Development and experiment of the rapid detection device of the nutrient ion concentrations for fertigation system

Abstract: Precise nutrient management has been widely expected to improve nutrient use efficiency in facility agriculture. The acquisition of nutrient data has been one of the most important steps during nutrient management in the fertigation system. The concentrations of macronutrients are of particular interest to the decision-making on t nutrient management, particularly in the ions form of potassium, nitrate, ammonium, and phosphate. Considering that these ions can be directly absorbed by plant roots, it is very necessary to rapidly and accurately detect these ions. However, rapid, accurate, and cost-saving detection is still lacking in the nutrient solution and leaching tail water. In this study, a rapid and accurate detection device was developed for the four kinds of available ions (K+, NO3-, NH4+, and PO43-) concentrations in the fertigation system during production. The capacitively coupled contactless conductivity detection (C4D) and capillary electrophoresis (CE) were combined to rapidly quantify the four kinds of ions. The device consisted of a high-voltage system, a C4D detection module, a capillary channel, a data acquisition card, and a host computer. Moreover, the device was also applied in the field via a touch screen with simple operations. The specific detection procedure was as follows. The sample was injected into the capillary channel that was filled with the buffer solution by an electro-injection process, and then a high voltage was applied across the capillary channel. As such, the ions were moved toward the C4D detector at the end of the capillary under the combined influence of electrophoresis and electroosmosis. After that, various nutrient ions were separated into the different ion groups, according to the migration rates. The real-time signal outputs of the C4D detector were recorded through the data acquisition card, where the signal peaks of various ions were also obtained at different times. Finally, the concentration was calculated in the host computer using the peak area of the ions within a few minutes. A phase-locked amplification was applied for the C4D detection circuit to alleviate the various interference noises on the detection signals, indicating rapid and reliable on-site detection. The output of the C4D detection module was tested under different amplitude Vp and operating frequency f. The optimal operating parameters were also achieved, where f =490 kHz, Vp=75 V (cation), and Vp=60 V (anion). The standard solution of K+, NO3-, NH4+, and PO43- ions was selected to estimate the measurement, where the detection limit of 1 μg/L was achieved for the four kinds of ions. In addition, the fertilizer solution and tail water samples were tested on-site in a strawberry planting greenhouse with a fertigation system, in order to verify the effectiveness of the device on-site detection. The relative standard deviations were all less than 7% for the detected K+, NO3-, NH4+, and PO43- ions. Consequently, the device can be widely expected to effectively realize the accurate on-site detection of the fertilizer solutions and tail water samples. Anyway, the new device can greatly contribute to the accurate and quantitative detection of the concentrations of macronutrients ions in the fertigation system. The finding can also provide strong support to the efficient decision-making of fertilizer management.