Continuous Monitoring of Soil Redox Potential to Compare Layouts for Post-Rice Crops

The effect of different bay layouts (beds vs flat) on crop growth was investigated using continuous monitoring of redox potentials to explain soil O2 and nutrient (N and P) availability. Paddy rice typically leaves the soil in an anaerobic and water-logged state with low or negative redox potential. These conditions transform nutrients in the soil: free nitrate is reduced to di-nitrogen gas, and iron, phosphorous and manganese are reduced to their soluble forms. Following drainage of bays for rice harvest, re-oxidation of the soil causes other transformations, such as phosphorous and iron precipitation. These may negatively affect cereal crops planted straight after rice harvest. Raised beds may provide better soil conditions for crops after rice, making double cropping more viable and lifting the total water productivity of Australian rice farming systems. A wireless measurement and data logging system was developed and used to monitor redox potentials continuously. Very high input-impedance amplifiers were used to ensure the data logging system did not influence redox potentials by drawing current through the electrode-soil interface. Connectivity between multiple loggers and the internet was provided using a WiFi hotspot and cellular data connection. There was no significant effect of layout type on rice yield, but monitoring of redox potentials showed the soil drained and oxidised faster under the beds when bays were drained for harvest. This effect was short lived, as the onset of wet conditions in May 2016 re-saturated the soil and pushed redox potentials down to levels indicative of soil O2 being limiting to crop growth (i.e. <350 mV) in both beds and flat treatments. These conditions persisted through winter and the wheat crop eventually failed. Soil sampling for soluble N and P showed the primary reason the crop failed was low fertility, presumably caused by denitrification under reduced conditions. An expected tie-up of phosphorous by oxidised iron was not observed. Layout type had no significant effect on nutrient concentrations. Continuous monitoring of redox potential using the loggers developed for this project provided a low-cost and effective method of evaluating treatments aimed at improving the total water productivity of the Australian rice rotation.