Low-Error Soil Moisture Sensor Employing Spatial Frequency Domain Transmissometry.

A new type of soil moisture sensor using spatial frequency domain transmissometry (SFDT) was evaluated. This sensor transmits and receives ultrawideband (1 to 6 GHz) radio waves between two separated antennas and measures the propagation delay time in the soil related to the dielectric constant. This method is expected to be less affected by air gaps between the probes and the soil, as well as being less affected by soil electrical conductivity (EC), than typical commercial sensors. The relationship between output and volumetric water content (θ), and the effects of air gaps and EC were evaluated through experiments using sand samples and the prototype SFDT sensor. The output of the SFDT sensor increased linearly with θ and was not affected by even a high salt concentration for irrigation water, such that the EC of the pore water was 9.2 dS·m-1. The SFDT sensor was almost unaffected by polyethylene tapes wrapped around the sensor to simulate air gaps, whereas a commercially available capacitance sensor significantly underestimated θ. Theoretical models of the SFDT sensor were also developed for the calibration equation and the air gaps. The calculation results agreed well with the experimental results, indicating that analytical approaches are possible for the evaluation of the SFDT sensor.

Monitoring nitrogen leaching under different fertiliser applications during the early growth stage of sugarcane using GS3 sensors: a case study on Negros Island, Philippines.

Nitrogen (N) is normally applied twice during sugarcane cultivation on Negros Island in the Philippines. As the first application is carried out during the early growth stage, there is a high possibility that N is not absorbed sufficiently by the sugarcane and a large amount of N may be leached with percolating water. To investigate this leaching, we monitored the vertical movement of soil water and soil solutes using GS3 dielectric moisture and electrical conductivity (EC) sensors in a sugarcane field on Negros Island. The monitoring was conducted after the first N application at different application rates (50% or 100% of the recommended rate) and timings (immediately or 30 days after planting [DAP]). The EC values increased when 100%-N was applied immediately after planting, indicating that N leached into the deeper soil layer. The monitoring results suggest that the applied N did not remain in the root zone (due to leaching) and was not replenished until the second application, when the absorption ability of the sugarcane was high. Conversely, when 100%-N was applied at 30 DAP, the applied N remained in the root zone until the second application. When 50%-N was applied at 30 DAP, the increase in EC was small compared to the 100%-N application. These results indicate that applying N immediately after planting at the recommended rate led to an insufficient nutrient supply for the sugarcane, as well as the loss of N fertiliser via leaching.