Materials and methods

water, and remote sensing sensors would be incapable of providing soil-related results.

Moreover, flooded soils like paddy fields are not easily accessible for soil sampling due to the almost permanent inundated condition. Despite these limitations, on-the-go sensor measurement of soil variability under flooded conditions is essential to implement PA practices in wetland crop cultivation systems. Consequently, a soil sensing system is needed to acquire information about the spatial variability of inundated paddy soils.

Therefore, the objective of this study was to develop, operate and evaluate a floating sensing system capable of providing relevant soil information to support PA under flooded paddy field conditions.

4.2 Materials and methods

4.2.1 The Floating Soil Sensing System (FloSSy)

Low-land paddy cultivation is typical for a flood prone country like Bangladesh.

Extensive soil information from these regularly flooded paddy soils is limited due to water logging. To acquire high resolution soil data efficiently in wet field conditions, a Floating Soil Sensing System (FloSSy) was developed. The FloSSy was designed so that it can be used in flooded fields even under seasonal rainy conditions. Therefore, the housing for all electronics and communication cables was made waterproof. To provide the geographic locations of the recorded soil measurements, the system was equipped with a GPS receiver.

Moreover, the FloSSy simultaneously processed real-time raw ECa and GPS information and converted those to the format required by the user. This required a software component, the FloSSy logger, to function in fully automated on-the-go mode. The graphical user interface (GUI) of the logger displayed the traversing path of the sensor in real-time otherwise it would be impossible, in a flooded field, to track the previously measured path in order to keep a constant distance between measured lines. A last condition that was fulfilled was related to the weight of the sensor carrying platform: light weight material that could float on water was used for the construction of the platform. At the same time, it was made stable enough against water waves to prevent accidental turning over.

As such, the FloSSy consists of an EMI soil sensing instrument, a GPS, a field laptop, a real time sensor data processing and path guidance software, a waterproof housing for the soil

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sensor, a floating platform for the sensor and the GPS and a wetland cultivation vehicle to pull the entire sensing platform on water (Fig. 4.1).

The EM38 soil sensor (Geonics Limited, Ontario, Canada) was used because of its non-invasive nature, structural simplicity, light weight and small physical dimension: all characteristics important for a floating sensor platform. Technical details and working principle of the EM38 can be found in McNeill (1980b). The sensor signal is a function of soil parameters like soil salinity, organic matter content, soil moisture content, bulk density, clay content and the amount of charge on the clay particles (Rhoades et al., 1999; Triantafilis et al., 2002). If one or more parameters are reasonably homogeneous, the influence of a more variable parameter becomes more important. For instance, for a given salinity, ECa increases as the volumetric water content at field capacity, gravimetric water content of a saturated soil and soil bulk density increase (Rhoades et al., 1999). Such correlated soil properties can be expected to be determined from the ECa measurements. The EM38 sensor can be operated in two orientations each having a different depth response profile. The horizontal orientation receives its major influence from the near-surface (0 - 0.3 m) soil layers while the vertical dipole orientation receives a dominant influence from the deeper (0.3 - 0.6 m) soil layers (Saey et al., 2008a).

Figure 4.1 FloSSy with: (i) laptop (protected by a plastic sheet), (ii) GPS antenna, (iii) waterproof sensor housing with an EM38 inside, (iv) floating platform and (v) power tiller.

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The GPS receiver NL-422MP (the NAVILOCK®, Berlin-Zehlendorf, Germany) was used to georeference the sensor-collected ECa information. Both the EM38 and the GPS were connected to an all-terrain grade laptop (Dell ATG 600 model), resistant against harsh environmental hazards. Separate USB 2.0 cables were used to transmit ASCII data streams to separate communication ports of the computer.

The floating platform was sufficiently lightweight to be trailed by a 12 HP vehicle operating on a muddy inundated field. First, a watertight housing was made according to the dimensions of the EM38 sensor from a cylindrical PVC (polyvinyl chloride) tube. The ends of the tube were closed with T-shaped PVC socket caps. The socket joints between the caps and the cylinder allowed opening and closing the hollow chamber whenever needed. The EM38 was placed inside the PVC cylinder and removable foam packing was used on either sides of the sensor to ensure safety against vibration. Another 1.8 m long PVC pipe was joined vertically to the sensor carrying pipe which enabled the data communication cables to be connected to the sensor. On top of this vertical pipe the GPS receiver was attached.

Therefore, the position indicated by the GPS corresponds to the centre of the EM38 sensor on the ground. A custom designed wooden sled was made to carry the sensor-bearing PVC tube.

One rectangular shaped wooden board (1.5 m by 0.60 m by 0.05 m) resting on top of two wooden planks was attached with 8 wooden screws to produce the platform structure. The platform can be attached to the back of a vehicle with a rope. In order to have noise free signals a distance of 1.8 m between the sensor head position and the closest metal part of the vehicle was ensured. A special type of motor vehicle called a ‘power tiller’ that is operated in wetland paddy cultivation systems was used to pull the sensor-carrying platform.

4.2.2 Software development

The computer language G from LabVIEW (National Instruments, 2003) was used to design and implement the FloSSy logger, software for ECa and GPS data logging and processing. The software architecture was designed for real-time soil sensor and GPS communication, data acquisition, decoding of the information byte, processing and display of acquired ECa data.

The processed data are written to a text file with headers and tab delimited values at a frequency of 4 times per second. The logged file contains the data on geographic location

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ordinates in WGS 84 datum format along with the time-stamped ECa values and respective serial number for each entry.

The communication port settings on both the GPS and the EM38 are configurable from the GUI of the FloSSy logger. These settings need to be adjusted at the beginning of each survey after connecting the instruments and the computer. The GUI displays the raw and the processed data from both the EM38 and GPS. The traversing path of the FloSSy was displayed on a computer screen in real time to guide the distance between measured parallel lines. For this, the laptop was secured inside a box and covered with a polyethylene jacket to guard against the splashes of field mud and raindrops. The laptop screen was visible to the vehicle driver.