Variable rate technology (VRT) means matching fertiliser or soil amendment rates to soil types, fertility, and yield potential. Trials in Western Australia (WA) showed a gross margin increase up to $50/ha using VRT. Costs might stay the same but resources are better allocated. This can translate to improved yield.
Because yield dictates nutrient demand, VRT needs sound yield estimates in different production zones and accurate soil fertility mapping. Measuring soil nutrient levels in different production zones is the first step.
Soil sampling for VRT
Use yield or zone maps to guide where different soil fertility areas might be. The WA trial divided the farm into low, medium, and high production zones.
Collect soil samples from each of the zones. In a new paddock, if there isn’t any historical yield data, use aerial photos or NDVI to work out zones. VRT is viable when there is substantial variation in soil fertility or soil constraints. If your soil results are fairly consistent across the paddock, VRT may not be worth the hassle. If you are managing a variable paddock, combine the soil information with yield estimates to work out fertiliser rates.
For a detailed study, collect samples from areas with:
- known differences in yield
- different soil types
- past management practices.
Avoid sampling areas that might have unusual soil nutrient levels such as:
- problem areas
- old fertiliser dumps
- areas near trees, fence lines, and roads
- areas prone to waterlogging.
Update your farm records with the new data. Refine your zones if the new information causes a rethink.
In soil testing, the greatest potential for error is in how you take the soil sample. Use consistent methods and tools. Always record sample locations and create a sample map as you go. Use GPS where you can and keep the location information with the test results.
Making the most of VRT
VRT is not just for early-season nitrogen applications. It can be used for targeted nutrition throughout the season. Vary soil amendments such as lime and gypsum to target problem areas within a paddock.
In the WA project, the most profit came from low and medium yield zones. In high production zones, consider the season before putting on higher fertiliser rate.
Photo: VRT depends on accurate soil fertility and production zone mapping’, courtesy of Wayne Pluske, Equii.