A New, Sensor-Based, Fertilizer Management Strategy

Precision ag research team at Oklahoma State University have developed sensor based, variable rate fertilizer management strategy for field crops.

Nitrogen fertilizer is one of the largest seasonal variable costs for farmers. The most common approach to determining a nitrogen (N) fertilization rate, in many regions, has been to estimate a yield goal (usually equal to or slightly greater than the 5-year average) and multiply that times an assumed N requirement for each unit (e.g., bushel) of yield expected. Available soil test-N is usually subtracted from the total requirement to arrive at a rate for the area, usually a preplant application. The attractive part of this approach was that it relied mainly on knowing the realistic yield goal for the field and it could often reliably separate field N needs in relation to productivity. While this has been a much better approach than simply guessing, it has several limitations that reduce farmer profitability and increase risk of N loss to the environment.

Traditional N application strategy:
There are four constraints with the traditional strategy of N fertilizer, and two of them have to do with what kind of a ‘production’ year it will be. The four constraints are:

1. Determining the potential yield for a field prior to a growing season.
   
2. Determining how much non-fertilizer N (soil-N) will be available for the potential yield.
   
3. Applying all, or most, of the seasonal nitrogen requirement in most cases, before we plant the crop.
   
4. Applying single rates of N for areas of the field that have different potential yields.

These “faults” of the traditional strategy are a result of assuming it will be an “average” production year and that the field is perfectly uniform. In rain-fed farming, to assume it will be an average year is to assume the weather will actually be average for the year. Everyone that has farmed for a few years knows that average weather conditions seldom occur. Likewise, when whole fields are viewed from a distance (as from an airplane at 10,000 feet), they seldom appear to be very uniform.

Temporal variability.
Results of 30 years continuous wheat research at the Oklahoma State University North Central Research Station in Lahoma, Oklahoma, show that when we fertilized for the average yield (2 lb N/bu):

• 60 % of the time we incorrectly estimated the actual yield by at least 10 %.
• Additionally, because available non-fertilizer N changes from year-to-year, 30 % of the time the average rate was the correct rate.
• 37 % of the time the average rate was short by at least 20 lb N/acre.
• 33 % of the time 20 to 80 lb excess N/acre (average = 38 lb) was applied.

• Topdress N was about 35 % more efficient than preplant N.

Changing tradition.
To improve on the traditional approach we need a strategy, or plan, that gives us a chance to evaluate the effects of current-year weather conditions on the potential yield achievable and the amount of non-fertilizer N that was supplied by “nature” (rainfall and that mineralized from soil organic matter). This reading of “nature” needs to be done during the growing season at a point when we will still have time to add needed nitrogen. The reading is made possible by reducing, or eliminating, preplant nitrogen except for a strip (spreader width) through the field at a rate that will assure nitrogen will not be limiting. This “Nitrogen-Rich Strip” can then be read, or compared, to the condition of the rest of the field, mid-season when there is still time to topdress any needed N.

Reading nature.

	Hand-held optical sensor

Although differences in the Nitrogen-Rich Strip and the rest of the field could be “read” by any measure of crop condition (height, color, tillers, etc..), Oklahoma State University researchers have used a commercially available (NTech, Inc.), hand-held optical sensor that integrates components of crop health (biomass and functioning chlorophyll). The sensor calculates an index (normalized difference vegetative index, or NDVI) from reflected red and infrared light. Once the readings (NDVI) have been made, a response index (RI) is calculated by dividing the reading of the Nitrogen-Rich Strip by the reading representing management for the rest of the field. Using the optical sensor to read the N-Rich Strips results in an unbiased number that is not affected by who does it or when the strip is being read. The response index (RI) tells us how much of a yield response to expect from topdress nitrogen.

Field response variability. Oklahoma State University research on 10 winter wheat fields in 2002 found RINDVI values that ranged from 1.1 to 1.6. These readings mean that we could expect to get 10% to 60% yield increase from in-season topdress N applications. This wide range of values also shows that the response to topdress nitrogen differs greatly from field to field in the same year and in some instances, in the same general location. In order to take fulladvantage of this new strategy/technology, every field that has a different soil, management history, or growing environment, can be expected to respond differently to the needs for N. Consequently, like soil testing, every field should have a Nitrogen-Rich Strip. With time some fields may be found to respond alike and treated the same based on reading one Nitrogen-Rich Strip.

Treating spatial variability.
Oklahoma State University researchers have found that areas as small as about 6 square feet can be different from each other and require different input of N. The “Precision Ag Team” has promoted and researched development of technology to identify and treat field areas this small. The technology is now available to sense and treat every 4 square feet at 15 mph using conventional boom applicators and solution 28 (UAN).

New strategy economics.
The bottom line on implementing this new strategy is increased farmer profits. Estimates using the 30-year data on continuous wheat, show that if N was applied only as a topdress at rates based on a Nitrogen-Rich Strip there would be an average increased return of about $19/acre/year, compared to 80 lb N/acre preplant for a 40 bushel yield goal.

With 40 lb N/acre as preplant and additional N topdressed based on the Nitrogen-Rich Strip, the benefit is not as good because of estimated lower efficiency of preplant N and that some years 40 lb is excessive. The question is how much fertilizer to apply each year. The answer is given from the Nitrogen-Rich Strip. Accurate reading of the Nitrogen-Rich Strip is crucial to this new strategy. This technology (hand-held sensor) will likely be available through fertilizer dealers. This and the technology for spatial treatment of every 4 square feet will be an added expense to the farmer, paid from profit.

Field experience. Results from 10 field-scale treatments for the 2002 crop showed an advantage of 4 to 9 dollars per acre from using the N-Rich Strip and applying a “flat” rate, in a year of drought and delayed topdressing. Combined with the spatial treatment of every four square feet the average improvement compared to a “farmer practice” was consistently greater than $12/acre. Strategies: 80pp = 80 lb N applied preplant costing $0.15/lb. 40pp = 40 lb N preplant costing $0.15/lb, plus topdress N at $0.25/lb based on N-Rich Strip, plus $2/acre application cost. RI-TD = All N applied topdress at $0.25/lb based on N-Rich Strip, plus $2/acre application cost Wheat at $3/bu.

Results from 10 field-scale treatments for the 2002 crop showed an advantage of 4 to 9 dollars per acre from using the N-Rich Strip and applying a “flat” rate, in a year of drought and delayed topdressing. Combined with the spatial treatment of every four square feet the average improvement compared to a “farmer practice” was consistently greater than $12/acre.

G.V. Johnson, W.R. Raun, John Solie and Marvin Stone Professors in the Departments of Plant & Soil Sciences and Biosystems and Ag Engineering Division of Agricultural Sciences & Natural Resources Oklahoma State University