**6. Conclusions**

With the idea of saving resources and costs, this paper applied LPWAN technologies to an inventoried proximal soil sensor device by designing an attachment hardware system

(AHS) and realized technical upgrades. Compared with conventional sensors based on wired communication technologies (such as RS-485 and SDI-12), mobile cellular technologies (such as 2G, 3G, 4G, etc.), and short-range wireless communication technologies (such as Bluetooth, ZigBee, etc.), it not only retained the original mature sensing technology of the sensor device but also exhibited ultralow power consumption and long-distance transmission, while having the advantages of an LPWAN. At experimental level, it can be seen from the actual energy consumption test and analysis that a single-battery power supply could work for more than 3 years without natural attenuation; thus, meeting the requirements of general applications. Additionally, flame-retardant epoxy resin can be used for integral molding and pouring, and this would make the system more compact as a whole, with high mechanical strength, strong heat resistance, and easy deployment, as well as being maintenance-free, waterproof, and anti-corrosion. However, traditional sensors need to be powered by mains power supply, which were troublesome to deploy in applications and required a lot of maintenance in the later stages. Even if some used a battery power supply, the sensor devices were still based on the traditional application design, and their power consumption could not use a long-term battery power supply, and were troublesome to maintain. Furthermore, through the communication distance test, signal coverage test, and gateway capacity analysis, it was shown that in a typical suburban farming environment, a single gateway could carry more than 5000 nodes within 1100 m, which could easily and quickly deploy a large-scale wireless sensor network; whereas, the traditional types would require a huge cost to achieve a large-scale sensor network. Finally, the sensor designed in this paper could obtain data remotely in real time, while the latter needed to obtain data manually on site.

The technical means to instantly obtain various soil physicochemical parameters in a field is not only an important research direction in soil science but also an important technical support tool for the development of conventional laboratory-based physicochemical soil testing and analysis procedures for field-based measurements [2]. The development and application of LPWAN technology has enabled low-cost, low-power, wide-coverage, and real-time soil field monitoring. In this paper, an AHS with LPWAN technology based on LoRa was designed and applied to an inventoried soil moisture sensor, to upgrade the technology so that it, not only retained the performance, accuracy, and reliability of the original sensor, but also had the ultralow power consumption and long-distance wireless transmission function of an LPWAN. After loading the AHS, the sensor device could be built and deployed as a node in a wireless sensor network in an economical, flexible, and convenient manner; this not only expanded the applicability of the LPWAN, but also prevented the elimination of inventoried soil moisture sensors, due to their outdated technology. It is further concluded that not only soil moisture sensors, but also other inventoried proximal soil sensor devices based on conventional communication methods (such as RS-485, SDI-12 and other data communication methods) or devices whose outputs are standard voltages or currents could be designed with, or adopt, AHSs with technical designs that require ultralow power consumption; in this way, they can not only possess the technical advantages and application capacities of an LPWAN, but also retain their original mature sensing technology and give full play to the application value and economic value of inventoried proximal soil sensor devices, to avoid a waste of resources.

**Author Contributions:** The contributions to this manuscript were as follows: conceptualization: Y.T.; methodology: W.H.; hardware and software development: Y.T. and H.T.; data analysis: Y.T.; field trials: Y.T.; writing: Y.T.; review and editing: H.T.; supervision: W.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key Research and Development Project of China (Grant No. 2017YFF0108201), the Open Project of the State Key Laboratory of Soil and Sustainable Agriculture (Institute of Soil Science, Chinese Academy of Sciences (Grant number Y20160005), and the Nanjing Science and Technology project of China (Grant No. 2020011002), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2019312).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
