Low-cost soil moisture profile probe [Professional]

Paper Introduction

Soil moisture is an important property for agriculture, indicative of how much water is available for plants. Therefore, its measurement can be utilized for decision-making in irrigation.

Continuous in situ measurement of the soil moisture started around 1940 with the electrical resistance method using a gypsum block. Electrodes in the block measure the internal electrical resistance, which is correlated to soil moisture under an equilibrium condition. This method was simple and cheap, but had problems concerning accuracy, e.g., the electrical resistance of gypsum is affected by temperature, response of the electrical resistance to changes in soil moisture is slow, and relationship between resistance and soil is not universal for a variety of soils.

A neutron attenuation probe was developed as a better method to measure in situ soil moisture in the 1950s. The neutron probe measures a count of radioactive beams emitted from a source, which is correlated to soil moisture. The method was successful in field measurement, but a health hazard problem remained.

The measurement of in situ soil moisture advanced with the discovery of time domain reflectometry (TDR). TDR measures apparent permittivity of the soil from the time that an electromagnetic pulse takes to go through parallel electrodes inserted into soil. Because the apparent permittivity of water (≈80) is much larger than those of other soil constituents (≈1–12), the apparent permittivity of soil is strongly correlated to soil moisture.

Figure 2: Scheme of the network system with the developed soil moisture sensors.

The advantages of the TDR method are that it can measure soil moisture and electrical conductivity simultaneously, it is not susceptible to temperature and salinity, and a variety of sampling volumes is possible by changing the design of the electrodes. However, TDR equipment is expensive, e.g., 3000 U.S. dollars or more.
The capacitance soil moisture sensor is known to be a good alternative to TDR. Soil capacitance depends on the apparent permittivity of soil such that capacitance sensors can determine soil moisture as well as TDR. It has been reported that the capacitance sensors are relatively sensitive to soil temperature, do not work well in some soils, and have a small sampling volume. Even with these disadvantages, a variety of capacitance sensors have been developed and commercialized because they are less expensive than TDR and sufficiently reliable. For example, the capacitance sensors commercialized by Decagon Devices Inc. (Pullman, WA, USA) are known to be relatively inexpensive and reliable, and these sensors have been widely used in recent scientific research.

Figure 3: Details for (a) master node and (b) slave node of the sensor network system.

Agriculture in Japan is in transition from traditional farming, which relies on the individual farmers’ skills and experiences, to precision farming (also called smart farming). Precision farming utilizes a variety of sensors to monitor and control environmental and crop conditions in order to achieve stable crop production and reduce the excessive use of resources. For precision farming, the soil moisture must be measured at multiple locations and at multiple depths. However, with capacitance sensors, the measurement of soil moisture implies a significant investment. Therefore, precise management of fields and crops with a number of soil moisture sensors, i.e., a soil moisture sensor network, is still impeded by the sensor cost.

Figure 4: Dimensions of the greenhouse and locations of master and slave nodes.

There is a trade-off between the cost and accuracy of sensors, i.e., more accurate sensors are more expensive. Most of the currently commercialized soil moisture sensors tend to have good accuracy by sacrificing cost reduction as they have been developed for research use. For agriculture, price must have a higher priority than accuracy, i.e., a low-cost sensor even with a relatively weak accuracy is preferred. The development of low-cost soil moisture sensors has been prevented by the expensive and complicated circuits for the high-frequency measurement of the capacitance as well as by the difficulty of obtaining a flexible sensor design that can accommodate a variety of environment and soil types. However, it is possible for soil moisture sensors to be much less expensive than currently commercialized ones, owing to recent technological developments. Thus, the objective of this study is to develop a low-cost soil moisture sensor with the latest technologies, i.e., coplanar plate capacitors on a film substrate and a capacitive touch integrated circuit (IC). The performance of the developed sensors is also evaluated.^[1]

Figure 5: Relationship between volumetric water content (VWC) and electrical capacitance obtained from the calibration experiments.

Figure 6: Temperatures above the surface and at 10, 20, and 30 cm depths measured with the developed sensor A (i), volumetric water content (VWC) measured in ridges (ii), and in furrows (iii), and precipitation (iv) at the grape field from 25 October to 19 December 2015.

References

[1] Yuki Kojima,Ryo Shigeta, Naoya Miyamoto, Yasutomo Shirahama, Kazuhiro Nishioka, Masaru Mizoguchi and Yoshihiro Kawahara (2016) Low-Cost Soil Moisture Profile Probe Using Thin-Film Capacitors and a Capacitive Touch Sensor. Sensors 2016, 16(8), 1292; https://doi.org/10.3390/s16081292

Paper Introduction

References

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