Precision agriculture
Variable rate application (VRA) maps: from field zones to an ISOBUS-ready file
A variable rate application map — often called a VRA or prescription map — tells the machine what rate of fertiliser, seed or crop protection product to apply in each part of the field. The zones can be based on multi-year satellite imagery, yield maps and soil tests, while the rates come from the fertiliser plan. The finished prescription is then exported to a compatible ISOBUS terminal as ISOXML or Shapefile.
What is a variable rate application map
With uniform-rate application, the spreader uses the same rate across the entire field. A VRA map replaces that single setting with rates assigned to specific areas. Using GPS position, the terminal identifies the current zone and adjusts the rate as the machine moves through the field.
VRA stands for Variable Rate Application. You will also see the terms "prescription map", "application map" or "task map". They all mean the same thing: a file with polygons (or a grid of cells) and a rate assigned to each.
To use VRA, you need three things: a rate-controlled implement such as a spreader, drill or sprayer; a terminal that can read prescription maps; and a GPS signal. The hardware does not have to be top of the range, but the terminal and implement controller must be able to communicate — most commonly through ISOBUS.
Where the zones on a VRA map come from
A zone is a part of the field that behaves in a similar way and where a single rate makes sense. Good zones are stable: the same areas yield better or worse year after year. That is why they are not drawn from one image but from data covering several seasons.
| Source | What it shows | What to watch out for |
|---|---|---|
| Multi-year satellite imagery (e.g. NDVI from Sentinel-2) | Persistent differences in crop biomass between parts of the field | A single image may show short-term stress, weeds or a drilling error rather than a lasting difference |
| Yield maps from the combine | Actual yield in each part of the field | They need cleaning: headland turns, partial header width, sensor delay |
| Zone-based soil testing | P, K and Mg indices and soil pH | They give values for sampling zones, not for every metre of the field |
| Local knowledge | Waterlogged hollows, wet patches, sandy knolls, old field boundaries | Worth using to check the zones before they go to the machine |
If you want to understand what each satellite index tells you and why a single-day NDVI is not enough, read our guide to NDVI, NDMI and NDRE maps.
In Nirby, management zones are derived from patterns visible in satellite imagery across several seasons. The same zones can guide soil sampling and later become the basis for the fertiliser plan and VRA map. This keeps the laboratory result tied to the exact area for which the rate is calculated. We explain the sampling process in our guide to soil sampling by management zones.
How to set the rate for each zone
Zones tell you where the field differs. They do not tell you what to do about it. The rate for a zone comes from the fertiliser plan: crop requirements, expected yield, previous crop, soil nutrient status and legal limits such as the Nitrates Directive.
For phosphorus, potassium and magnesium, the starting point is the soil test results. A zone with a low index gets more, a zone with a high index gets less or none. Nitrogen is a harder call, because you have to choose a strategy.
| Strategy | Logic | When it works |
|---|---|---|
| Levelling (compensation) | More where a nutrient is lacking or the crop is weaker | P, K, Mg and liming according to soil indices; weaker areas where the deficiency can be corrected |
| Yield potential | More where the field consistently yields well, less where yield is limited by water or soil | Nitrogen in zones with permanently different potential, e.g. sandy knolls versus fertile hollows |
There is no single right answer. If a weak zone is weak because it lacks water, extra nitrogen will not help. If it is weak because of low pH, you need to lime first. That is why a VRA map is only as good as the plan it was built from.
In Nirby, the fertiliser plan calculates recommendations for each zone using soil nutrient status, crop requirements, Nitrates Directive limits and Polish eco-schemes. The completed plan can then be converted into a prescription map without redrawing zones or re-entering rates.
It is important to set the right expectation: variable rate application primarily redistributes fertiliser across the field and does not necessarily reduce total use. For example, a 20 ha field at a flat 200 kg/ha would use 4,000 kg. With zones of 6 ha at 150 kg, 10 ha at 200 kg and 4 ha at 260 kg, total use is 3,940 kg. The saving is small, but more fertiliser is directed to areas where it has a better chance of generating a return. The real effect can only be assessed after harvest by comparing yield with the management zones.
File formats: ISOXML or Shapefile
A terminal cannot read images or PDFs. It needs a file with geometry and rates in a format it understands. In practice two formats dominate: ISOXML and Shapefile. Nirby exports prescription maps in both.
| ISOXML | Shapefile | |
|---|---|---|
| What it is | The task data format of the ISOBUS standard (ISO 11783-10). A TASKDATA.XML file, usually in a TASKDATA folder, sometimes with binary grid files | A GIS format made up of several files with the same name: .shp (geometry), .shx (index), .dbf (attributes, including the rate), often also .prj (coordinate system) |
| Where it works | ISOBUS terminals with task management (Task Controller) | Terminals and software that do not read ISOXML or handle maps through their own import |
| Advantages | The task holds the field, product, unit and rate; the terminal can record what was applied in the same task | Simple, universal, easy to inspect in any GIS program |
| Typical pitfalls | Wrong folder structure on the USB stick, incompatible standard version, no Task Controller licence on the terminal | Missing .shx or .dbf file, wrong rate column name, unsupported coordinate system, Polish characters in names |
The rule of thumb is simple: if your terminal and machine support ISOBUS task management, start with ISOXML. If the terminal has its own map import or no task licence, use Shapefile. Check your terminal's manual to see which formats and versions it supports.
How to load a map onto an ISOBUS terminal, step by step
Menus differ between manufacturers, but the process itself looks similar on most terminals. Below is a general, brand-neutral version.
- Prepare a USB stick. Use an empty stick formatted as FAT32, because some terminals cannot read other file systems.
- Export the map for the right field, crop and product. Check the rate unit before saving the file.
- Save the files in the right structure. For ISOXML: a TASKDATA folder with TASKDATA.XML in the root of the USB stick (unless the terminal manual says otherwise). For Shapefile: the full set of .shp, .shx and .dbf (plus .prj, if present) in one folder, without renaming them.
- Import the data on the terminal. For ISOXML this is usually done in the task management module, for Shapefile in the prescription map or data import module.
- Select the rate column and unit (mainly for Shapefile). The terminal will ask which field in the .dbf file holds the rate. Choose the right one and confirm the unit.
- Assign the product and machine. Check that the terminal has recognised the spreader or drill and that the product has the correct settings, e.g. density or calibration factor.
- Set a default rate. This is the rate applied outside the map or if the GPS signal is lost. The average rate for the field is usually entered.
- Turn on section control, if the machine supports it, to reduce overlaps on point rows and headlands.
- Start the task and do a test run over the first few metres. Watch whether the rate on screen changes at zone boundaries.
Test your first map on one field before you load tasks for the whole farm. A few minutes at the terminal saves a whole day of corrections.
Common problems with VRA maps
Most problems with a prescription map only show up in the cab. These are the ones that come up most often.
- The terminal cannot see the file. Wrong folder structure, a missing Shapefile component, an unsupported file system on the USB stick, or the terminal is looking for data in a different folder.
- The map appears in the wrong place or not at all. The coordinate system is usually to blame. Terminals most often expect WGS84 geographic coordinates, and a map saved in, say, PUWG 1992 (the Polish national grid) without a .prj file lands far from the field.
- The rate is zero or absurdly high. The terminal has read the wrong column from the .dbf file or mixed up the units. Check the column name, as some terminals limit name length or do not handle Polish characters.
- Zones are smaller than the working width. A spreader with a 24 or 36 metre working width cannot deliver a rate in a zone 10 metres wide. Small islands need to be merged with a neighbouring zone, or the map smoothed.
- The rate changes too late. Spreaders and drills respond with a delay. Terminals have a look-ahead setting that compensates for the response time; it is worth calibrating.
- Headlands and field edges. On headlands the machine turns, and at the boundary border spreading kicks in. Here the map interacts with the machine settings, so the actual rate may differ from the plan.
- Excessive rate jumps. A change from 80 to 300 kg/ha between neighbouring zones may exceed how quickly the machine can respond. Smoother transitions are more realistic to apply accurately.
What to check after application
The process does not end when the machine leaves the field. The terminal will usually record the as-applied rate, treated area and product use. Compare those figures with the plan. If actual use differs by more than a few per cent, calibration is often the first thing to check rather than the map itself.
The second step is recording the application in the field history. In Nirby you record the completed operation in the system, so the whole process, from soil tests and the fertiliser plan to application, stays documented. The record also goes into your spray and application records and is available when you plan the next season.
The third step comes after harvest. A yield map set against the zones shows whether varying the rate made sense. If the zone that got more nitrogen did not yield more, that is valuable information for next year. After a few seasons you have your own data, not just assumptions.
Checklist before loading a VRA map
- Zones delineated from multi-year data, not from a single image.
- Up-to-date soil test results assigned to the zones (for P, K, Mg and pH).
- Rates come from the fertiliser plan and stay within Nitrates Directive limits.
- Rate unit clearly defined: product or nutrient, kg/ha or l/ha.
- Zones are no narrower than the machine's working width.
- File format compatible with the terminal (ISOXML or Shapefile) and the correct folder structure.
- For Shapefile: a complete set of files and a known rate column name.
- Default rate set and section control switched on.
- Machine calibrated for the specific product.
- A plan to record the application in the field history and assess the effect after harvest.
If you are just starting out, make your first map for one field with clear variability and for the nutrient where the decision is simplest, e.g. potassium based on soil testing. Once you have been through the whole process once, you will prepare the next maps much faster.
Frequently asked questions
What is a variable rate application map?
It is a file that assigns a rate of fertiliser, seed or plant protection product to individual parts of the field. The machine terminal reads its position from GPS and changes the rate according to the zone it is in. The map is built from field variability data, soil test results and the fertiliser plan.
How do I make a variable rate fertiliser map?
First delineate the field zones, ideally from multi-year satellite imagery or yield maps. Then take soil samples by zone and set the rates in the fertiliser plan. Finally, turn the plan into a prescription map and export it in a format your terminal supports, e.g. ISOXML or Shapefile.
How do I load a VRA map onto an ISOBUS terminal?
Save the map to a USB stick in the structure the terminal requires: for ISOXML a TASKDATA folder with TASKDATA.XML, for Shapefile the full set of .shp, .shx and .dbf files. Import the data on the terminal, select the rate column and unit, assign the product and machine, and set a default rate. You will find the menu details in the terminal manual.
What is the difference between ISOXML and Shapefile?
ISOXML is the task data format of the ISOBUS standard: it holds the field, product, unit and rate, and the terminal can record the completed application in it. Shapefile is a general GIS format with zone geometry and an attribute table. It is simpler and universal, but you have to select the rate column and unit manually.
Does variable rate fertiliser application pay?
It depends on how variable the field is. On a very uniform field, the benefit may be limited. Where soil conditions and yield potential vary clearly, VRA can shift fertiliser towards areas where it is more likely to generate a return and reduce it where additional input is unlikely to help. Total use does not always decrease, so the result should be evaluated after harvest using yield data.
