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Illinois Fertilizer Conference Proceedings
January 25-27, 1999

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Development and Field Testing of Real-Time Soil Nutrient
Sensors for Precision Fertilizer Application

S. J. Birrell, J. W. Hummel, R. G. Hoeft and T. R. Peck1 spacer

Introduction
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The objective of this work was to develop and test a real-time soil nutrient analysis system, based on ion-selective field-effect transistors (ISFETs). The development of a real-time soil nutrient sensor could allow the automated collection of soil nutrient data on a fine resolution to accurately characterize within-field variability for site-specific fertilizer application.

The application of the sensor in Illinois agriculture will allow producers to identify fields and soils where variability can be profitably addressed by site-specific management technology. In addition, the sensor will provide accurate maps of soil nutrients, so that geo-referenced nutrient applications can be made with precision. Improved correlation between measured soil nutrient levels, nutrient removal due to harvested crops, and spatially applied nutrient additions will foster increased confidence in soil nutrient analysis. Agricultural producers will benefit from more efficient utilization of purchased inputs, and Illinois consumers will benefit from reduced adverse impact of agricultural practices on the environment.

Materials and Methods
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Previously completed research (Birrell 1995) resulted in a multi-ISFET nitrate sensor that was integrated into a flow injection analysis (FIA) system to measure soil nitrates. A prototype automated soil extraction system was also developed and tested in conjunction with the ISFET/FIA system. The prototype automated extraction system did not consistently provide soil extracts that could be analyzed by the ISFET/FIA system, and the predicted concentrations were much lower than the actual soil nitrate levels, although the predicted levels did show the same trends between different soils. The major sources of error resulted from inconsistent sample flowrate to the FIA system due to blockages in the filtration process and variation in the volume of soil metered for individual cycles. Although the automated extraction procedure was not very successful, the results were encouraging and warranted further development.

Soil fertility testing, particularly in the case of immobile nutrients, is based on the assumption that the extraction procedure differentiates between the quantity of plant-available nutrient in the soil and the total amount of the nutrient in the soil. This research is leading toward a soil nutrient sensing system in which the chemistry of the testing procedures is not fundamentally different from that of current laboratory methods. Although separate sensing modules may be necessary for each nutrient, a system incorporating a common extraction system using a universal extract is a natural progression of our research.

This research effort was divided into three areas of work: 1) investigation of the suitability of different ion selective membranes and extracting solutions for soil nutrient analysis; 2) evaluation of extraction efficiency and extraction time over a range of soil types, sample volumes, and sample preparation techniques; and 3) evaluation of the extraction system and ISFET/FIA system for real-time soil analysis.

Results and Discussion
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The research has focused on determination of the time required for extraction of the nutrient from the soil sample. The test apparatus is complete and tests are proceeding, although not as rapidly as planned due to premature sensor failure. Additional effort has been expended to identify the factor(s) contributing to the problem. The testing procedure has been modified due to preliminary results from early tests. Sensor failures have forced the fabrication of additional mounting boards, followed by mounting of the ISFETs on the boards, and wire bonding to the boards. These specialized procedures had to be out-sourced to vendors with the proper equipment and expertise.

Further delays in achieving project research objectives have occurred because of circumstances beyond our control. Potential budget re-allocations within the USDA-Agricultural Research Service, only recently rescinded by the United States Congress, would have terminated Dr. Hummel's research project at the end of FY98. In addition, Dr. Birrell's employment situation at the University of Illinois was short-term. In light of the uncertainty associated with these two principal investigators, second-year project funds were returned to the Fertilizer Research and Education Council (FREC). These uncertainties have now been resolved, and we expect the project to continue with FREC support in 1999. The results reported here were made possible by the first-year funding of the project.

Investigation of ion selective membranes and extracting solutions

Progress has been made in the development of an automated testing apparatus. The system under development is capable of testing multiple membranes simultaneously and generating preliminary reports. This system will allow rapid testing of the various membranes and their interaction with different extraction solutions, and is also capable of testing soil extracts to evaluate the prediction capability of different membranes. Upcoming tests with different membranes will evaluate their sensitivity, detection limits, longevity, and interactions with extraction solutions. A potassium ISFET with good sensitivity (56 mV/decade) has been produced. Preliminary results of ISFET-predicted soil potassium concentration versus actual soil concentration of manually extracted samples show promise (Figure 1) (Birrell and Hummel 1997). A pH ISFET has been developed and tested using standard buffer solutions (Figure 2).

The sensitivity response of the pH ISFET was 28 mV/decade change in hydrogen ion concentration (r2 = 0.997).

Evaluation of extraction efficiency and extraction time

A system has been developed to test nutrient extraction performance. The system was designed to evaluate the effect of soil type and texture, soil moisture content, soil sample volume, sample preparation, and rate of extraction solution injection into the sample on extraction efficiency and extraction time. Extracting solution is forced through small circular soil cores of different sizes and then drawn through a filter into a flow cell containing an ISFET sensor. The ISFET output voltage provides a time trace proportional to nitrate concentration profile (Figure 3). Concentration profiles through the ISFET flow cell and total mass of nitrates extracted are being compared to the soil nutrient concentration determined using standard extraction and analysis methods. Preliminary results for nitrates are encouraging, showing a definite relationship between ISFET output and total mass of nitrate in the soil sample (Figure 4). In addition, the concentration peak generally occurs within 5 s of the start of sample introduction. However, for soils with low levels of nitrate (< 15 ppm), the concentration profile approaches the detection limit of the sensor. Upon completion of the nitrate extraction tests, the same system will be used to test extraction efficiency and extraction times for potassium using a potassium ISFET detector.

Evaluation of extraction system and ISFET/FlA system for real-time soil analysis

All research thus far has utilized conventional peristaltic pumps and injection valves. A miniaturized system using micro-valves should provide significant improvement in performance. This technology has the potential to reduce both system switching time and sample dispersion. Although development of a miniaturized system cannot proceed until extraction efficiency tests are completed, a collaborative effort with faculty of the Microelectronics Laboratory, College of Engineering, who have expertise in MicroElectroMechanical systems (MEMs), is being explored.

Summary
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Progress has been made in the development of an automatic testing system that can rapidly test various membranes and their interaction with different extraction solutions. The system can also test soil extracts to evaluate the prediction capability of different membranes. Preliminary tests of both pH and potassium membranes have been promising. The system for testing nutrient extraction performance has been developed, but testing has been delayed due to premature failure of sensors. However, preliminary results with the nitrate sensor are encouraging, indicating that the concentration peak generally occurs within 5 s of the start of sample introduction. Collaborative efforts are being explored, so that a miniaturized system for nutrient extraction might be designed as soon as extraction efficiency tests are completed.

Figures
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Figure 1. pH ISFET response peak height versus standard buffer solution concentration with 5 s injection and washout time.

Figure 2. Potassium ISFET predicted versus actual soil concentration for manually extracted soil solutions with 5 s injection and washout time.

Figure 3. Soil extract concentration profile with respect to time.

Figure 4. Peak ISFET response versus total mass of nitrate (NO3-N) in soil cores

Footnotes and References
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1 S.J. Birrell is Assistant Professor, Agri. and Bio. Engr. Dept., Iowa State Univ., Ames IA; J.W. Hummel is Agricultural Engineer, USDA Agri. Res. Ser. and Professor, Agri. Engr. Dept., Univ. of Illinois; R.G. Hoeft is Professor, Crop Sciences Dept., Univ. of Illinois; T.R. Peck is Professor, Nat. Res. and Environ. Sciences Dept.. Univ. of Illinois, Urbana IL.

Birrell, S.J. 1995. Multi-ISFET sensor system for soil analysis. Unpublished Ph.D. Dissertation. Library, University of Illinois at Urbana-Champaign, Urbana, IL.

Birrell, S.J., and J.W. Hummel. 1997. Multi-sensor ISFET system for Soil Analysis. In: Precision Agriculture, 97, vol. 2: pp 459-460. Bios Scientific Publ., Oxford, United Kingdom.

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*University of Illinois at Urbana-Champaign
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