We need to do better with nitrogen

By Harold Watters and Greg LaBarge, Ohio State University Extension

Making better nitrogen rate decisions can help the pocketbook and the environment. Nitrogen is one our highest cost variable inputs to produce an acre of corn with around 15% of the variable cost. Nitrogen also gets a lot of attention in water quality discussions. If we apply more nitrogen than the corn needs in a year, it is likely headed out the tile and downstream.

Long-term nitrogen rate trials conducted at OSU’s Western Agricultural Research Station and Northwest Agricultural Research Station show how variable the right N rate for corn is from year to year. Soil types at two locations are different with Western being a silt loam soil and Northwest a lake-bed clay. Figure 1 has a trend line drawn by location with all treatments shown by individual symbol at each rate. Note that the highest yield varies tremendously from year to year, varying by nearly 150 bushels per acre. The yield response curves look very similar with around 200 pounds of nitrogen per acre maximizing yield even though the final yield varies, due to soil type and environment. 

An interesting point from the data is the average yield at zero pounds of nitrogen applied. For Western, no N yields 124 bushels per acre and 79 bushels per acre is the average yield for Northwestern. Nitrogen mineralization due to soil organic matter and rainfall differences are reflected at the zero rates. Corn yields never go to zero with no applied nitrogen, soil supplied nitrogen is always present under Ohio conditions. Putting a strip of zero nitrogen in corn fields gives farmers an opportunity to understand the amount of soil nitrogen supplied in their systems over a growing season and an opportunity to think about long-term changes to their nitrogen management.

The current Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat and Alfalfa, (2020) Bulletin 974 uses the same method of determining N rate that has been adopted across most of the upper Corn Belt since 2006. The approach, called the maximum return to nitrogen (MRTN), is favored over trying to maximize corn yields because of the economic volatility in both corn grain and nitrogen fertilizer prices. Indiana, Michigan, and Ohio corn N rate recommendations (and those of Iowa, Illinois, Minnesota, and Wisconsin) are based on extensive N response trials conducted over several years in each state. These trials have determined the N rate at which the last pound of added nitrogen fertilizer returns a yield increase large enough to pay for the cost of the additional fertilizer. 

The MRTN rate is calculated along the yield response curve using four inputs: 1) location, 2) previous crop grown (corn; soybean or small grain), 3) price of nitrogen fertilizer, and 4) price expected per bushel of corn. When corn prices are low and/or fertilizer prices are high, nitrogen rates are reduced; when corn prices rise or N fertilizer prices fall, recommended nitrogen rates increase. 

The corn N rate recommendations do not account for N fertilizer application timing, but rather assumes best management practices are used. Therefore, the recommended N fertilizer rate represents the total N to be applied over the growing season, regardless of timing of N application. Application timing and placement practices that result in N loss will often require higher N rate applications to maximize profit. Soybean or crop rotation credits are based on field trials in each state and are already built into the recommendations. 

Table 1 shows the different combinations of nitrogen fertilizer and corn prices for a quick look at N rate recommendations. For more detailed output on the MRTN rate farmer can access the corn N rate calculator at cnrc.agron. iastate.edu. The tool provides both a single recommended N rate and a Profitable N Rate Range, that is within range of $1 per acre plus or minus of the MRTN rate. Other charts that are useful to understanding the rate information and risk of use are also included.

Table 1. Ohio Recommended Nitrogen Rate got corn following soybeans based on price of corn grain and nitrogen fertilizer

 Price of Nitrogen Fertilizer ($/ Lb)
Price/Bushel Corn$0.30$0.35$0.40$0.45$0.50
$3.25185176168162155
$3.50187180173166160
$3.75191184176170164
$4.00195186180174168
$4.25199190184177171
$4.50200193185180175

A comprehensive study looking 31 N rate tools was reported on in 2018. In that study the MRTN was ranked in the top 6 for determining a N rate at planting and in split applications when compared to the Economically Optimal Rate. Other tools such as Pre-Plant Nitrogen Test, Pre-Sidedress Nitrogen Test and crop reflectance sensing were also mentioned. One study conclusion was there was no one best tool and more work is needed.  

The MRTN tool helps to facilitate N rate decisions across each state, but farmers are encouraged to use other available information such as weather, soil type, pre-sidedress N tests, management history and previous performance to help refine a localized N rate for any given field. Emerging technologies such as crop sensors and weather-driven soil-crop models may also be considered as information to guide N management decisions.

Is manure part of your nutrient management plan?

Here are some suggested adjustments to the MRTN rate by soil nitrate test levels in a manure system. Purdue University has done extensive work on using information from pre-sidedress nitrate tests to modify your corn nitrogen rate. Sample using a 1-inch probe 12 inches deep when corn is 6-12 inches tall (in the 4- to 6-leaf stage) using a random zig-zag pattern across the field. Use the NO3-N number to determine nitrogen level, then reduce from the MRTN by the amount shown in Table 2.

Table 2, adjustments to the MRTN corn N rate depending on soil nitrate level.

Soil NO3-N in ppmAdjustment to MRTN recommendation. N rate lb/A
0-10Full rate
11-15-30
16-20-45
21-25-90
>25No sidedress N

In eastern Ohio and use PennState presidedress recommendations? PSU Research has shown that when the soil nitrate-N level is above 21 ppm, there is little chance of an economic response to adding additional N to the field.

What’s next?

Want to try some of that emerging technology to manage nitrogen? Try these ideas to change your plans for managing nitrogen, too. If you study the problem of managing nitrogen well, you will learn we need a lot more information than we have on April 1 to know that right N rate.

  • Crop sensors (crop color sensors): NDVI, NDRE  — as hand held, applicator mounted, drone or satellite based
  • Weather-driven soil-crop models use satellite information and soil information for your fields and include Adapt-N developed at Cornell, now Agronomic Technology Corp, Climate FieldView Plus from Bayer and Encirca developed at Pioneer is now part of Granular solutions from Granular Agronomy
  • Biological or root-attached soil bacteria: There are at least a couple of companies out there that provide advanced bacteria to make a bit of extra N in season. Maybe try one and reduce your N rate by 25 to 30 pounds per acre
  • Genetic answers gene editing: Not there yet, but I know that a gene jockey (or several) are working right now to find a way to allow the corn plant to make it’s own nitrogen.

Harold Watters and Greg LaBarge, OSU Extension Agronomy Field Specialits, are rarely in their offices, so they can best be reached at watters.35@osu.edu or by phone at 937 565-6064. Greg LaBarge is available at labarge.1@osu.edu.

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