Development of On-Site Rapid Detection Device for Soil Macronutrients Based on Capillary Electrophoresis and Capacitively Coupled Contactless Conductivity Detection (C4D) Method

: The acquisition of nutrient data on a precise scale has played a vital role in nutrient management processes for soils. However, the lack of rapid precise and multi-index detection techniques for soil macronutrient contents hinders both rational fertilization and cost reduction. In this paper, a rapid detection method and device were devised, combining capillary electrophoresis (CE) and capacitively coupled contactless conductivity detection (C4D), and presented to detect macronutrient contents of soil. The device consisted of a capillary channel, C4D detector, high-voltage system, etc. It separated macronutrient ions using capillary electrophoresis and then measured the ion concentration based on the C4D principle. Lime concretion black soil samples from a complete ﬁeld were collected and detected. NO 3 − , NH 4+ , H 2 PO 4 − and K + in sample solutions could be detected in 5 min with relative standard deviations (RSDs) from 1.0 to 7.51%. The injection voltage was set to 10 kV for 5 s, and the separation voltage was set to 14 kV. This demonstrated the excellent performance of the C4D device on the detection of soil macronutrients, which could help to guide fertilization operations more effectively.


Introduction
Commonly, soil macronutrient (N, P and K) management has generally been tailored, to the extent possible, on a scale of tens of hectares [1]. Inevitably, the common broad distribution of fertilizers ultimately leads to under-and overapplication, with the former lowering optimal crop yields and the latter being a detriment to the surrounding ecosystems and creating unnecessarily high input costs. As a result, aiming at achieving the growth in crop yield and reducing polluting emissions, it has become urgent to monitor the fertility of soils on a more precise scale in time and space. Only through thorough understanding of soil fertility on a more precise scale could a reasonable local fertilization strategy be formulated, leading to the reduction of pollution and the improvement of agricultural efficiency.
The availability of soil macronutrients N, P and K in the ionic form of NO 3 − , NH 4 + , H 2 PO 4 − and K + was deemed to be of special interest [2], as these ions are directly available for plant roots. Traditional soil detection methods have mainly relied on laboratory testing, which requires complicated and long-term pretreatment processes and large, expensive soil sample testing instruments. To satisfy detection requirements, it was essential to simultaneously adopt several large soil testing instruments, e.g., atomic absorption spectrometers,

Principle of C4D
To achieve high-sensitivity detection, the C4D detector was applied for high-precision measurement of ion concentration. The schematic diagram of the C4D detector is shown in Figure 1; it consisted of an excitation circuit, a receiving circuit and two tubular ring electrodes attached to the surface of the capillary. The detection cell was placed in an aluminum shell to shield against external electromagnetic interference. The equivalent circuit of the detection cell consisted of a resistor (R stands for the ohmic resistance of the solution in the capillary section) and capacitors (C1 and C2 were the wall capacitance of the capillary). When a sinusoidal AC voltage U s was generated by the excitation circuit and applied to an electrode, an AC current I a could be measured by the receiving circuit at the other electrode. As the frequency of U s increased, the influence of C1 and C2 could be ignored. The conductivity of the solution could be calculated by the formula: K = Q × I a /U s (Q is a constant related to the detection cell). As the conductivity was sensitive to all charged sample components, the concentration of ions could be accurately obtained by measuring I a . Due to noncontact between the electrodes and the sample solution, the C4D detector had the characteristics of high stability and long service life.
the present time in China [23,24]-exhibited poor soil fertility, unstable pr formances and great annual fluctuations in crop output. To prove the validi device, detection was performed on lime concretion black soil samples.

Principle of C4D
To achieve high-sensitivity detection, the C4D detector was applied fo sion measurement of ion concentration. The schematic diagram of the C4 shown in Figure 1; it consisted of an excitation circuit, a receiving circuit an ring electrodes attached to the surface of the capillary. The detection cell wa aluminum shell to shield against external electromagnetic interference. T circuit of the detection cell consisted of a resistor (R stands for the ohmic res solution in the capillary section) and capacitors (C1 and C2 were the wall c the capillary). When a sinusoidal AC voltage Us was generated by the exc and applied to an electrode, an AC current Ia could be measured by the rec at the other electrode. As the frequency of Us increased, the influence of C1 be ignored. The conductivity of the solution could be calculated by the form Ia/Us (Q is a constant related to the detection cell). As the conductivity was s charged sample components, the concentration of ions could be accurately measuring Ia. Due to noncontact between the electrodes and the sample solu detector had the characteristics of high stability and long service life.

Excitation Circuit and Receiving Circuit of C4D
The excitation circuit generated a sinusoidal excitation AC voltage Us Figure 2a. Increasing the amplitude and reducing the total harmonic distor Us could contribute to improving the output signal-to-noise ratio of the C4D traditional laboratory method applied a signal transmitter and a commercia signal amplifier in combination. However, the equipment was large in size cated in operation, which made it inconvenient to use on-site. In our metho olution direct digital synthesis (DDS) chip was adopted to synthesize a lowoidal signal Ue with lower distortion and better stability. The amplifier A1 R1 and R2 composed an inverting amplifier for voltage amplification. A2

Excitation Circuit and Receiving Circuit of C4D
The excitation circuit generated a sinusoidal excitation AC voltage U s , as shown in Figure 2a. Increasing the amplitude and reducing the total harmonic distortion (THD) of U s could contribute to improving the output signal-to-noise ratio of the C4D detector. The traditional laboratory method applied a signal transmitter and a commercial high-voltage signal amplifier in combination. However, the equipment was large in size and complicated in operation, which made it inconvenient to use on-site. In our method, a high-resolution direct digital synthesis (DDS) chip was adopted to synthesize a low-voltage sinusoidal signal U e with lower distortion and better stability. The amplifier A1 and resistors R1 and R2 composed an inverting amplifier for voltage amplification. A2 was a buffer amplifier with a gain of 1 for current amplification. T1 was a high-frequency transformer with a ratio Chemosensors 2022, 10, 84 4 of 13 of 1:20. The working frequency range of the T1 was limited to 10 KHz-1 MHz to reduce the harmonic noise. By optimizing the parameters of the excitation circuit, the amplitude of U s could finally reach 400 V. The THD was less than 0.8%, which could meet the requirements of nutrient ion detection. hemosensors 2022, 10, x FOR PEER REVIEW amplifier with a gain of 1 for current amplification. T1 was a high-frequency tr with a ratio of 1:20. The working frequency range of the T1 was limited to 10 KH to reduce the harmonic noise. By optimizing the parameters of the excitation c amplitude of Us could finally reach 400 V. The THD was less than 0.8%, which c the requirements of nutrient ion detection.  Ia reflected the ion concentration in the solution, which was a weak AC cu receiving circuit converted the Ia to the C4D signal and uploaded it to the com recording and analysis. As shown in Figure 2b, the Ia was first converted into a age Ua by the transimpedance amplifier circuit, which consisted of A3, R3 and C a high-input impedance FET op-amp, which could reduce the interference caus current and resistance thermal noise. The peak detection circuit converted Ua t age Ub, which consisted of A4, A5, D1, D2, C4 and R4. Ub contained a backgrou caused by the conductivity of the buffer solution, which would reduce the ra C4D detector. The zero-adjustment circuit could eliminate the background vol which consisted of an instrument amplifier A6, R5 and VR1. The microcontrol periodically collected Uc through the AD port and sent C4D signals to the through the USB port. The receiving circuit was able to realize the measuremen ampere current with frequency below 1MHz, which made the C4D detector hig tive to low-concentration ions in solution.

High-Voltage System
The high-voltage system had two functions: sample injection and capilla phoresis. The system was divided into three modules (boost module, control mo put module), as shown in Figure 3. The boost module contained a multi-voltag circuit, which could magnify voltage amplification by several hundred times. T module was the core of the system and was composed of a controller, a touch s an interface circuit. On the one hand, the controller could be programmed to s of the boost module according to the user's settings on the touch screen. On hand, the control module monitored the actual output voltage of the boost m corrected the set value to ensure the accuracy of high-voltage systems. In the I a reflected the ion concentration in the solution, which was a weak AC current. The receiving circuit converted the I a to the C4D signal and uploaded it to the computer for recording and analysis. As shown in Figure 2b, the I a was first converted into an AC voltage U a by the transimpedance amplifier circuit, which consisted of A3, R3 and C3. A3 was a high-input impedance FET op-amp, which could reduce the interference caused by bias current and resistance thermal noise. The peak detection circuit converted U a to DC voltage U b , which consisted of A4, A5, D1, D2, C4 and R4. U b contained a background voltage caused by the conductivity of the buffer solution, which would reduce the range of the C4D detector. The zero-adjustment circuit could eliminate the background voltage in U b , which consisted of an instrument amplifier A6, R5 and VR1. The microcontroller (MCU) periodically collected U c through the AD port and sent C4D signals to the computer through the USB port. The receiving circuit was able to realize the measurement of nanoampere current with frequency below 1MHz, which made the C4D detector highly sensitive to low-concentration ions in solution.

High-Voltage System
The high-voltage system had two functions: sample injection and capillary electrophoresis. The system was divided into three modules (boost module, control module, output module), as shown in Figure 3. The boost module contained a multi-voltage rectifier circuit, which could magnify voltage amplification by several hundred times. The control module was the core of the system and was composed of a controller, a touch screen and an interface circuit. On the one hand, the controller could be programmed to set the gain of the boost module according to the user's settings on the touch screen. On the other hand, the control module monitored the actual output voltage of the boost module and corrected the set value to ensure the accuracy of high-voltage systems. In the lab, each module was powered by a switching power supply. When the device was used in the field, the high-voltage system was powered by a lithium battery. The high-voltage connector was used for the connection of the high-voltage system with the capillary. The output module was placed between the high-voltage connector and the boost module. It contained high-voltage switches, which could realize automatic switching of positive and negative voltages and high-precision timing output. The performance parameters of the high-voltage system were as follows: (1) the output voltage was adjustable from −15 kV to 15 kV; (2) the output voltage ripple was less than 0.05%; (3) the timing accuracy was 1 ms, which could ensure the stability of the electrophoresis process and the accuracy of the injection volume; (4) low power consumption, meaning that the system could run for more than 8 h on battery power. hemosensors 2022, 10, x FOR PEER REVIEW negative voltages and high-precision timing output. The performance para high-voltage system were as follows: (1) the output voltage was adjustable to 15 kV; (2) the output voltage ripple was less than 0.05%; (3) the timing a ms, which could ensure the stability of the electrophoresis process and the a injection volume; (4) low power consumption, meaning that the system more than 8 h on battery power.

Instruments and Reagents
The proposed rapid soil macronutrient detection device, which cons parts (capillary device with C4D detector, high-voltage system and laptop Figure 4a. The capillary device was equipped with a fused quartz capillar length and 50 µm inner diameters. The high-voltage system could be config erated on a touch screen. The C4D detection cell was located near the end of The C4D detector with the excitation circuit and the receiving circuit was con C4D detection cell by coaxial cable, as shown in Figure 4b. The excitation cir posed of a DDS circuit, inverting amplifier, current amplifier and high-fre former. The receiving circuit was composed of transimpedance amplifier, p circuit, zero adjustment circuit and microcontroller. The microcontroller sen to the laptop via a USB cable. The analysis software running on the laptop data and calculated the contents of soil macronutrient ions. (a)

Instruments and Reagents
The proposed rapid soil macronutrient detection device, which consisted of three parts (capillary device with C4D detector, high-voltage system and laptop), is shown in Figure 4a. The capillary device was equipped with a fused quartz capillary with 35 cm length and 50 µm inner diameters. The high-voltage system could be configured and operated on a touch screen. The C4D detection cell was located near the end of the capillary. The C4D detector with the excitation circuit and the receiving circuit was connected to the C4D detection cell by coaxial cable, as shown in Figure 4b. The excitation circuit was composed of a DDS circuit, inverting amplifier, current amplifier and high-frequency transformer. The receiving circuit was composed of transimpedance amplifier, peak detection circuit, zero adjustment circuit and microcontroller. The microcontroller sent C4D signals to the laptop via a USB cable. The analysis software running on the laptop recorded the data and calculated the contents of soil macronutrient ions.
All chemicals were of analytical pure grade, and all chemical solutions were prepared with deionized water with resistivity of 18.25 mΩ·cm −1 from an ultrapure water system (AWL−100-M, Chongqing Yiyang Enterprise Development Co., Ltd., Chongqing, China). Standard solutions of NO 3 − , NH 4 + , H 2 PO 4 − and K + with 1000 µg/mL were purchased from Guobiao (Beijing) Testing & Certification Co., Ltd. (Beijing, China). Different concentrations of standard solutions of NO 3 − , NH 4 + , H 2 PO 4 − and K + were prepared to evaluate the ability of the device to quantify ion concentrations.
The pTAE buffer solution was first prepared with 3.03 g Tris(hydroxymethyl)methyl aminomethane (Tris), 0.18 g ethylene diamine tetraacetic acid (EDTA) and 0.3 mL acetic acid dissolved in 50 mL deionized water. The electrophoresis cationic buffer solution was prepared with 2 mL pTAE buffer solution and 0.6 g polyvinyl pyrrolidone (PVP) dissolved in 100 mL deionized water. The buffer solution for phosphate detection was prepared with 0.026 mol/L acetic acid. The buffer solution for nitrate detection was prepared with 4.65 g/L DL-histidine (His) and 0.426 g 2-(N-morpholino)ethanesulfonic acid (Mes) in 50 mL deionized water. The C4D detector with the excitation circuit and the receiving circuit was connected to t C4D detection cell by coaxial cable, as shown in Figure 4b. The excitation circuit was com posed of a DDS circuit, inverting amplifier, current amplifier and high-frequency tran former. The receiving circuit was composed of transimpedance amplifier, peak detectio circuit, zero adjustment circuit and microcontroller. The microcontroller sent C4D signa to the laptop via a USB cable. The analysis software running on the laptop recorded t data and calculated the contents of soil macronutrient ions.

Sampling and Soil Sample Preparation
In the present study, lime concretion black soil samples were collected from Long Kang farm, Anhui Province, China. Twenty-four topsoil samples were collected in an S-type sampling way, which was 15~20 cm below the earth's surface. A total of 1000 g of each soil sample was collected, then naturally air-dried and sieved less than 1 mm.
Additionally, 3 g of air-dried soil was weighed in a specimen cup. Then, 30 mL of deionized water was added to the specimen cup and shaken on a reciprocal shaker for 30 min. After shaking, the solution was filtered using 150 mm quantitative filter paper and syringe filter (0.45 µm/25 mm) (Sangon Biotech (Shanghai, China) Co., Ltd.). The filtrates were collected and stored at 4 • C until taken out for injection into the device.

Measuring Procedure
Both ends of the capillary tube and platinum electrodes were cleaned with deionized water. The capillary channel was cleaned with sodium hydroxide solution, deionized water and electrophoresis buffer solution for 5 min each. After rinsing, the two ends of the capillary were completely immersed in the electrophoresis buffer solution in the detection tank and storage bottle.
The collected data were recorded by the data collection software for 4 min after the high-voltage power supply. If the baseline was stable, injection mode could be prepared. The soil sample solution was moved to the position of the capillary injection port. The injection voltage was set to 10 kV for 5 s. The electric injection procedure of the instrument was then begun.
The electrophoresis voltage was set to +14 kV for potassium ion and ammonium ion detection. Meanwhile, the electrophoresis cationic buffer solution was applied to simul-Chemosensors 2022, 10, 84 7 of 13 taneously detect potassium ions and ammonium ions. For nitrate ion and phosphate ion detection, the electrophoresis voltage was set to −14 kV. The buffer solution for phosphate detection was used to detect phosphate ions. The buffer solution for nitrate detection was used to detect nitrate ions. The sample volume was 2 mL.

Soil Sample Ion Separation
An extracted soil sample solution with three kinds of electrophoresis buffer solutions was tested. Typical measurement results of the electrophoretic analysis with C4D detection are presented in Figure 5. Each ion peak of the extracted soil sample solution could be clearly separated. The peaks of four kinds of soil macronutrient ions are highlighted with their labels in Figure 5. The target ion peaks were identified according to standard addition method. By comparing the significant differences of peak areas before and after adding the standard solution of single ions, the target ion peak could be easily identified. The corresponding cation curve contained peaks of potassium ions and ammonium ions. The detection times were all less than 6 min. The peak of potassium ions appeared at 2.58 min followed by the peak of ammonium ions at 2.97 min. The second peak, which appeared at 4.15 min, was the peak of nitrate ions, as shown in Figure 5b Chemosensors 2022, 10, x FOR PEER REVIEW 7 with their labels in Figure 5. The target ion peaks were identified according to stand addition method. By comparing the significant differences of peak areas before and adding the standard solution of single ions, the target ion peak could be easily identi The corresponding cation curve contained peaks of potassium ions and ammonium i The detection times were all less than 6 min. The peak of potassium ions appeared at min followed by the peak of ammonium ions at 2.97 min. The second peak, which peared at 4.15 min, was the peak of nitrate ions, as shown in Figure 5b.

Determination of Standard Concentration of Single Ion
To investigate the analytical performance of C4D on four kinds of macronutrient in electrophoresis buffer solutions, standard solutions of single ions were used to mea the reproducibility. All these measurements were considered in subsequent evaluati The peak areas in each electropherogram were acquired, and the performances of the

Determination of Standard Concentration of Single Ion
To investigate the analytical performance of C4D on four kinds of macronutrient ions in electrophoresis buffer solutions, standard solutions of single ions were used to measure the reproducibility. All these measurements were considered in subsequent evaluations. The peak areas in each electropherogram were acquired, and the performances of the detection device were evaluated for each ion. The results ( Figure 6) show that the peak area of each ion was positively correlated with the solution concentration, which meant that the larger the concentration, the larger the peak area. The repeatability of analysis was evaluated by relative standard deviations (%RSD) of peak areas. The peak area repeatability of each ion was determined by performing repeat analyses of the same sample using a standard protocol. The RSD for peak area was determined to be 0.45-5.88% for four kinds of macronutrient ion solutions with standard concentrations. These values demonstrated the excellent repeatability and analytical performance of the C4D for quantitative analysis.
The results ( Figure 6) show that the peak area of each ion was positively correla with the solution concentration, which meant that the larger the concentration, the la the peak area. The repeatability of analysis was evaluated by relative standard deviat (%RSD) of peak areas. The peak area repeatability of each ion was determined by forming repeat analyses of the same sample using a standard protocol. The RSD for p area was determined to be 0.45-5.88% for four kinds of macronutrient ion solutions w standard concentrations. These values demonstrated the excellent repeatability and a lytical performance of the C4D for quantitative analysis.
The fitting curves of the concentration and peak area of the four different kind ions are shown in Figure 6. The function of curves and correlation coefficients were spectively expressed as: The value of R 2 greater than 0.97 indicated good fitting (between concentration peak area). Furthermore, the fitting curve of phosphate ions was observed to be lin while the peak area of the other three ions grew slowly, and the trend of curves grew when the concentration reached a certain value. Owing to the ionic conductivities total amount of injected ions at a certain injection voltage and time, sample ions at h concentrations did not always show linear fit results. New injection methods, rather t electric injection, will be considered in future works.

Soil Macronutrient Analysis
For the quantitative analysis of four kinds of macronutrient ions in the extracted sample solutions, a quantitative method was introduced. In this method, an electroph sis analysis was carried out after a small amount (0.1 mL) of a high-concentration standard solution (C0 = 10 mg/L) was added to the sample at the end of the test. Figure 7 illustrates the electropherograms of phosphate in a soil extract sample be The fitting curves of the concentration and peak area of the four different kinds of ions are shown in Figure 6. The function of curves and correlation coefficients were respectively expressed as: The value of R 2 greater than 0.97 indicated good fitting (between concentration and peak area). Furthermore, the fitting curve of phosphate ions was observed to be linear, while the peak area of the other three ions grew slowly, and the trend of curves grew flat when the concentration reached a certain value. Owing to the ionic conductivities and total amount of injected ions at a certain injection voltage and time, sample ions at high concentrations did not always show linear fit results. New injection methods, rather than electric injection, will be considered in future works.

Soil Macronutrient Analysis
For the quantitative analysis of four kinds of macronutrient ions in the extracted soil sample solutions, a quantitative method was introduced. In this method, an electrophoresis analysis was carried out after a small amount (0.1 mL) of a high-concentration ion standard solution (C 0 = 10 mg/L) was added to the sample at the end of the test. Figure 7 illustrates the electropherograms of phosphate in a soil extract sample before and after the addition of a high-concentration ion standard solution. From Figure 7, the peak area was significantly improved after the addition of a high-concentration ion standard solution. By comparing the peak areas of addition (or not), we obtained the final ion concentration (C t ) in the sample according to the following formula: where the volumes of solutions before and after the addition of a high-concentration ion standard solution were V 1 mL and V 2 mL, respectively. The electrophoresis peak areas before and after the addition of a high-concentration ion standard solution were S 1 and S 2 .

Soil Macronutrient Analysis
For the quantitative analysis of four kinds of macronutrient ions in the extracted soil sample solutions, a quantitative method was introduced. In this method, an electrophoresis analysis was carried out after a small amount (0.1 mL) of a high-concentration ion standard solution (C0 = 10 mg/L) was added to the sample at the end of the test. Figure 7 illustrates the electropherograms of phosphate in a soil extract sample before and after the addition of a high-concentration ion standard solution. From Figure 7, the peak area was significantly improved after the addition of a high-concentration ion standard solution. By comparing the peak areas of addition (or not), we obtained the final ion concentration (Ct) in the sample according to the following formula: where the volumes of solutions before and after the addition of a high-concentration ion standard solution were V1 mL and V2 mL, respectively. The electrophoresis peak areas before and after the addition of a high-concentration ion standard solution were S1 and S2.
Furthermore, the contents of four kinds of soil macronutrients were obtained according to the following formula: Soil macronutrients contents = 10 × Ct.  Furthermore, the contents of four kinds of soil macronutrients were obtained according to the following formula: Soil macronutrients contents = 10 × C t .
Additionally, three lime concretion black soil samples were randomly picked for macronutrient measurement a total of three times. The RSD (n = 3) for the detections ranged from 1.0 to 7.51% (Table 1), which demonstrated good repeatability.  Table 2 shows the macronutrient measurements of 24 lime concretion black soil samples in a complete field. Figure 8 shows the content distributions of the four macronutrients in this field. These spatially continuous distribution maps of soil macronutrients were generated based on the interpolation of known nutrient data, a prediction method and coordinate locations of 24 soil samples in Table 2. The prediction method was realized by using the sample data around the unmeasured points and the weight value, in which the sample point closest to the predicted position was assigned a larger weight. According to the weight value of different soil sample points, the soil nutrient data of all unmeasured points in the target area were calculated to form the distribution map of soil nutrients. The results prove that the contents of the four macronutrients varied greatly in the same field. Therefore, the C4D device will help to measure the macronutrients of lime concretion black soil on a more precise scale and guide fertilization operations more effectively. using the sample data around the unmeasured points and the weight value, in which the sample point closest to the predicted position was assigned a larger weight. According to the weight value of different soil sample points, the soil nutrient data of all unmeasured points in the target area were calculated to form the distribution map of soil nutrients. The results prove that the contents of the four macronutrients varied greatly in the same field. Therefore, the C4D device will help to measure the macronutrients of lime concretion black soil on a more precise scale and guide fertilization operations more effectively.

Conclusions
The lack of rapid precise and multi-index detection techniques for soil macronutrient contents has hindered rational fertilization on a precise scale. To address this problem, this paper presented a rapid detection device, which combined capillary electrophoresis and the C4D method to achieve the detection of four soil macronutrient contents. The obtained results in this study are as follows: The rapid detection device (including a high-sensitivity C4D detector and a highperformance high-voltage system that could be used on-site) was designed. A highresolution DDS chip and a high-frequency transformer were adopted in the C4D detector to improve the sensitivity of macronutrient detection. The high-voltage system was batteryoperated, modular-designed and highly integrated, making it suitable for on-site use and meeting measurement requirements for soil macronutrients.
The standard solutions of single ions were measured to investigate the analytical performance of C4D. The RSD was determined to be 0.45-5.88% for four kinds of macronutrient ion solutions with standard concentrations, which demonstrated the excellent repeatability and analytical performance of the C4D for quantitative analysis.
A total of 24 topsoil lime concretion black soil samples from a complete field were collected and extracted by deionized water. NO 3 − , NH 4 + , H 2 PO 4 − and K + in soil extract solutions could be separated well and detected in 5 min with three kinds of electrophoresis buffer solutions, with the injection voltage set to 10 kV for 5 s and the separation voltage set to 14 kV. According to the electrophoresis peak areas, the contents of four kinds of soil macronutrients were obtained, and the RSD values of these soil samples ranged from 1.0% to 7.51%. This demonstrated the excellent performance of the C4D device in the detection of lime concretion black soil. By detection of soil samples from a complete field, the content distributions of the four macronutrients in a plot were proven to vary greatly, especially NO 3 − , H 2 PO 4 − and K + . The C4D device proposed in this paper will help to measure the macronutrients of lime concretion black soil on a more precise scale and guide fertilization operations more effectively. Moreover, the C4D device will also be extended to other types of soil detections. See Supplementary Materials.