What is Variable N-Rate Application (VRA)? How does it work?

Precision fertilizing is made possible by Variable Rate Application (VRA). With this technology, the amount of fertilizer applied is precisely adapted to the location or characteristics of an area.


Field conditions can vary considerably due to soil characteristics, humus content, water supply, different previous crops, etc. You can respond to these field conditions by applying nitrogen at variable rates and achieve these benefits:


  • Increased nitrogen use efficiency
  • Less nitrogen left in the soil after harvest, reducing the risk of leaching
  • Reduced nitrogen balance surplus
  • Easier compensation of growth differences within a field
  • Reduced risk of crop lodging
  • Easier harvesting due to more homogeneous crop growth
  • Higher yields and protein content in relation to weather conditions
  • Save materials and costs
  • Protect the environment



How does it work in Atfarm?


The Variable N-Rate Application (VRA) feature can be accessed via both the Atfarm web app in your desktop browser and the Atfarm mobile app on your mobile device. However, not all options available in the Atfarm web app, are available in the mobile app.


Once you've added your field to Atfarm, you'll see a recent satellite image with a biomass layer overlaid. Atfarm uses data from the Sentinel-2 A and B satellites of the European Space Agency's Copernicus program to calculate a vegetation index and produce a biomass map that can be used for the creation of application maps. The satellite images must be cloud-free in order to calculate a vegetation index.


You start by selecting a biomass map date (latest cloud-free satellite image) and providing crop specific inputs such as growth stage, target N-rate and fertiliser to be applied. You can then either let Atfarm generate the application map using the VRA algorithms, or you can draw the application map from scratch (web only) and define the fertilization zones and amounts with the brush editor. Either way, you'll be able to adjust the fertilization zones and rates using the brush editor.


You can then export the generated application map to your terminal or, if you don't have compatible equipment, send it to your mobile device and simply use the free Atfarm mobile app as your terminal. With the Atfarm mobile app, you can then spread either by flow rate if your spreader can adjust the flow rate while driving, or by speed if it can't.





What makes Atfarm's application maps special?


When Atfarm generates the application map, it uses different algorithms to calculate the optimum N-rate for the field, based on information about canopy density and N-uptake provided by the vegetation indices from the satellite images. The algorithm used depends on the crop type of your field and your country.



VRA Basic algorithm


This algorithm is available for many countries and crops. The application map is generated from the Normalized Difference Vegetation Index (NDVI) calculated from the latest cloud-free satellite image and your inputs. The VRA Basic algorithm is available in the Atfarm web app. In the Atfarm mobile app, the VRA N-Sensor algorithm is used.



VRA N-Sensor algorithm


This algorithm isn't only based on a vegetation index calculated from a cloud-free satellite image but also on data from years of conducted field trials with the Yara N-Sensor. The application map is generated based on the Yara N-Sensor target rate algorithm which sets Atfarm apart.


The VRA N-Sensor algorithm is based on thousands of samples and lab tests and adapts to the crop and growth stage. If we've conducted enough field trials in your country with the Yara N-Sensor to calibrate the algorithm, the VRA N-Sensor algorithm will be used. A blue badge in the VRA Map Builder saying 'Powered by N-Sensor technology' will indicate that the VRA N-Sensor algorithm is used.


The Yara N-Sensor Index is what sets Atfarm apart. The index measures not only red and near-infrared radiation, but also the spectral ranges in between. With this fine gradation and the inclusion of crop and regional data, the N-uptake can be determined even for advanced growth stages. This is particularly important for late nitrogen applications. Read more about the Yara N-Sensor Index here.



What variable rate strategies are available?


The 2 variable rate distribution strategies available are levelling for compensatory fertilization and protein dressing for potential fertilization.


Either you or Atfarm choose the strategy, depending on the algorithm used to calculate the application map. Which algorithm is used depends on the crop type of your field and your country. If the VRA Basic algorithm is used, you choose the strategy. If the VRA N-Sensor algorithm is used, either you or Atfarm selects the strategy depending on the growth stage.


For example, in cereals, Atfarm automatically applies the levelling strategy for the 1st and 2nd application (BBCH 20-29, BBCH 30-36). For the 3rd application (BBCH 37-51) you can choose the strategy. If this is the last application, protein dressing should be selected. If a 4th dressing is planned, levelling is recommended for the 3rd application. From BBCH 59 (flowering) Atfarm automatically applies the protein dressing strategy.





The levelling strategy applies higher nitrogen rates to areas with lower biomass/nitrogen uptake and lower nitrogen rates to areas with higher biomass/nitrogen uptake. The area is homogenized and fertilizer is redistributed from well developed areas to less developed areas. It's also assumed that there is enough water in the soil to dissolve the fertilizer and absorb the nutrients.


This strategy is best used when nitrogen is the growth limiting factor to ensure that all areas of the field have sufficient nitrogen. Levelling should be applied when the crop can still compensate for growth differences, usually during tillering, stem elongation and booting.


The Biomass Cut-Off ensures that nitrogen isn't wasted on areas that are unlikely to yield due to crop damage, waterlogging, etc. Instead of applying very high rates of nitrogen because of low biomass, it's assumed that the crop is uneconomical to treat, and the rate is reduced. When the biomass measurement falls below a certain level (<30 % of the average field N-uptake), the Biomass Cut-Off instructs the redistribution algorithm to reduce the application rate to the minimum rate (lower limit).



Protein dressing


The protein dressing strategy applies higher nitrogen rates to areas with higher biomass/nitrogen uptake and lower nitrogen rates to areas with lower biomass/nitrogen uptake. The area is differentiated and fertilizer is redistributed from less developed areas to more developed areas.


This strategy is best used when other factors, such as water availability, are limiting crop growth and yield potential to ensure consistent protein concentrations and prevent protein dilution in high yielding areas.


The protein dressing strategy is designed for the final application at later growth stages. You can select protein dressing for late growth stages. If you want to apply the protein dressing strategy to earlier growth stages, you can draw the application map from scratch and then change the fertilization zones and enter your own rates.

Was this article helpful?

24 out of 26 found this helpful

Have more questions? Submit a request



Please sign in to leave a comment.