Using Reference Bands to Determine Nitrogen Fertilizer Requirements – Sterling Journal-Advocate


In the late 1990s, Oklahoma State University (OSU) developed a method to determine nitrogen (N) fertilizer requirements. The nitrogen management strategy developed addresses two conditions that greatly affect the amount of nitrogen fertilizer a crop will need for maximum yield: (a) variability within a field (spatial variability) and (b) year-to-year variability (temporal variability).

For the past 20 years, spatial variability has been at the center of research in precision agriculture. This led to the development of variable rate fertilizer applicators. In contrast, temporal variability was largely ignored until a group of OSU scientists assessed and described it for wheat and several other crops.

Several studies have shown that the demand for N fertilizer changes from year to year due to variations in environmental conditions (precipitation and temperature). Certainly, the crop’s response to N fertilizer is unpredictable when pre-plant nitrogen is applied. For example, when winters and springs are wet and warm, enough N can be mineralized from soil organic matter before and during the growing season. This mineralized N may be sufficient to meet demands for maximum yields.

If the optimal rate of N fertilizer changes from year to year, how can a farmer accurately calculate the correct amount of N fertilizer to apply? Soil nitrogen before planting (ammonia and nitrate concentrations) does not take into account temporal variability. As a result, crops are frequently under- or over-fertilized, which makes it difficult to understand the economic and environmental impacts. Under-fertilized crops could result in low yields and lost profit, while over-fertilized crops could result in high fertilizer expenditure without increasing yield and without contamination of surface and ground water.

One way to know how much N has been made available between planting and the time of application could be to use an N reference strip. This method takes into account temporal variability. The N Rich Strip is an area of ​​the field that receives a high N rate. Nitrogen is not lacking during the growing season, regardless of rainfall and other environmental conditions. The rest of the field does not receive an N or the standard preplant rate. The growing crop can accurately show the amount of nitrogen delivered “free” by comparing the green color and growing biomass in the N-rich band with those in the control zone without N or N.

The most basic use of the N-rich band is to help farmers decide on the need for nitrogen in the season. If the rich N band is clearly visible and shows a darker green color and more growth than the rest of the field or the N control, it means that there is a need for N. On the other hand, if it doesn’t there is no difference, there is no need to apply any N fertilizer.

The N-rich strip is used in conjunction with the GreenSeeker â„¢ handheld sensor to determine in-season N levels. The GreenSeeker â„¢ measures the Normalized Vegetative Difference Index (NDVI) which is calculated from the light reflected from the crop canopy. This value provides a very precise estimate of the plant biomass. Therefore, the yield potential can be predicted during the season using both sensors and some known climate data (precipitation, growing degree days and soil moisture) from planting to detection. By knowing the yield potential of the N Rich Strip and the yield potential of the rest of a field, the N rate can be calculated.

Two N Rich Strips are recommended per field. They are simply carried out by a double or triple pass of the applicator during the application of the pre-implantation N. Also, it is recommended to place the strips in different management or yield zones of the field. In this way, a rate N can be prescribed for each zone. The nitrogen rate before planting should be at least 1/3 of the total nitrogen needed for the yield target. The N-rich band must be at least 125% of the total recommended N to meet the performance target.

When farmers know their crop’s fertilizer needs and potential benefits, they have a better chance of maximizing their inputs rather than misjudging the needs of that particular crop during this season. This method makes nitrogen management easier for growers. Sensor-based nutrient management gives growers the ability to assess crops during the growing season and make decisions instead of guessing the correct fertilizer rate even before the season begins. This practice helps producers to maximize their input costs, as well as to use nitrogen fertilizers more efficiently.


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