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Nitrogen-Rich Strips - A New Way to Determine N Fertilizer Requirements

Posted Apr. 30, 2005

For decades, farmers and ranchers have fertilized uniformly based on soil test results and yield goals. Recently, Oklahoma State University developed a new way to determine nitrogen (N) fertilizer requirements. Nitrogen-rich strips (N-rich strips) supplement the use of yield goals as a basis for making fertilizer-N recommendations.

This newly developed N management strategy addresses two conditions that greatly affect how much fertilizer-N a crop will need. They are: point-to-point variability within a field (spatial variability) and year-to-year variability over time (temporal variability).

Spatial variability has been the focus of "precision ag" research for the past 10 years and has led to development of variable rate fertilizer applicators. Temporal variability was largely ignored until scientists evaluated and described it in 2000 for winter wheat production in Oklahoma.

Bermudagrass forage yields from non-fertilizer sources greatly affect year-to-year N fertilizer need. Most N from non-fertilizer sources is believed to come from release of soil organic-N just before and during the growing season. This N is also called mineralized-N. An N-rich strip is used during the growing season to estimate the potential yield for that field, that year. The amount of mineralized-N supporting the yield potential is gauged by comparing the field, in general, to the condition of the N-rich strip.

Bermudagrass forage yields vary greatly from year to year, but mineralized-N changes even more. Whats more, the two are unrelated, meaning that the soil can mineralize non-fertilizer N, making it plant available, but the weather conditions at that time determine if there will be a positive forage response or not. These surprising findings by Oklahoma State University scientists led them to look for a way to help farmers better manage fertilizer N inputs.

Questions about Nitrogen-Rich Strips

What is an N-rich strip?
Use of mineralized-N by the crop can be estimated only if the crop has not already received all of the fertilizer-N needed to meet its requirement for the growing season. Thus, the new strategy is to apply little or no fertilizer-N to bermudagrass at green-up, except for a spreader width the length of the field that receives enough early season fertilizer-N in order to assure the crop is not deficient of N. This spreader-width application is the N-rich strip for that field.

How much N fertilizer should be applied in the N-rich strip?
At the start of the growing season, apply either twice the normal recommended fertilizer-N rate or 300 pounds of actual N per acre. After each hay cutting or grazing event through the growing season, apply an additional 50 pounds of actual N per acre to the strip.

Where should the N-rich strip be in the field?
The N-rich strip should be located through a representative part of the field. If possible, it is also useful to place the strip where it will be convenient to periodically observe whether it looks different from the rest of the field. After appropriate marking, the strip can be used every year.

How is the N-rich strip compared to the rest of the field?
The N-rich strip and an adjacent similar part of the field that did not receive as much fertilizer-N are each individually "read" by using either an optical, hand-held GreenSeeker sensor or the unaided eye. If using the sensor, it can predict how much fertilizer-N is required for that subsequent cutting or grazing event. If using visual observation, it can still tell you if a response will be obtained by adding additional N fertilizer.

When is the N-rich strip read?
The N-rich strip is read just before applying fertilizer-N and will identify the proper fertilizer rate to use. Earlier than "normal" application is indicated when bermudagrass in the N-rich strip appears to be in better condition than the rest of the field. Decisions about early N fertilization should be made at this time. When the N-rich strip looks the same as the rest of the field, either use sensor readings or the unaided eye to confirm there is no difference. If no true difference is observed, then no additional fertilizer-N is needed for the rest of the field.

What exactly do the sensor readings tell us?
Sensor readings, together with the number of days the crop has been growing, are used to estimate yield. The yield without fertilizer-N, how much N should be applied and the potential yield with fertilizer-N application are all calculated. Ongoing research at the Noble Research Institute, in collaboration with Oklahoma State University, is working on verifying these calculations for bermudagrass.

What is the value of using the N-rich strip for making fertilizer recommendations?
Research shows an increase in profit using the N-rich strip and sensor readings to determine N fertilizer rates compared to using yield goals. The increased profit is related to the fact that yield goals (averages) are easy to calculate, but are only experienced about one-third of the time. The chance of using the correct amount of fertilizer-N each year is increased when using the N-rich strip technology, and only applying N when it is needed will improve fertilizer use efficiency.

For further information regarding the use of N-rich strips or sensor technology, contact either a Noble Research Institute soil and crops specialist by calling the Ag Helpline at (580) 224-6500 or, if in Oklahoma, your county Extension agent.