A strategic soil nitrogen test for flooded rice (2005)

Angus, J


This CRC report presents the results of the project undertaken to investigate s strategic soil nitrogen tests for flooded rice. The trial was undertaken between 1998 through until 2002. The projects aim was to develop a system to forecast the optimum N supply for pre-flood application and minimize the amount being topdressed which has been a safe, but inefficient system. The method undertaken included using wet chemistry to assist in developing a test by comparing the near infrared reflectance specatra with crop productivity and N Mineraliseation. Whilst 22 previous experiments measuring yield response to N applied at sowing were also used. The study undertook seventeen methods of mineralisation and the most reliable was found to be anaerobic incubation at 40°C for 21 days. This method predicted the optimum N requirement with a standard error of about 75 kgN/ha, which is clearly unsatisfactory for an industry where the average amount of N fertiliser applied is 145 kgN/ha. There was some evidence that sowing date and deficiencies of other nutrients were partly responsible for the variability of the N response. The project concluded that there were close relationships of the NIR spectra with crop productivity and N mineralisation but because of the small data set the relationships had little predictive value. However the close relationships found between NIRS, N mineralisation measured in the laboratory and crop performance produced encouragement for further studies. The study also concluded that there were two options proposed for more reliable application of N fertiliser at the time of sowing. The first to use the existing soil test only to identify soils with large amounts of potentially mineralisable N. Such a test could be the basis of a recommendation to apply little or no N fertiliser before sowing. Rice growers would still have the option of topdressing N fertiliser at the panicle initiation stage. The results in this project suggest that yield responses are more accurately predicted by sodicity than by the soil N test. The recommendation were to undertake further studies in this area.