C.O.R.N. Newsletter: 2024-02

Soil available and added phosphorus (P) nutrient impacts Ohio's crop production and environment. Fertilizer P can increase crop yields. Yet, excessive P can have negative impacts on water quality, resulting in toxic algal blooms. To properly calibrate the use of P for maximum crop yield efficiency and environmental safety, it is important to monitor P use trends, understand the changes to P recommendations from 1995 to today, review changes in soil test phosphorus (STP), and identify the sources of P used. Two recent factsheets examine P nutrient use at two scales: statewide and regional. The regional results were summarized by Crop Reporting Districts (CRD), shown in Figure 1. The two publications and their links are Phosphorus (P) Nutrient Use in Ohio ANR-0143, found at https://go.osu.edu/ohiop, and Ohio Phosphorus (P) Use by Crop Reporting District ANR-0144, found at https://go.osu.edu/ohiocrdp.

These two reviews of P use reveal several trends impacting agronomic management and environmental outcomes:

• Ohio purchased P fertilizer use is trending downward (Figure 2). During the highest use period (1993–1997), 206,000 tons of P₂O₅ were applied annually. In the most recent period (2018–2022), annual applications averaged 137,000 tons. Agriculture reduced the average yearly P₂O₅ application by 33% between the two periods.

• All nine Ohio crop reporting districts (CRDs) show a declining trend in P₂O₅ usage from 1994 to 2022. The reduction rate ranges from 50 to 868 tons annually (Table 1).

• Table 1. Annual change in trendline fertilizer usage by Crop Reporting District 1994-2022.

  	Annual Change (tons P2O5)	R2
  CRD10	-868	0.56
  CRD20	-453	0.35
  CRD30	-255	0.24
  CRD40	-657	0.44
  CRD50	-589	0.22
  CRD60	-50	0.32
  CRD70	-98	0.05
  CRD80	-70	0.15
  CRD90	-129	0.56

• Seven of nine Ohio CRDs show increased availability of P₂O₅ sourced from manure. CRD 30 and 80 show a decline. The 2017–2022 average annual increase in manure P₂O₅ is 28 to 355 tons for the seven districts with an increase.
• Sixty-five percent of Ohio counties had decreasing trends in mean soil test P (STP) levels between 1993 and 2015. Since 2003, Ohio's P₂O₅ removal through crop harvest has exceeded P applied as a nutrient, resulting in a net annual removal of 8 pounds of P₂O₅ per acre. Continued net removal would indicate that STP should continue trending downward, which has positive environmental impacts.
• The P₂O₅ mass balance of applied nutrients through fertilizer and manure applications minus nutrients removed through crop harvest ranges from -14 to 1 pound per acre in Ohio’s nine CRDs (Table 2).

• P fertilizer recommendations developed in 1995 were modified, resulting in the reduced application of P fertilizer while still meeting crop production needs. Tri-state fertilizer recommendations were validated and re-released without significant changes for P use in a 2020 publication, Ohio State University Extension Bulletin 974 Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa (Culman, et al., 2020).

Environmental Implications of CRD Data

Ohio's nonpoint source goals for improving water quality are related to nutrient management and reducing erosion. Figure 3 shows the map of CRDs, along with Ohio's major watersheds. Several watersheds align with one or more CRDs. For example, CRDs10 and 20 represent nutrient application activity in the Western Lake Erie Basin. The Great Miami River is impacted by CRD 40 and 70, and CRD 50 represents most of the acreage in the Scioto River basin. The data shown here can be used to anticipate water quality changes around P nutrient use.

The trend toward less applied P reduced the acute P loss around surface applications. One higher P loss scenario occurs when rainfall resulting in concentrated surface flows occurs near the time of application. Reduced application tonnage reduces loss risk from this pathway. The trend to remove more P through crop harvest than is applied in fertilizer and manure should result in continued declines in Soil Test P. Soil test P is one factor that can mitigate chronic P loss at the edge of a field. (LaBarge & Cochran, 2023).

Reason for Reduced P Use

Several factors have impacted farmer decisions in nutrient use since 1987. One factor was a 1995 collaborative effort to regionalize P recommendations (LaBarge, 2023). A second factor is the higher per-unit cost for P fertilizer since 2018. A third factor is increased education on fertilizer use from the implementation of Fertilizer Applicator Certification in 2014. Farmers are required to use the Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa, Bulletin 974 for their P recommendations when participating in Natural Resource Conservation Service cost-share programs, the 2017 4R Certified Ag Retailer voluntary third-party audit program, and the 2019 Ohio H2Ohio program. Increased farmer knowledge of P use, technical assistance programs, and economic returns are all factors driving reductions in P use.

References:

Culman, S., Fulford, A., Camberato, J., & Steinke, K. (2020). Tri-state fertilizer recommendations for corn, soybean, wheat, and alfalfa, bulletin 974. College of Food, Agricultural, and Environmental Sciences, The Ohio State University.
extensionpubs.osu.edu/tri-state-fertilizer-recommendations-for-corn-soybean-wheat-and-alfalfa

LaBarge, G. & Cochran, R. (2023). Understanding how soil test phosphorus impacts water quality [Fact sheet]. Ohioline. https://go.osu.edu/stpwq

LaBarge, G. (2023). Phosphorus nutrient use in Ohio [Fact sheet]. Ohioline.
https://go.osu.edu/ohiop