Space age fertiliser spreading

What have GIS, DSS, NDVI, pixels, the Mitscherlich equation and gamma-ray spectrometry got to do with fertiliser? They may not yet be part of every farmer's lexicon, but they may soon become part of more cost-effective application systems.

While today's soil testing and fertiliser prediction are a far cry from the old rule of thumb of applying between one bag and half a bag to the acre, an ongoing difficulty remains the variation in nutrient level across a paddock.

Soils in the western region are notoriously variable in their physical properties and in their nutrient content. Yield mapping shows it is not uncommon for yield within a paddock to vary ten-fold within only short distances.

This leads to inefficient fertiliser use as rates are too low in some places and too high in others. This holds true for K as well, as natural soil levels vary such that response to fertiliser K may be large in parts of paddocks, but non-existent in places only a few metres away.

Better than average?

Mike Wong and his collaborators in Western Australia believe that we can do better than averaging fertiliser application, and have set out to prove it with K.

Using a 'Geographic Information System' (GIS), they mapped an 80-hectare paddock at Corrigin in Western Australia measuring soil K on a 100 m x 100 m grid. They also needed to know the achievable yield across the paddock. First, the mean maximum achievable yield for the paddock was calculated using the French and Schultz equation (other models of potential yield could be substituted). In this case it was 3,480 kg/ha.

Relative yield (i.e. the variation in yield) across the paddock was estimated from satellite images. This agreed well with the farmer's estimate and can be enhanced as annual yield maps of the paddock are accumulated. A decision support system then links achievable yield and soil K level to provide a K fertiliser requirement for each 'pixel' of the paddock.

In addition, Dr Wong and his co-workers have shown that on-ground gamma-ray spectrometry can reduce the cost of soil testing by identifying areas unlikely to respond to K.

The first tests of the system are promising and it may not be long before fertiliser spreading means hooking up the variable rate fertiliser spreader, turning on the in-cab computer and dialing up the satellite to monitor the position of the rig.