the watershed. These repeated litter applications have resulted in an accumulation of phosphorus (P) in agricultural soils, increasing the potential of nutrient losses to waterways. 

West Virginia’s eastern panhandle has been identified as a high phosphorus loading area of the Chesapeake Bay. West Virginia’s Phase II Chesapeake Bay Watershed Implementation Plan (WIP) noted that the agricultural sector is responsible for 49% of the total delivered nitrogen load and 62% of the total delivered phosphorus load. A portion of this nutrient loading is attributed to the repeated land application of poultry litter as a soil amendment, resulting in high levels of phosphorus accumulation and saturation in pasture, hay and cropland soils.

At the beginning of September 2018, the WVU Soil Testing Lab changed from the Mehlich 1 to the Mehlich 3 soil test extraction method. The Mehlich 3 method provides new information to customers who are using the lab to make soil fertility decisions. Included with the nutrient analysis is a phosphorus saturation ratio (PSR) or Psat. Phosphorus saturation ratio is defined as the ratio between the amount of phosphorus present in the soil and the total capacity of that soil to retain phosphorus. The ability of phosphorus to be bound in the soil is primary a function of iron (Fe) and aluminum (Al) content in that soil. Mehlich 3 extraction allows iron and aluminum levels to be determined and then, a ratio of phosphorus to the sum of iron plus aluminum results in a percentage being reported. This tells the farmer how much iron and aluminum are currently holding phosphorus and how much is left to accept additional applied phosphorus. 

Research has shown that there is a phosphorus saturation threshold level where a soil changes from a sink for phosphorus to be a source for phosphorus loss to the environment. Results from these studies included a group of Mid-Atlantic soils. The threshold level was determined using the relationship between water soluble phosphorus and a soil's PSR (Figure 1).  The soil phosphorus saturation threshold where soils switch from a phosphorus sink to a phosphorus source was determined to be .1 or 10%. The water-soluble phosphorus concentration increased significantly once the threshold PSR was exceeded (Dari et al, 2018).

Figure 1. Relationship between water soluble phosphorus (WSP) and phosphorus saturation ratio (PSR) for all soil samples in this study. Threshold PSR is 0.10; 95% confidence interval = 0.05 to 015; p,0.0001. Group 1: Mid-Atlantic United States; Group 2: Ozark Plateau Group; Group 3: Southeastern Piedmont; Group 4: Southern-most Atlantic Coast and Gulf Coast Plain; Group 5: Various regions (Dari et al 2018).

Results

To help farmers in the eastern panhandle better understand the environmental risks of phosphorus saturated soils, a WVU student was hired in the fall of 2019 to collect soil samples from crop, hay and pasture fields. Our survey area included Grant, Hardy, Mineral, Hampshire, Pendleton and Jefferson counties. A total of 92 fields had samples collected. The 38 pastures and 26 hayfields had a sampling depth of 0 to 2 inches, and the 28 crop field samples were collected at the 0- to 6-inch depth. The WVU Soil Testing Lab recommendation system ranks soils as low, medium, optimum and excess (Table 1).

Table 1. Fertility rating break points in WVU fertilizer recommendation system.

As the student collected soil samples, field histories were requested, and each field was categorized as high, medium and low for historic litter applications to the fields. Results from the crop fields showed heavy historic use of litter with a corresponding excessive level of fertility across all fields. The degree of phosphorus saturation was ten percent or above for all fields except for one (Table 2). Crop field phosphorus levels ranged from 60 ppm to over 700 ppm, an excess fertility rating for all sampled fields except one (Table 2). The threshold PSR of 10% was exceeded on all but one of these crop fields. Twenty-six hay fields were sampled with 40% having soil test phosphorus levels in the excess range and above the PSR Threshold level of 10% (Table 3). Of 38 pastures, 52% had phosphorus levels in the excessive range and above the 10% phosphorus threshold level (Table 4).

Table 2. Row crop fields with their location, crop, soil test phosphorus, and phosphorus saturation percentage ranked in order of their soil test phosphorus concentration.

Table 3. Hay fields with their location, soil test phosphorus, and phosphorus saturation percentage ranked in order of their soil test phosphorus concentration.

Table 4. Pastures with their location, soil test phosphorus, and phosphorus saturation ranked in order of their soil test phosphorus concentration.

Table 5. WVU Soil Testing Lab’s degree of phosphorus saturation recommendations.

Conclusions

Having farmers better understand phosphorus soil chemistry is key to reducing or eliminating applications of poultry litter to phosphorus-saturated fields. The Mehlich 3 soil test enables famers to identify fields that are vulnerable to phosphorus losses and then, take steps to manage the fields to minimizes losses. The WVU Soil Testing Lab includes phosphorus saturation guidance for low, medium and high levels of phosphorus saturation (Table 5). These recommendations should be included in all nutrient management plan narratives for each field. These recommendations are in alignment with current phosphorus research in the Mid-Atlantic region (Sims et al 2002, Dari et al 2018). The Mehlich 3 soil test method will allow the addition of a field’s PSR as a factor into the WV Phosphorus Site Index.        

References

Dari, B., V. Nair, A. Sharpley, P. Kleinman, D. Franklin and W. Harris. 2018. Consistency of Threshold Phosphorus Saturation Ratio across a Wide Geographic Range of Acid Soils. Agrosystems, Geosciences & Environment. Agrosyst. Geosci. Environ. 1:180028 (2018) doi:10.2134/age2018.08.0028, https://acsess.onlinelibrary.wiley.com/doi/pdf/10.2134/age2018.08.0028

Sims, J. T., R. O. Maguire, A. B. Leytem, K. L. Gartley, and M. C. Pautler. 2002 Evaluation of Mehlich 3 as an Agri-Environmental Soil Phosphorus Test for the Mid-Atlantic United States of America. Journal of the Soil Science Society of America

USDA, 2020 Annual Statistical Bulletin Number 51,  https://www.nass.usda.gov/Statistics_by_State/West_Virginia/Publications/Annual_Statistical_Bulletin/index.php

WV watershed Implementation Plan 2, Appendix C. Sources of N and P in WV Potomac Counties http://www.wvchesapeakebay.us/WIP/files/phase2/Appendix_C_Sources_of_N&P_in_WV_Potomac_Counties.pdf

Authors: Tom Basden, Retired WVU Extension Specialist – Nutrient Management; Ed Rayburn, Retired WVU Extension Specialist – Forage Agronomy; Louis McDonald, WVU Professor of Environmental Soil Chemistry and Soil Fertility; and Emily Morrow, WVU Extension Agent – Jefferson County

Project Participants: Paden Rightsell, WVU student, and WVU Extension agents Brad Smith, Alex Smith and David Seymour

Last Reviewed: October 2021

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