Research And Test Of A Rapid Measuring System For Nitrogen,Phosphorus And Water Content In Soil Using Both Flow Inject Method And Type J Probe Based On Time Domain Reflection

Excessive nitrogen and phosphorus chemical fertilizer inputs enter groundwater and lakes due to leaching and surface runoff,and nitrogen and phosphorus eutrophication in water bodies poses a serious threat to people’s health.With the construction of smart agriculture and the construction of agricultural non-point source nitrogen and phosphorus pollution prevention and control systems,rapid determination of nitrogen and phosphorus in soil is very urgent,and nitrogen and phosphorus in soil are closely related to soil water content.Therefore,in order to solve key issues and technical difficulties in rapid determination of nitrogen,phosphorus,and water in soil,the paper creatively proposes an integrated reactive flow analysis method for the design of a fully automated soil nitrogen and phosphorus testing system.At the same time,it studies and designs a time-domain reflection J-type soil water content testing probe to achieve high-precision detection of soil water content with uneven spatial distribution of water content.Through experiments,the testing system is validated and data analysis is conducted,providing technical support for the control and prevention of agricultural non-point source pollution and the implementation of precision agriculture.The main research contents of this paper are as follows:(1)The theory and related parameter calculation of a rapid testing system for soil nitrogen,phosphorus,and water content were systematically analyzed.The analysis of rapid testing methods and technologies for soil nitrogen,phosphorus,and water content,especially the characteristics and advantages of flow injection technology and time domain reflection technology,provides a theoretical basis for the construction of a rapid testing system for nitrogen,phosphorus,and water content.This paper studies the relationship between soil water content and dielectric constant,theoretically analyzes the testing mechanism of time domain reflection technology,and proposes the design idea of J-type probe.(2)A rapid testing system for soil nitrogen and phosphorus based on flow injection analysis was designed and integrated with reactive flow analysis.Combined with absorbance spectroscopy,rapid and automated analysis of nitrate,ammonium,and phosphorus in soil was achieved.The test system consists of three modules: a quantitative sampling unit,a chemical reaction chamber,and a modular photoelectric detection unit.The quantification of sample and reagent sampling is ensured through high-precision injection pumps and liquid level switches.The peristaltic pump accelerates the reaction by disturbing the reactants in the reaction chamber,ultimately achieving a stable state of the reaction.After the reaction,the absorption spectrum of the solution is tested by a photoelectric detection unit to determine the concentration of the substance to be measured in the sample.(3)The Modbus protocol control system is used to achieve data transmission and automatic control of each module unit in the test system,ensuring stable operation of each module of the system.Design a reliable mechatronic testing system from the aspects of sample quantification module,sample drive module,control circuit layout,sample pipeline material selection,and perform pressure simulation on the box structure of the testing system,which meets design expectations.To facilitate installation,debugging,and maintenance,the test unit and the electrical control unit are installed on two rotatable mounting plates.The paper evaluates the performance of the testing system by evaluating the light source stability,detection limit,repeatability,and spiking recovery rate.The test results show that the detection limits for ammonium nitrogen,nitrate nitrogen,and phosphorus in the nitrogen and phosphorus rapid testing system of the paper are 0.0086mg/L,0.0094 mg/L,and 0.0078 mg/L,respectively.The repeatability relative standard deviation is 0.8%,5.7%,and 0.9%,respectively.The spiked recovery rates are between97% to 105%,96% to 109%,and 99% to 105%,respectively.The sensitivity of the nitrogen and phosphorus testing system using cobalt chloride standard solution is Ka=5.34× 10-2(A /(g·L-1))..Then,on-site application testing was conducted.For the same soil sample,the instrument was tested separately with a conventional ultraviolet visible spectrophotometer.The on-site test results showed that the correlation coefficients R2 for the measured values of ammonium nitrogen,nitrate nitrogen,and phosphorus were 0.9461,0.9954,and 0.8889,respectively.The test system performed well.(4)Based on the time domain reflection method,a J-type soil water content probe is designed,which increases the probe length at the same measurement depth,increases the return time of high-frequency electromagnetic waves,and improves the detection accuracy of soil water content with uneven spatial distribution of water.The paper analyzes the parameters of J-type probe from the aspects of probe type,material,length,diameter,and spacing,and simulates the distribution of electromagnetic field under different lengths and spacing conditions of J-type probe.Comparative analysis of soil water content was conducted with the laboratory drying method to obtain the variation rule of measured dielectric constant values.Through data regression analysis,a “soil water content dielectric constant relationship model” based on a J-type probe was obtained.After calibration,the probe performance test results show that the relative standard deviation of repeatability is about 2%,and the relative standard deviation of accuracy is between 0.5% and 3.6%.The J-type probe was applied to field measurement and the expected results were obtained.