MRTN – Maximum return to N

So how do we make nitrogen (N) recommendations in Ohio? Current recommendations from Ohio State University use an economic model to set our corn nitrogen rate. The Maximum Return To N (MRTN) concept was developed by soil fertility specialists from across the north central region: this is a regional Corn Belt wide approach for nitrogen rates.

For us, we use data from trials in Ohio so we also have our weather included as part of the equation and we factor in the price of nitrogen and the value of corn to bring in the economics. Chart 1 shows that our best economic return to nitrogen for $3.50 corn and $0.40 per pound of N is about 168 pounds of N per acre with a range of about 15 pounds to either side giving us about the same economic return –— within $1. You may also gain efficiency by delaying the bulk of your N application until sidedress timing. From last year’s experience, you’d better put 30 to 50 N units with your corn at planting, though, and then you can sidedress the remainder later.

The calculator for this MRTN is available on the Iowa State University website: You can go there to get the best suggestion on your nitrogen rate, even run some different scenarios. You’ll need to know:

  • Your state
  • Previous crop – corn or soybeans
  • Price of N
  • Expected sales price for a bushel of corn.

The MRTN housed at Iowa State does use Ohio data from a range of years that we provided after conducting trials on Ohio research sites and farms across several years.


Have you read the Tri-State Recommendations for corn, soybeans, wheat and alfalfa?

Most folks say they have not. But if you had, you could say you understand the principles behind the recommendations and trust them to be quite valuable. While no longer printed, it is available as a pdf on-line:

A few of the principles in the Tri-State for P and K management:

  • You should soil test every three to four years, results are presented in ppm, and correct application rates as necessary.
  • Expected grain nutrient removal as: For P2O5 – corn 0.37, soybean 0.80, and wheat 0.63-pound removal per bushel. For K2O removal  — corn 0.27, soybean 1.40, wheat 0.37-pound removal per bushel.
  • Recommendations are designed to provide adequate nutrition for the crop, and to create or maintain a soil capable of providing sufficient nutrient without fertilizer addition for one or more years.
  • We follow a philosophy of build up for low testing soils, maintain levels above the critical level in the maintenance plateau, or drawdown at high nutrient levels to maximize crop yield
  • And key to these recommendations is field calibration and correlation studies that have been conducted over the past 40 years

Recently our Ohio State University soil fertility specialist along with his research team — Dr. Steve Culman with Muhammad Tariq Saeed and Anthony Fulford — dug through the data from Ohio that was used to develop the tri-state recommendations. The Tri-State Fertilizer Recommendations, published in 1995, provided a unified soil fertility framework between Michigan, Ohio and Indiana.

At Ohio State, Dr. Jay Johnson was the Soil Fertility Specialist who conducted field trials and developed the fertilizer recommendations. From 1976 to 1999, Dr. Johnson reported the results of his field trials from each field season in an annual report. The group went through these reports and pulled out every field trial that looked at phosphorus or potassium fertilization. They found 85 Phosphorus (P) trials conducted over eight sites: 47 in corn, 33 in soybeans and five in wheat. And they also found 102 Potassium (K) trials conducted over eight sites: 68 in corn, 32 in soybeans and two in wheat.

For each trial, they 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, they use relative yield to show us how much fertilization increased or decreased grain yields. For each trial, they 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 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 vertical 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 dashed line have a very low chance of a yield response to fertilizer, and so little to no fertilizer is recommended.

While we have confidence in past trial work, Steve suggests we can consider this information “historic” or “old” and that efforts underway will help produce ‘current’ information to see if the fertilizer recommendations need to be revised. You can imagine how much collective effort will be required to generate robust information across the state. Steve, and all of the agronomic crops team, is looking for farmer cooperators to conduct on-farm strip trials to help generate additional information. More information on how to participate in these field trials can be found here:

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