Understanding How Soil Test Phosphorus Impacts Water Quality

Phosphorus (P) is an essential nutrient in crop production. However, excessive amounts of P in streams and lakes near crops can also lead to higher aquatic plant growth. This process, called eutrophication, results in depleted dissolved oxygen in water bodies. A soil test for P (STP) defines the agronomic need for a nutrient application of P to support crop yields and can indicate the increased risk of P loss in surface and tile water leaving a field. When STP exceeds 120 ppm, conservation practices beyond 4R nutrient management (right source, right rate, right time, and right place) may be needed to reduce field P losses. Implementing appropriate water management conservation practices that retain or filter water can reduce P losses to achieve water quality goals. Identifying appropriate sites for water management practices is important to water quality improvement efforts.

Measuring the amount of plant-available P from an 8-inch-deep soil core is a standard agronomic practice to determine if enough soil P is readily available or if P needs to be added to reduce the risk of crop yield loss. The soil test measures plant-available P forms in soil solution, plus forms that easily solubilize into solution during the growing season. In addition to plant uptake, soil solution P can be lost in subsurface or surface drainage water leaving the field. As STP increases, the risk of soluble P losses to the environment increases due to higher soil solution P concentrations.

The amount of dissolved reactive phosphorous (DRP) was measured in 39 Ohio production fields to determine the P loss potential based on STP. These fields ranged from 4 to 45 acres. Figure 1 shows the relationship of STP values to soluble P (labeled as DRP) concentrations that left the fields through tile drainage (Osterholz, et al., 2020). The figure shows P loss increasing slowly until STP reaches 225 ppm. After STP reaches 225 ppm, the P loss accelerates. Understanding that this change point for increased P loss occurs is important as we assess P loss risk.

The precision of the 225 ppm change point identified in this study is uncertain because only two fields measured above 150 ppm STP. Studies from other locations near Ohio identified STP change points for subsurface drainage between 112 to 200 ppm (Table 1). Guidelines for nutrient management planning in Ohio identify a procedure for assessing P loss risk which defines the high-risk P loss category at 120 ppm STP (USDA, 2020). For STP above 120 ppm, the recommendation is to limit nutrient application and increase other conservation practices if manure is applied.

STP also impacts surface water DRP concentrations. For example, the Ohio study showed that concentrations of DRP in surface water increased with higher STP values (Osterholz, et al., 2020). In summary, STP indicates P loss risk through tile and surface discharges from a field. A higher STP indicates a greater risk of elevated P concentration in drainage water leaving a field site.

Sound nutrient management practices are a long-term solution to reduce P loss in fields with high STP values. However, with the soil's ability to buffer STP, it may take decades with no P application before fields with higher STP values return to the agronomic range of 20-50 ppm. Figure 2 compares various initial STP values to those after 15 years with no P application (Fiorellino, 2017). Depending on the initial STP value, it will take 5 to 44 years to return to 100 ppm STP. Under Ohio conditions, a similar amount of time is expected. Guidelines in Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa (Culman, 2020) use a relationship of 20 pounds of P₂O₅ to increase (or decrease) STP 1 ppm. Long-term Ohio trials on three different soil types in a corn-soybean rotation had annual drawdown rates of 0.8–2.1 ppm with no P application.

The slow reduction in STP translates into decades of high P losses from drainage water leaving a field with higher STP values. Using conservation practices that retain, reduce the volume of water leaving, or filter, reduce the concentration of P in drainage water are needed to reduce P losses and reach water quality goals until the STP value returns to agronomic ranges.

Soil test P is one indicator of environmental P loss potential from a field. STP values above 100 ppm are not widespread. A summary of 2020 soil test results for Ohio finds that approximately 10% of soil samples were above 100 ppm (soiltest.tfi.org/). STP values often vary across a field, with high STP values only in some zones. Figure 3 is an example of a zone-sampled field with STP values ranging from 25 to 131 ppm. The proportion of a field with a high STP will influence DRP and total P losses measured from that field. Other factors also impact measured losses. For example, tillage, manure or fertilizer application, soil texture, and distance from a water body influence P loss measured at the edge of a field.

An Ohio State University research project monitors annual phosphorus losses from fields with high STP values. The researchers screened nine fields with STP between 75 and 550 ppm for P loading (Figure 4). Only six of the nine fields screened had losses that exceeded annual loading thresholds. Loading thresholds (green line) to improve water quality entering Lake Erie from the Maumee River are defined by the Great Lakes Water Quality Agreement (GLWQA) (epa.gov/glwqa). This data shows that relying on STP levels alone can be misleading when determining site conservation practice needs. STP levels can identify potential risks, but other screening tools, such as analysis of water grab samples from tile, should be used before deciding on a conservation practice. Using appropriate screening methods to quantify the P loss is a wise strategy to determine where to use limited conservation funds.

Conservation Practices for Fields with High STP Levels

In-field conservation practices to manage high STP fields to reduce P losses are limited and it can take several years to bring STP back into acceptable agronomic ranges. Some in-field practices to consider include:

• Do not apply additional P.
• Harvest high yielding crops or increase rotation intensity to harvest multiple crops over a year to maximize P removal. Harvesting forages often remove more P than when only grain is harvested.
• Increasing field water retention with soil-improving practices, such as cover crops and no-till, to improve organic matter can reduce P loss.

Water management practices that reduce or filter runoff can directly impact the P losses from a field. However, due to the cost of some water management practices, it is essential to identify fields where more significant P reductions can be achieved. Water management practices have flexible design options that can reduce interference with agronomic production, such as tillage or planting.

Consider these water management practices for high STP fields. Complete information on each practice can be found on the AgBMPs website (Best Management Practices, n.d.):

1. Blind Inlet (NRCS 620)

A blind inlet, similar to a French drain, is a structure that replaces a tile riser. The blind inlet is placed at the lowest point of a farmed depression or pothole to reduce the amount of sediment, nutrients, and other contaminants transported to receiving ditches or streams.

2. Controlled Drainage/Drainage Water Management (NRCS 554)

Controlled drainage adjusts the elevation of a drainage system outlet. This practice controls the volume of water leaving the field and reduces nutrient losses. Controlled drainage is considered an in-field practice.

3. Phosphorus Removal Structure (NRCS 782)

A phosphorus removal structure (PRS) is an edge-of-field practice that removes dissolved phosphorus (DP) from drainage water leaving the field.

4. Saturated Buffer (NRCS 604)

A saturated buffer has a subsurface drainage control structure that diverts water flow from the tile outlet to a perforated distribution pipe that runs along the buffer. As a result, the water table is raised in the buffer where the soil filters and removes nutrients before the water naturally enters an adjoining ditch or stream. A saturated buffer is considered an edge-of-field practice.

5. Water Control Structure (NRCS 587)

A water control structure is a water management system that conveys water, controls its direction or rate of flow, or maintains a desired water surface elevation.

6. Constructed Wetlands (NRCS 656)

A constructed wetland is an ecosystem that can hold water and contains water-tolerant native or non-invasive vegetation.

For a more general discussion of conservation practices, see the Ohio AgBMP best management practices at agbmps.osu.edu.

STP is an initial screening tool to identify sites with a higher P loss potential. Additional site assessment procedures, such as analyzing drainage water grab samples, can be used to confirm actual P loss. Using in-field practices to reduce STP may be limited depending upon the initial STP. Using water management conservation practices that retain, or filter water, can reduce P losses to achieve water quality goals where high P contributions over a prolonged period are expected. These edge-of-field practices can allow the continuation of crop production, which will lower STP over time while reducing downstream P impacts.

References

Best Management Practices. (n.d.). AgBMPs, Ohio State University Extension. Accessed January 17, 2023.
agbmps.osu.edu/bmp.

Culman, S., Fulford, A., Camberato, J., & Steinke, K. (2020). Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa (Bulletin 974). The Ohio State University.
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Fiorellino, N., Kratochvil R., & Coale F. (2017). Long-Term Agronomic Drawdown of Soil Phosphorus in Mid-Atlantic Coastal Plain Soils. Agron. J. 109(2):455–461.
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Osterholz W. R., Hanrahan B. R., & King K. W. (2020). Legacy Phosphorus Concentration–discharge Relationships in Surface Runoff and Tile Drainage from Ohio Crop Fields. J. Environ. Qual. 49(3):675–687.
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McDowell, R.W. and Sharpley, A.N. (2001) Phosphorus Losses in Subsurface Flow before and after Manure Application to Intensively Farmed Land. Science of the Total Environment. 278, 113–125.
doi.org/10.1016/S0048-9697(00)00891-3.

Sharpley, A. N., McDowell R. W., & Kleinman P. A. (2001). Phosphorus Loss from Land to Water: Integrating Agricultural and Environmental Management. Plant and Soil. 237: 287–307.
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United States Department of Agriculture (USDA). (2020). Assessing Nutrient Loss Risk in Ohio. Natural Resources Conservation Service, Ohio Nutrient Management Technical Note.
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Wang, Y. T., Zhang T. Q., O'Halloran I. P., Tan C. S., Hu Q. C. & Reid D. K. (2011). Soil Tests as Risk Indicators for Leaching of Dissolved Phosphorus from Agricultural Soils in Ontario. Soil Sci. Soc. Am. J. 76(1):220–229.
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