Use of Reference Strips to Determine Nitrogen Fertilizer Requirements

CSU Extension Website

Article by Wilma Trujillo

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

In the past 20 years, spatial variability has been the focus of “precision agriculture” research. It has led to development of variable rate fertilizer applicators. In contrast, temporal variability was largely ignored until a group of scientists from OSU recently evaluated and described it for wheat and several other crops.

Several studies have demonstrated that the demand for N fertilizer changes from year to year due to variations in environmental conditions (precipitation and temperature). Certainly, yield response to N fertilizer is unpredictable at the time when pre-plant N is being applied. For example, when winters and springs are wet and warm, sufficient N can be mineralized from soil organic matter before and during the growing season. This mineralized-N can be enough to meet the demands for maximum yields. However, the only way to know how much N was made available between plant¬ing and time of top-dress would be by using a reference N strip.

If the optimal N fertilization rate changes from year to year, how can a farmer accurately calculate the correct amount of N fertilizer to apply? Nitrogen soil testing (ammonia and nitrate concentrations) pre-planting does not account for temporal variability. As a result, fields frequently are either under- or over- fertilizer which make it difficult to detect the economic and environmental impacts. Under fertilization could result in low yields and loss of profit, while over-application could end up in high fertilizer expense with no increase in yield and contamination of surface and ground waters.

The newly developed methods account for the temporal variability effect on N requirements, by making better N management decisions in-season. The growing crop can accurately show how much N was delivered for free when comparing the N Rich Strip with an area that represents the farmer’s practice (N check). Soil testing is still necessary for other nutrients. This method does not account for availability of phosphorous, potassium, sulfur and other macro- and micro-nutrients.

The N Rich Strip is an area in the field that receives enough N fertilizer. Nitrogen is not deficient during the growing season no matter what the precipitation and other environmental conditions. The rest of the field that receives no N or the standard pre-plant rate is called the Farm¬er’s Practice or N Check.

The most basic use of the N Rich Strip is to help farmers to decide the need of N in-season. If the N Rich Strip is clearly seen and shows a darker green color and more growth than the rest of the field or the N check, it means there is a need for N fertilizer. In contrast, if there is no difference, there is no need to apply any N fertilizer.

The N-rich strip is used in conjunction with the GreenSeeker™ hand held sensor to determine in-season N rates. The GreenSeeker™ measures nor¬malized difference vegetative index (NDVI) which is calculated from the reflected light from the canopy of the crop. This value provides a highly accurate estimate of plant biomass. Therefore, yield potential can be predicted in-season using both sensor and some known climatic data (precipitation growing degree days, soil moisture) from planting to sensing. 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 by field. They are simply made by a double or triple pass of the applicator when pre-plant N is being applied. Also, it is recommended to place the strips in different management or yield zone of the field. This way an N rate can be prescribed for each zone. Pre-plant N rate should be at least 1/3 of the total N needed for yield goal. The N Rich Strip should be at least 125% of the total N recommended to achieve yield goal.

When farmers know their crops’ fertilizer needs and potential benefits, they have a better chance of maximizing their inputs rather than misjudging the needs for that particular crop during that season. This advanced technology was developed to make N management easier for producers. Sensor-based nutrient management gives producers the ability to evaluate crops during the growing season and make decisions as opposed to guessing the correct fertilizer rate before the season even begins. The past few years, producers are more familiar with sensor-based nutrient management to maximize their costs of inputs, as well as utilize N fertilizer more efficiently.

For more information on N management tools, please contact your local Colorado State University Extension offices or visit us at: Other resources are found at:

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