Efficient fertilizer management starts with the basics—applying the right source, at the right rate, in the right place, at the right time. These are the 4R’s. While soil-based zones are the most common basis for a 4R nutrient stewardship program, in-season plant-based zones can boost agronomic efficiency.

Imagery from drones or satellites can be a powerful tool to monitor in-season crop performance and nitrogen status. But going beyond just a “pretty picture” or “colorful map” requires understanding how to use imagery as a source of agronomic information.

First, it is important to keep in mind what an early-season NDVI image is measuring. At the most basic level, NDVI (Normalized Difference Vegetation Index) differentiates between areas where a healthy crop is growing and where bare soil is located. Increasing as the growing season progresses, NDVI indicates a higher proportion of healthy crop canopy. When NASA scientists developed NDVI in the 1970s, this is the key insight that they were leveraging!

 

 

While NDVI isn’t directly measuring crop nutrient status (as compared to a tissue or soil test), it still can be used as a tool to diagnose and locate different rates of crop growth to develop plant-based management zones. This approach leverages the same principle of indirect measurements that agronomists and growers use to develop soil-based management zones. Indirect measurements—such as electrical conductivity mapping, topography or bare-soil imagery—are used to map relative differences in soil texture, hydrology and fertility. In the case of plant-based zones, we are just mapping the same relative differences in crop growth without necessarily knowing the source of variation.

For both soil-based and plant-based management zones, understanding how to interpret relative differences requires both field-specific knowledge and expert agronomic knowledge. Just because you can detect differences doesn’t mean that you can also decide what to do about them!

As an example, let’s consider an NDVI image capture for corn at V6 that an agronomist would like to use to develop a plant-based management zone prescription for a variable-rate side-dress nitrogen application. While the NDVI map provides the basis for creating zones based on variation in crop growth, it doesn’t alone provide the insights needed to decide what to do. Many agronomists leverage reference strips, tissue or soil samples—or their own eyes and experience—to diagnose what the source of crop growth variation might be. In many cases, it’s a safe bet to assume that nitrogen (N) might be the root cause of NDVI variation; however, variation in crop growth could be due to any number of agronomic factors.

 

 

With plant-based management zones now created a variable-rate side-dress N prescription can be created and applied. However, deciding what rate of N to allocate to each prescription zone still requires the expertise of an agronomist and should be based on the goals for the field. Sometimes, variable-rate prescriptions are developed to “cut back” on fertilizer applications to areas of the field where yield potential has been severely reduced (for instance, a drown out). In other cases, a variable-rate prescription can be used to “double down” on areas to maintain maximum yield potential (for example, to counteract leaching losses).

Communicating the agronomic basis of a variable-rate side-dress application is a key piece of the process to provide confidence to growers that management decisions and recommendations are being made as efficiently as possible to help them achieve their goals.

Regardless of which variable-rate strategy is selected, the key to determining if a variable-rate N prescription is “correct” is to use follow-up monitoring with satellite-based NDVI and end-of-season harvest data.

Plant-based management zones using imagery are powerful tools for an agronomist to leverage for boosting agronomic efficiencies. The Sentera FieldAgent® platform translates NDVI imagery into actionable insights that acutely prescribe where, when and how much nitrogen fertilizer to apply, helping streamline workflows and reduce costs.

 

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