In-Soil Potassium Sensor System SHERLAN G. LEMOS, †,‡ ANA RITA A. NOGUEIRA,* ,† ANDRE Ä TORRE-NETO, § ALEIX PARRA, # JUDIT ARTIGAS, # AND JULIAN ALONSO # Grupo de Ana ´lise Instrumental Aplicada, Embrapa Pecua ´ria Sudeste, C.P. 339, 13.560-970 Sa ˜o Carlos, SP, Brazil; Departamento de Quimica, Universidade Federal de Sa ˜o Carlos, Sa ˜o Carlos, SP, Brazil; Embrapa Instrumentac ¸ a ˜o Agropecua ´ria, Sa ˜o Carlos, SP, Brazil; and Grup de Sensors i Biosensors, Departament de Quı ´mica, Universitat Auto `noma de Barcelona, Bellaterra, Spain A potentiometric sensor system based on potassium ion-selective electrodes was developed for agricultural purposes. Sensors were built using PVC ion-selective membranes over an inner solid contact prepared with graphite-epoxy composites. A copper plate was used as a reference electrode. A two-stage electronic circuit composed of current and voltage amplifiers was designed to interface the sensors to a distributed data acquisition system. Three ion-selective sensors and three off-the- shelf temperature sensors and their associated circuits were mounted in a PVC tube to set up a soil probe. The electronic controls were placed in an airtight box fixed at the upper part of the probe. The system was evaluated in the field, where the sensors presented sensibility within the range of 69- 71 mV dec -1 . Extracts of soil samples were analyzed by a current flame photometry approach, and the results, compared with the probe measurements, showed a linear relationship (r 2 ) 0.992 and 0.995, respectively, to 5 and 20 cm depths), which implies viability and instrumentation reliability for agricultural applications. KEYWORDS: Potassium; in-situ; soil analysis; soil fertility; potentiometric sensor INTRODUCTION Potassium is one of 16 nutrient elements needed by plants for healthy growth. It is the second one, surpassed by only nitrogen, in the amounts required by plants and is a key nutrient involved in many vegetal metabolic processes such as enzymatic activation, osmotic control of water economy, carbohydrate production, and cation/anion balance (1). New agricultural practices have been focused on an economi- cal and environmental point of view. These include the use of fertilization methods to optimize economical costs and to minimize negative environmental impacts. In this way, efforts dealing with concepts such as sustainable agriculture or precision agriculture (2, 3) have gained relevance. Current agriculture demands continuous in-situ information of soil physical and chemical parameters, such as macro- and micronutrients, owing to modulation of the amounts of fertilizers to be added. Some commercial systems allow physical data to be obtained, but not much instrumentation has been developed to determine concentrations of key parameters such as nitrogen, phosphorus, and potassium in soil. These and other parameters are normally obtained by off-line methods (4-8); these methods provide some data of great accuracy and precision about the soil composition, but they do not allow the migration behavior of nutrients from soil surface application through the soil profile to be monitored, when submitted to different interaction processes such as sorption, leaching, bioaccumulation, and volatilization. Scarce examples of soil in-situ analysis (9) can be found in the literature. The most promising analytical instrumentation developed belongs to the sensor field. In recent years, instrumentation based on direct contact or remote sensing performed in close proximity to the crop has been introduced. The advantages of sensors are their robustness, reduced size, versatility, and low mass- production costs. They are simple devices, compared with other analytical techniques such as chromatography or spectroscopy, and, moreover, they offer the possibility of designing in-situ analysis systems (10). The aim of this work was to develope analytical instrumenta- tion that permits potassium monitoring directly in soil, obtaining continuous and in-situ information for this nutrient. To ac- complish this purpose, a K + soil probe containing three all- solid-state ISEs located at different probe heights was developed. Instrumentation capable of transmitting digital data signals via radio was coupled to the probe. The developed system was evaluated to provide an understanding of the relationship among free potassium in soil nutrients reserves, soil texture, and root growth, which could optimize fertilizer consumption. The results were compared by means of a standard method based on potassium extraction from soil with an ion-exchange resin and its subsequent determination by flame photometry (11). * Author to whom correspondence should be addressed (fax 5516 3361 5754; telephone 5516 3361 5611; e-mail anarita@cppse.embrapa.br). † Embrapa Pecua ´ria Sudeste. ‡ Universidade Federal de Sa ˜o Carlos. § Embrapa Instrumentac ¸ a ˜o Agropecua ´ria. # Universitat Auto `noma de Barcelona. 5810 J. Agric. Food Chem. 2004, 52, 5810-5815 10.1021/jf0492924 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/21/2004