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Ohio State University Extension


Ohio Long-Term Phosphorus and Potassium Fertilizer Trials: Grain Yields

Additional Authors:  Anthony Fulford, Clay Dygert,

Ohio corn, soybean and wheat check-off dollars are currently funding Ohio State Extension research to update the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat and Alfalfa ( As part of this effort, this is the first of several articles on the results of a nine-year corn and soybean trial conducted in Clark, Wayne, and Wood counties. The main goal of the study was to evaluate grain yield response to phosphorus and potassium fertilization. Corn and soybean were grown in 2 rotations (corn-soybean and corn-corn-soybean) for 9 years (2006–2014) at 3 sites for a total of 54 site-years. At each site and in each rotation, there were 3 rates of phosphorus (0, 1x, 2x) and 3 rates of potassium (0, 1x, 2x). Fertilizer rates for each rotation were determined by the estimated nutrient removal rates of corn and soybean grain at harvest. In other words, 1x represents nutrient removal rates with average yields and 2x represents the estimated nutrient removal when average yields are doubled.

At the start of the study in 2006, soil test phosphorus levels were within or above the maintenance range (15–30 ppm Bray P) at all three sites (27 ppm at Clark County; 37 ppm Wayne County; 30 ppm at Wood County). Likewise in 2006, soil test potassium levels were within or above the maintenance range (100–155 ppm ammonium acetate K) at all three sites (115 ppm at Clark County; 133 ppm Wayne County; 209 ppm at Wood County).

Corn and soybean grain yield responded positively to phosphorus and potassium fertilization in 14 of the 54 yield trials conducted from 2006 to 2014. A positive yield response to phosphorus fertilization was documented in 18% of corn trials and 5% of soybean trials. A positive yield response to potassium fertilization occurred in 9% of corn trials and in 19% of soybean trials.

Trends were examined to see if a lack of fertilization became more pronounced over time. There was a clear relationship between phosphorus and years since fertilization, but no clear relationship between potassium and time (Table 1).

Table 1. Number of occurrences that corn or soybean grain yields increased with fertilization.


Early (2006-2008)

Mid (2009-2011)

Late (2012-2014)

Total (2006-2014)


0 out of 18

2 out of 18

5 out of 18

7 out of 54


3 out of 18

3 out of 18

1 out of 18

7 out of 54





 The general lack of positive fertilizer responses suggests that the Tri-State recommended maintenance range for soil test phosphorus and potassium values are still relevant at these three sites, although some modifications may be necessary as we continue to collect more data. On-farm fertilizer response trials are currently being conducted across the state to evaluate these maintenance ranges for sites with different management histories. These on-farm trials will complement the information shown here. Farmers, crop consultants and agronomists interested in contributing to this state-wide project are encouraged to contact Steve Culman at


Reference: Vitosh, M.L., J.W. Johnson, and D.B. Mengel. 1995. Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat and Alfalfa. Ext. Bull. E-2567. Available online at:

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