# Development and Performance Test for a New Type of Portable Soil EC Detector

Xiaoshuai Pei, Lihua Zheng, Yong Zhao, Menglong Zhang, Minzan Li

► To cite this version:

Xiaoshuai Pei, Lihua Zheng, Yong Zhao, Menglong Zhang, Minzan Li. Development and Performance Test for a New Type of Portable Soil EC Detector. 6th Computer and Computing Technologies in Agriculture (CCTA), Oct 2012, Zhangjiajie, China. pp.418-424, 10.1007/978-3-642-36124-1_50 . hal-01348126

HAL Id: hal-01348126

https://inria.hal.science/hal-01348126

Submitted on 22 Jul 2016

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Development and performance test for a new type of portable soil EC detector

Xiaoshuai Pei, Lihua Zheng, Yong Zhao, Menglong Zhang, Minzan Li *

Key Laboratory of Modern Precision Agriculture System Integration Research, Ministry of Education, China Agricultural University, Beijing 100083, China.

*Email: limz@cau.edu.cn

**ABSTRACT:** The soil electrical conductivity (EC) refers to the capability for soil to conduct current. It is a comprehensive reflection of soil salinity and moisture. Therefore, acquiring soil EC rapidly and accurately can provide better guidance for farming production. Based on improving four-electrode method, a new portable soil EC detector with six electrodes was developed and its performance was tested. Inside two electrodes and outside two electrodes were used to measure soil EC near the surface and in deeper soil, respectively. And middle two electrodes were used to input a constant current to soil. The stability tests of the current source showed that the amplitude fluctuation was less than 3%.

**Keywords:** Soil EC detector; Four-electrode method; Precision agriculture

## 0 Introduction

China is the largest agricultural producer and consumer in the world. With the rapid development of agriculture, large amounts of chemical fertilizers and pesticides were used to increase the crop yield. It has resulted in soil erosion, agricultural pollution and groundwater pollution and decreased soil productivity consequently. Precision agriculture, as a new farming approach, aimed at avoiding waste and excessive fertilizer spraying caused by blind input in farmland[1,2].

Precision agriculture technology is an information-based agricultural management system. Implementing spatial-temporal variable management could achieve true sense of the “intensive farming” based on information and
advanced technology[3]. Soil researches has shown that soil electrical conductivity (EC) contains a wealth of information for analyzing soil nutrients and physicochemical properties[4] and can reflect the soil salinity, moisture, organic matter content, soil texture, structure and porosity, etc. Acquiring soil EC effectively is of great significance to determine the spatial-temporal distribution of soil parameters[5].

In this paper, we tried to improve the traditional current-voltage four-electrode method to better fit for in-situ measuring, and aimed at developing a in-situ soil EC detector with low price, easy operation, high measurement precision, integral control procedures and data processing procedures.

# 1 Materials and methods

## 1.1 Principle of soil EC detector development

Figure 1 is a typical structure of the current-voltage four-electrode approach, including two current electrodes (J and K) and two voltage electrodes (M and N). By loading constant amplitude current as excitation signal through two current electrodes, the soil EC can be calculated according to the voltage collected from two voltage electrodes.

<figure><img src="image_2.png" /><figcaption>Figure 1 Principal diagram of current-voltage four-electrode method</figcaption></figure>

The earth is complex object with uncertain cross-sectional area and length. Researches showed that the earth EC could be measured according to formula (1)[6].

$$ \sigma = \frac{\left( \frac{1}{d_{JM}} - \frac{1}{d_{JN}} \right) - \left( \frac{1}{d_{KM}} - \frac{1}{d_{KN}} \right)}{2\pi} \frac{I}{V_{MN}} = k(a, b)V_{MN}^{-1} \quad (1) $$

where, $\sigma$ (with unit of S/m) is the value of EC; $I$ (with unit of A) is the current
provided by the constant current source. $V_{MN}$ (with unit of V) is the voltage measured between the M and N electrode; $k(a,b)$ (with unit of m) is the function with variables of $d_{JM}$, $d_{JN}$, $d_{KM}$ and $d_{KN}$; Besides, $a=d_{JM}=d_{KN}$, $b=d_{MN}$.

To measure the EC in deep soil layer, two electrodes were added on the basic four electrode theory, which is shown in Figure 2. Moreover, accurate circuits were designed to ensure constant alternating current could be loaded between two current electrodes, and the voltage of inside and outside electrodes could be collected accurately.

<figure><img src="image_3.png" /><figcaption>Figure 2 Schematic of improved current-voltage four-electrode method</figcaption></figure>

# 2 Results and discussions

## 2.1 Soil EC in-situ detecting system design

In-situ measurement system of soil EC was designed as shown in Figure 3. It mainly includes signal generating circuit, output signal conditioning circuits and data acquisition circuit.

<figure><img src="image_4.png" /><figcaption>Figure 3 System diagram</figcaption></figure>
## 2.2 Circuits development

### 2.2.1 Sinusoidal signal generating circuit

ICL8038 was used as waveform generator to output sinusoidal signals. After processed by LM324, the circuit produced stable alternating current source avoiding distortion caused by oversized signal.

Figure 4 is 300Hz sinusoidal signal generating circuit. C₁₅ and R₂ were used to implement low-pass filter, playing the role of cutting off direct current and conducting alternating current. Resistors of (R₁₄+Rₓ) (R₁₅+R₁₃-Rₓ) were used to adjust the amplitude of current source of I₁, I₂ and duty cycle square wave. Tests indicated that in order to minimize the total harmonic distortion of signal (TDH), it was necessary to keep (R₁₄+Rₓ) and (R₁₅+R₁₃-Rₓ) equal. R₁₆ was used to adjust the shape of the sinusoidal signal. Tests showed that when R₁₆=82 kΩ, TDH kept the minimum. C₃ and (R₁₄+Rₓ) were used to adjust the frequency of output sinusoidal signal calculated by formula (2).

$$f=0.3/[(R_{14}+R_{x})/C_{3}] \quad (2)$$

<figure><img src="image_5.png" /><figcaption>Figure 4 Sinusoidal signal generating circuit</figcaption></figure>

### 2.2.2 Constant current source circuit

Experiments and literatures suggested that the soil EC measuring accuracy mainly depended on the constant amplitude alternating current source when using current-voltage four-electrode method[7,8]. Figure 5 is the circuit diagram of stable
alternating current source controlled by LM324. LM324 series are devices of the four op amp with differential input and have some advantages compared with standard op amp. With 3~32V supply, the quiescent current is only one fifth of the MC1741's. In digital systems, it can easily provide the necessary interface circuitry without extra supply. In Figure 5, the benchmark (pin 3 in LM324, the input signal) of the AC source is Sinusoidal signal, which is the output signal from the sinusoidal signal generator ICL8038.

This circuit was with high output impedance and the output current could be controlled by programming digital potentiometer. It could be automatically adjusted with different impedance conditions and designed to fit for the soil EC with low

<figure><img src="image_6.png" /><figcaption>Figure 5 Constant current source circuit</figcaption></figure>

### 2.2.3 Differential circuit

Unbalanced texture and structure of soil are likely to cause the instability of the signal. A subtraction circuit was designed to reduce signal instability, remove unreasonable signal, enhance useful signal and improve the accuracy. The differential circuit is shown as Figure 6. It was the combination amplifying circuit of inverting input and non-inverting input, and implemented the subtraction between Vin+ and Vin-. In the ideal conditions, two input voltages of the op amp were equal. It meant that there were virtual short and common-mode voltages between both ports of the op amp. When R5=R6 and R7=R8=R9=R10, Vin can be calculated by formula (3).

$$ Vin=1+2R_{17}/R_{16}[(Vin+)-(Vin-)] \quad (3) $$
Because of common-mode voltage, the op amp with higher common-mode rejection ratio should be chosen and the bias voltage of the op amp should be taken into consideration. In the case of the above two factors and cost reason, LM358 and OP07 were used in this paper. This circuit was with high input impedance and low output impedance, which is suitable for the signal conditioning circuit.

<figure><img src="image_7.png" /><figcaption>Figure 6 Differential circuit</figcaption></figure>

## 2.3 Design and development of in-situ soil EC detector

The electrode and structure and structure of the detector, signal generating circuit, signal conditioning circuit, LCD module and storage module were designed and developed. In addition, the system was set aside many peripherals used for future extensions.

### 2.3.1 Hardware integration and development

The structure of the in-situ soil EC detector is shown in Figure 7. The instrument consisted of handle, straight pipe, electrodes, data acquisition controller and other components. After measuring the voltage of the electrodes, the detector calculated EC, and then displayed and stored data.
<figure><img src="image_8.png" /><figcaption>**Figure 7** Schematic of the in-situ soil EC detector</figcaption></figure>

### 2.3.2 Software design and development

The overall software flow chart is shown in Figure 8.

<figure><img src="image_9.png" /><figcaption>**Figure 8** Overall flow chart of the system software</figcaption></figure>
ACKNOWLEDGEMENTS

This research was financially supported by National Science and Technology
Support Program (2011BAD21B01) and NSFC program (61134011).

REFERENCES

1. [1] Wang Maohua. Development of precision agriculture and innovation of engineering technologies, Transactions of The Chinese Society of Agricultural Engineering, 1999, 15(1): 1-8

2. Zhao Chunjiang. Research and Practice of Precision Agriculture. 2009, Science Press, Beijing.

3. Minzan Li. Precision agriculture characteristics, current status and development. Agricultural Machinery Market, 2008. 3; 38-41.

4. Kitchen N R, Sudduth K A, Drummond S T. Soil electrical conductivity as a crop productivity measure for claypan soils. J Prod Agric, 1999, 12: 607-617.

5. Sudduth K A, Drummond S T, Kitchen N R. Accuracy issues in electromagnetic induction sensing of soil electrical conductivity for precision agriculture. Computers and Electronics in Agriculture, 2001, 31: 239-264.

6. Yurui Sun, Maohua Wang. A mathematical model and its experimental study for a kind of measurement method of soil electric conductivity. Transactions of the Chinese Society of Agricultural Engineering, 2001, 17(2): 20-23.

7. Menglong Zhang, Minzan Li, Yong Zhao. The research and development of vehicle-mounted soil EC monitoring system. Proceeding of CSAE2011, 2011-10-22.

8. Junning Zhang, Minzan Li, Dexiu Kong, Qizhang Zou. Artificial intelligence in real-time evaluating electrical conductivity of greenhouse substrate. Artificial Intelligence Applications and Innovations (IFIP International Federation for Information Processing), 2005, 187: 609-615.