The demand for fertilizer nitrogen changes from year to year, even in long-term plots where the same fertilizer N rates are applied each year.  When winters are wet and warm, sufficient N can be mineralized from soil organic matter, and enough to nearly meet the demands for maximum yields (see 1973, 1979, 2001, and 2005)(Figure 1).  In these years, this is compounded by increased N deposition in the rainfall.  Our long-term winter wheat data confirms that the demand for N is variable from year to year, and that is unpredictable.  This exact same phenomenon was observed in the RC-RS long-term corn experiment conducted by the late Professor Robert A. Olson at the University of Nebraska where near maximum yields in the check plot were produced in 1978 (Figure 2).  Where did all the N come from if these check plots had not been fertilized for nearly 10 years?  The answer is N mineralization via an extremely wet and warm spring-summer. 

The influence of temporal variability on the need (or lack thereof) for fertilizer N is huge, yet our fertilizer N rate recommendations seldom account for this.  Soil testing (NH4-N and NO3-N) works, but when collected preplant, applying a static value on N demands that will become dynamic mid-season, makes no sense.   For precisely this reason, we have developed methods that account for the influence of temporal variability on N demands, by making better fertilizer N decisions mid-season when the growing crop can tell us precisely how much N was delivered for free (comparison between the N Rich Strip and the Farmer Practice, mid-season). 

The GreenSeeker sensor via NDVI measurements provides a highly accurate estimate of plant biomass, as a result, total N uptake can be estimated from mid-season measurements.  More importantly, yield potential can be predicted mid season using both sensor and known climatic data from planting to sensing.  By estimating yield, we can then estimate total grain N removed.  Yield potential (YP0) is then multiplied times the response index (estimated in-season from NDVI readings collected in the N Rich Strip and Farmer Practice) to obtain the yield obtainable if N fertilizer is to be applied (YPN).  The fertilizer N rate is determined by subtracting N uptake at YP0 from N uptake at YPN, and divided by an expected NUE. This entire algorithm is completely delineated in the "Optical Sensor Based Algorithm" article below. 

Optical Sensor Based Algorithm for Crop Nitrogen Fertilization  Commun. Soil Sci. Plant Anal. 36:2759-2781 (pdf)

Automated Calibration Stamp Technology for Improved In-Season Nitrogen Fertilization Agron J. 97:338-342. (pdf) 

Figure 1.  Wheat grain yields in the check (0 kg N/ha) and the N rich (112 kg N/ha) plots, and the associated response index (grain yield N rich, divided by the grain yield in the check) from 1971 to 2007 in the Stillwater 222 long-term winter wheat experiment.

Figure 2.  Corn grain yields in the check (0 kg N/ha) and the N adequate or N rich (180 kg N/ha) plots, and the associated response index (grain yield N rich, divided by the grain yield in the check) from 1969 to 1983 in the RC-RS long-term corn experiment conducted by Professor R.A. Olson near Mead, Nebraska.

Figure 3.  Wheat grain yields in the check (0 kg N/ha) and the N rich (112 kg N/ha) plots, and the associated response index (grain yield N rich, divided by the grain yield in the check) from 1971 to 2010 in the Lahoma 502 long-term winter wheat experiment.

Relationship between RI_NDVI and RI_Harvest at Feekes 5 across 62 locations in Oklahoma, 1998-2003.

Relationship between the estimated responsiveness to applied N or response index (RI-NDVI) from sensor readings (RI-NDVI = NDVI in the N Rich strip divided by the NDVI in the farmer practice) and the response index determined at harvest (Grain yield in the N rich strip plot divided by the N Rich strip in the farmer practice), 2004 Corn Trials.  Clearly, the responsiveness to N can be determined from mid-season sensor measurements collected between V8 and V9.

2007: Relationship between the response index measured in season, (NDVI of fertilized plot / NDVI of check plot) from readings of cotton at growth stages from 60 to 80 days after planting, and measured lint yield and the response index measured at harvest (yield of fertilized plot / yield of check plot) from all site years.

N Rich Strip Adopters

Steinert’s near Hennessey

Brent Rendel (see link below where he has developed several rampers)

Jimmy Wayne Kinder, near Walters

Panhandle via extension presentations Brian gave in Boise City and Woodward

Jason Lawles via Western Equipment (several hundred)
Levi and Curtis Johnson Brothers (developed the J2 ramper, , see below, Helena, OK)
Joe Biggerstaff, Engineering Student of Solie’s (every field they owned and rented)

Curtis Fischer (Kingfisher)

Pictures above taken March 20, 2007 showing both RAMPS using the configuration delineated in the figure below

How to obtain highly accurate mid-season fertilizer N rates using the Ramped Calibration Strip 

The new Ramped Calibration Strip Applicator applies 16 different N rates (10 to 20 foot intervals), over 160 to 320 feet (actual rates and distances can be adjusted depending on the crop).  Companies interested in building variants of this applicator can adjust distances, N rates, intervals, etc., as they see fit.  The Ramped Calibration Strip is used to visually determine precise mid-season fertilizer N rates for wheat and corn production.  Although the use of hand-held GreenSeeker sensors are not required, the sensors offer the opportunity to sense the entire "RAMP" and thus accurately determine where the peak in NDVI exists over the range of N rates applied.  Without the sensor, farmers can simply walk from one end of the RAMP to the other and stop where they no longer see any differences.  Whether determined visually, or with a handheld sensor, the point where no differences no longer exist, is the TOPDRESS N Rate. 

This methodology is a variant of what has been developed using the N Rich Strip and the handheld sensing technology that relies on predicted yields and the Sensor Based Nitrogen Rate Calculator (http://www.soiltesting.okstate.edu/SBNRC/SBNRC.php)

Looking at the graph above, you can see exactly why this is the case.  By stopping at the point (recording distance in feet) where there are no longer visible or recorded differences in NDVI, you can plot a line directly down to "distance" that has a predetermined N rate associated with it.  That N rate (depending on how you set up the intervals to change the rate, and the actual rates chosen) is the TOPDRESS N Rate to be recommended.  For the 3 fields above where NDVI values were plotted with distance (green squares, red triangles, and blue circles), the recommended topdress N rates would have been 40, 80, and 100 lbs N/ac, respectively.  Why?  You have to remember that the Ramped Calibration Strip is applied "ON-TOP" of the farmer practice (whatever that may be).  Assuming that we can "catch up" and/or achieve maximum yields from the mid-season N application, and assuming that yield potentials were not severely restricted by excess early season N stress, the RAMP interpolated rate is how much you would need to apply on the rest of the field to achieve the same "visible" or "NDVI recorded" response. 

In corn, some would argue that this methodology is flawed, because you don't know whether or not this rate will "run out" later in the season.  This is to a certain extent correct, but if the farmer is wanting to avoid that potential "risk" the topdress N rate can be increased by whatever amount he/she deems appropriate.  However, the RAMP is an incredible starting point for determining the optimum mid-season N rate.  Why?  Because we have now evaluated enough corn and wheat data to know that in several long-term experiments (corn, NE, WI, MX; wheat, OK), there were years where the check plot (No N Fertilizer ever applied) produced near maximum yields.  A RAMP N Rich Strip in those fields would have told us "MID-SEASON" that there were no visible differences between the 0-N and all other plots within the RAMP receiving N.  Wouldn't farmers want to know that?  Wouldn't this be important to know?  If the check plots with no fertilizer N looked as good as the fertilized plots, where was their N coming from?  Over the years, we have learned that warm wet winters (winter wheat) and warm wet springs and early summer (corn) are conducive to increasing the amount of N mineralized from soil organic matter, and that N deposition in rainfall increases significantly in these years.  Because we have years where the demand for fertilizer N is less (and highly dependent on the environment), and other years where it is cool and dry and the demand for fertilizer N is greater.  The only way to determine how much the environment delivered for free is to have a Ramped Calibration Strip in each and every field. 

For those farmers/producers interested in using the sensors for determining their topdress N rates, all they have to do is mark the start and end of the Ramped Calibration Strip (preplant), and collect sensor data using the handheld GreenSeeker sensor walking at a constant speed over the length of the ramp.  Using the program below, they can automatically read that file, and it will compute the optimum N rate.  We recommend the use of the sensors simply because our eyes are not as sensitive in picking up these differences.

NOTE:  This approach (interpreting the Ramped Calibration Strip) is for determining field rate application (flat rates) and accounts for only temporal variability.  Simultaneous consideration of temporal and spatial variability requires on-the-go prediction of yield potential (YP0) and use of the response index for each 0.4m2 area (wheat), and every 2-3 plants in corn.

Ramp Data Processor (excel version 1.5)
Example File (Morrison)
Example Data (Salmon) below

Relationship between RI (determined at harvest) and the nitrogen use efficiency observed over time, Experiment 502, 1971-2005.

The relationship between RI NDVI and the final N recommendation using SBNRC is listed above.

The Ramped N Reference Strip will allow researchers to refine mid-season fertilizer N rates by knowing exactly what the RI was (NDVI Max, divided by NDVI Min), and by integrating the area under the curve over the distance included within the ramp (currently set at 150 feet, but than can be altered to increased rate change intervals).

Click on the Question to See the VIDEO.

Tom Denker, Farmer near Enid, OK
Question: What is the benefit of GreenSeeker Technology?
Question: What do you think about OSU working with private industry to make this type of technology available to producers?
Question: Would you like to add anything?

Jerrel Powell, Farmer near Council Hill, OK
Question: How have you used GreenSeeker Technology?
Question: Why is Nitrogen Important?
Question: What would you like to tell producers in OK about GreenSeeker Technology?

Response Index Picture Page

Response Index (Defined)

Managing N Fertilizer Using a N-Rich-Strip: Projected Profitability

Reducing N fertilizer field losses by managing spatial and temporal response variability (GVJ_Chem_Society)

• Net Revenue Estimates