Precision farming insights – Part 1: First step in Variable Rate Technology

Iulian Stefan Iulian is precision agriculture professional, currently working for Syngenta in Romania.
NDVI sensor mounted on a tractor. - Photo: Yara
NDVI sensor mounted on a tractor. - Photo: Yara

Every year farmers are asking me for advice on what should be the first step in variable rate technology. Every time my advice goes to Nitrogen fertilizer VRA as one of the most crucial, albeit expensive, tasks for a farmer in the application of nitrogen-based fertilizers. The following lines will help you gain a better understanding of the best methods to achieve this using VRA (Variable Rate of Application) technology and it will summarize my advice for them.

Variable Rate Application (VRA) for nitrogen fertilizers is best performed when the fields contain the plants, based on NDVI (Normalized Difference Vegetation Index). This information can be calculated using an array of sensors, ranging from handheld sensors to machine or drone-mounted sensors, and perhaps most commonly, from satellites.

Let’s go into the pros and cons of different ways of applying the VRA nitrogen, based on my experience in the field.

Application on-the-go is one of the most promoted and well-known methods of application. NDVI sensors are mounted on the tractor (and optionally on the application equipment) to read the ‘greenness’ and health of the plant. Based on this index, more or less nitrogen-based fertilizer is applied, depending on the crop’s condition and set parameters. Let’s explore the advantages and disadvantages of this application method.


  • It is probably the fastest and simplest method.
  • The precision of the NDVI value can be very good.

While on-the-go application has its advantages, there are some limitations to consider.


  • The application will not be done with a high degree of accuracy because when we use such tools, we have two working methods from which we have to choose:
    • 1. Where plants are greener and well-developed, we apply more fertilizer to help these plants produce more. For example, in areas of the fields where there are fewer plants or they are less developed due to factors like water logging or weak emergence
    • 2. We apply more fertilizer in areas where plants are not as well-developed because they lack sufficient nutrients, and thus, we apply larger quantities in different areas.

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Example image of a satellite used to scan the Earth for NDVI data
Example image of a satellite used to scan the Earth for NDVI data

Weather conditions

The farmer (or the consultant) should take the decision for each field individually, before the work is starting and the decision should be made based on the collected data, correlated with what is actually in the field to apply efficiently. It’s pointless to apply a lot of nitrogen where it hasn’t sprouted, for example.

Weather conditions influence the quality of the data read by the sensors. Therefore, once you enter a plot, it is ideal to work in the same light conditions throughout the plot, so you should not have clouds or the work should not last very long, so the sun does not change its position significantly.

The other way that this can be done and the way that I am recommending it in the most cases is: Scanning followed by application – this method involves scanning crops and making decisions about how to fertilize from the office (do we apply more or less where we have a good NDVI index?) depending on the situation in the field.

Collecting NDVI data

To collect NDVI data, we have three options, each with advantages and disadvantages.

  • Using dedicated sensors on a tractor or equipment will give us the highest precision, but we will have additional costs for purchasing the sensors and fuel, as well as time consumed.
  • Using drones equipped with NDVI sensors ensures lower costs in acquisition and operation, fairly good precision, but the collected data will have lower precision and will depend on the technical flying capabilities of the drone and the sensor used. In addition, we will also need a qualified operator to pilot the drone.
  • Satellite images are the last way we can have this NDVI information. The main advantages brought by satellites are the low cost (often free), the fact that you will not need equipment or personnel involved in data collection. The frequency with which you receive the data is high, but here we have several disadvantages. The major disadvantage is the resolution or accuracy of the data. Being so high in the sky, the sensors on satellites analyze entire blocks of land, which are much larger than the blocks you would scan with a drone, for example.

Experience for large crops

As per my experience lor large crops, in fields greater than 10 ha, I recommend using satellite data for NDVI, but for specialty crops or orchards I recommend using drone mounted NDVI sensors. The sensors mounted on the tractor are the best option, but I recommend it only to big farms, advanced technologically.

Once the data on how the crops are presented using one of the three methods mentioned, the agronomist will have to make the decision on how to apply, particularly for each plot. Do I apply more or less nitrogen where it is greener?

And as you made it that far into this article, I want to let you know that I will follow up on this with a real example and data from a farmer who chose to implement this approach for the first time in spring of 2023. I will let you know step by step on how he did it, the equipment used, the working method, encountered challenges, and the obtained results.

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