# Demonstrating the Potential of a Low-Cost Soil Moisture Sensor Network
###### Abstract  
Soil moisture is a key parameter of the climate system as it relates to plant transpiration and photosynthesis and impacts land-atmosphere interactions. Recent developments have seen an increasing number of electromagnetic sensors available commercially (EM) for soil volumetric water content (\(\theta\)). Their use is constantly expanding, and they are becoming increasingly used for agricultural, ecological, and geotechnical applications and climate research, providing decision support and high-resolution data for models and machine-learning algorithms. In this study, a soil moisture sensor network consisting of 10 Sense Cap capacitance-based sensors is evaluated. Analytical performance of the sensors was determined based on laboratory and field measurements with dielectric permittivity (\(\varepsilon\)) standards and soil media substrates. Sensor response normalisation to standards of known \(\varepsilon\) was found to reduce intersensor variability and provide robust estimates of \(\theta\) in soil samples with known \(\theta\). Cross-comparison with a time-domain reflectometry (TDR) instrument carried out in two soil media demonstrates good agreement between the two probes throughout the tested range. The data communication performance of the network was evaluated in terms of packet drop rate at different ranges and sampling frequencies. It was noticed that the drop rate increased with distance from the gateway, while sampling frequency had no effect. Sources of errors associated with probe installation were identified and recommendations are provided for sensor deployment. The off-the-shelf all-in-one solution provided by Sense Cap is low cost, user friendly and suitable for implementation at temporal and spatial scales once the identified shortcomings are addressed. The evaluation presented aims to aid stakeholders and users involved in soil and land management practices including crop production, soil conservation, carbon sequestration and pollutants transport. dielectric permittivity; LoRaWAN; TDR; soil moisture +
Footnote †: journal: Sensors  
0202222987780  
data remain crucial for the calibration and validation of remote sensing products [10]. Such data can be obtained using gravimetric sampling, which is a labour-intensive, destructive technique, or through the use of EM sensors which can provide continuous high-frequency data and spatial distribution when used in a network [11]. EM sensors respond to the soil dielectric permittivity (\(\varepsilon\)) which is used to determine \(\theta\). Time-domain reflectometry (TDR) and transmissometry (TDT) sensors operate in the GHz range, while the impedance and capacitance sensors operate at lower frequencies from 20-300 MHz [12; 13]. In general, low-frequency sensors are much cheaper but more sensitive to cofounding effects of salinity, temperature, soil texture variability and imaginary dielectric permittivity [12; 14]. Though the TDR-method-based sensors are regarded as the most accurate EM-based technique for \(\theta\), their use in distributed sensor networks is limited due to the high cost. Instead, low-cost sensor networks are being increasingly employed for climate research [11], irrigation management [15; 16; 17] and validation of spatial methods [10; 18]. Soil moisture sensors operating within wireless sensor networks (WSNs) provide a significant reduction in wiring and harness and offer enhanced flexibility in network design and implementation, are easier to deploy and cost effective. Over the past few decades, a variety of wireless communication protocols have been designed and adopted for different applications. Each of these protocols are designed to meet specific requirements suitable for a particular set of applications. Mostly, wireless communication networks are evaluated based on their characteristics such as transmission range, data rate and power consumption. Wireless communication systems with high data rate (due to usage of higher frequency) can only support short-range transmission (due to limited wavelength).