Variable Rate Phosphate


Electromagnetic (EM) map showing the variability in conductivity in the top 50cm of the soil profile, low conductivity (red) is typically deeper sand whilst the higher conductivity (yellow through dark green) is shallow duplex soil with varying depth of sand over clay.


The EM map is broken into statistical management zones and trial strips of varying rates of phosphate applied to cross each statistical zone to evaluate each soil type for phosphate responsiveness. Phosphate rates used were 5kg, 10Kg, 15Kg and 20Kg/ha, a yield map was then used to assess the yield response to increasing rates of phosphate by soil zone. If each soil zone is not yield responsive to increasing rates of phosphate over and above rates that are already being applied we can now move to implementing a phosphate replacement model based on the phosphate removal of the previous crop, good quality yield maps are essential for this process.


Often soil testing sites are selected on a random basis and do not fully cover the range of soil variability across a paddock or farm. Once an EM or radiometrics survey has been undertaken and soil management zones are in place, long term soil and plant testing sites can be put in place that now cover the full range of soil variability.


EM zones overlayed with yield map, average yield determined per soil zone and replacement phosphate applied based on previous crops removal.


Whole farm zoned based on EM zones, all phosphate applied on previous crops phosphate removal.


Phosphate rate and cost savings by implementing variable rate phosphate over the farms 2009 cereal crops compared to a traditional standard flat rate of applied phosphate prior to variable rating.


Phosphate rate and cost saving with variable rate on the same farm during 2010.The cumulative phosphate savings over two seasons 2009 and 2010 = $75,000 or $37.22/ha.