By Steve Culman
The Tri-State Fertilizer Recommendations, published in 1995, provided a unified soil fertility framework between Michigan, Ohio and Indiana. These recommendations are based on, “field calibration and correlation studies that have been conducted over the past 40 years.” Although it was a collective scientific work, the extensive data that went into these recommendations were not systematically compiled and shared with the public.
At Ohio State, Dr. Jay Johnson was the Soil Fertility Specialist who conducted field trials and helped develop fertilizer recommendations. From 1976 – 1999, Dr. Johnson reported the results of his field trials from that field season in an annual report. We went through these reports and pulled out every field trial that looked at phosphorus and potassium fertilization. We found 85 P trials conducted over 8 sites: 47 in corn, 33 in soybeans and 5 in wheat. We found 102 K trials conducted over 8 sites: 68 in corn, 32 in soybeans and 2 in wheat.
For each trial, we calculated the percentage of relative grain yield by dividing the yield of the unfertilized plots by the yield of the fertilized plots and multiplying the result by 100. Since yields can vary greatly over sites and years, we use the relative yield to shows us how much fertilization increased or decreased grain yields. For each trial, we then took the relative yield and graphed it against the soil test P or K level. Figure 1 shows this relationship with P and Figure 2 shows the relationship with K. Each dot represents a single field trial from one year. The solid black horizontal line at 100% represents no change between unfertilized and fertilized plots. The dotted black line at 90% shows a 10% reduction in yield. The read dashed vertical line shows the Tri-State critical levels of 15 ppm Bray P1 (Figure 1) and 100 ppm Ammonium Acetate K (Figure 2). These are the data from Ohio that helped establish the critical soil test P and K levels found in the Tri-State Fertilizer Recommendations. A field with soil test levels to the left of the red dashed line has a reasonable chance of a yield response to fertilization and so fertilizer is recommended, while fields with soil test levels higher than the red line have a very low chance of a yield response to fertilizer, and so little to no fertilizer is recommended.
We can consider this information a ‘historic’ foundation and efforts are now underway to produce ‘current’ information to see if the fertilizer recommendations need to be revised. You can help with this effort by representing your region as a farmer cooperator and conduct on-farm strip trials. There is some financial support available for your time. More information can be found here: go.osu.edu/fert-trials
This work is supported by the Ohio Soybean Council and the Corn and Small Grains Marketing Program.
Figure 1. Relative Grain Yield vs. Soil Test Phosphorus
Figure 2. Relative Grain Yield vs. Soil Test Potassium