Review
Review of Novel and Emerging Proximal Soil
Moisture Sensors for Use in Agriculture
Marcus Hardie
Tasmanian Institute of Agriculture, University of Tasmania, Hobart, TAS 7000, Australia;
marcus.hardie@utas.edu.au
Received: 9 November 2020; Accepted: 29 November 2020; Published: 4 December 2020
Abstract:
The measurement of soil moisture in agriculture is currently dominated by a small number
of sensors, the use of which is greatly limited by their small sampling volume, high cost, need for
close soil–sensor contact, and poor performance in saline, vertic and stony soils. This review was
undertaken to explore the plethora of novel and emerging soil moisture sensors, and evaluate their
potential use in agriculture. The review found that improvements to existing techniques over the
last two decades are limited, and largely restricted to frequency domain reflectometry approaches.
However, a broad range of new, novel and emerging means of measuring soil moisture were
identified including, actively heated fiber optics (AHFO), high capacity tensiometers, paired acoustic
/ radio / seismic transceiver approaches, microwave-based approaches, radio frequency identification
(RFID), hydrogels and seismoelectric approaches. Excitement over this range of potential new
technologies is however tempered by the observation that most of these technologies are at early
stages of development, and that few of these techniques have been adequately evaluated in situ
agricultural soils.
Keywords:
matric potential; capacitance; soil moisture probes; dielectric constant; soil humidity;
soil water FDR; TDR
1. Introduction
Knowledge of soil moisture is important for supporting agricultural production, catchment
hydrology, flood forecasting, landslide prediction and other ecosystem services [
1
–
3
]. Globally,
agriculture is the largest water user accounting for approximately 70% of total water consumption [
4
].
Global demand for diminishing water resources has triggered renewed interest in the development
of proximal soil moisture sensors for improved management of irrigation and soil moisture in
agriculture. Proximal soil sensors are defined as being in contact with, or within proximity to the
soil (<2 m). Proximal sensors are usually classified as being (i) in-situ, stationary or point scale,
including both invasive or buried sensors, or (ii) noninvasive sensors which may operate on or
near the ground surface including being attached to a vehicle to generate ‘maps’ of soil moisture
variability [
3
,
5
]. Use of non-proximal platforms such as drones, aircraft and satellites are not in the
scope of this review, see [3,6–8].
An extensive range of proximal soil moisture sensors have been commercialized for use in
agriculture. Yet despite the efforts of government agencies, and private consultants, remarkedly few
farmers use sensors for monitoring soil moisture or scheduling irrigation. Literature on the adoption
of soil moisture sensors is scarce, studies report adoption rates of around 8–13% in Australia, [
9
],
18% in Washington State, USA, [
10
] and 3–4% in Southern Alberta, Canada [
11
]. Poor adoption
seems to be related to a combination of operational and soil constraints rather than issues with sensor
accuracy. Operational constraints include cost, data volume and interpretation, poor soil–sensor
contact, small measurement volume, lack of portability and installation hassle, creation of in-field
Sensors 2020, 20, 6934; doi:10.3390/s20236934 www.mdpi.com/journal/sensors
