REAL-TIME SOIL NITRATE SENSING

Authors: HUMMEL, JOHN; BIRRELL, STUART - UNIV OF ILLINOIS

Submitted to: North Central Extension Industry Soil Fertility Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/15/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: Concern about the intensive application of nitrogen fertilizers, and their potentially adverse effects on the environment, has provided impetus for development of improved fertilizer management. Different soil nitrate levels, and different soil types with differing crop producing capabilities within a field, suggest that application levels might be adjusted across the field, supplying just the supplemental amount of nutrient needs to produce the crop. The cost and time required for the intensive sampling needed, using conventional sampling and analysis techniques, makes implementation of a variable-rate nitrogen fertilizer management system based on soil nitrate tests impractical. On-the-go real-time nitrate sensors might be used to locate areas of fields where additions of nitrogen fertilizer will be beneficial, and other areas where soil nitrate levels are such that additions of nitrogen fertilizer may have no economic benefit and could result in environmental degradation. A sensor has been developed that can rapidly measure soil nitrate levels in soil extracts. When coupled with a high-speed soil sample collection and extraction system (yet to be developed), soil nitrate levels could be used to control the nitrogen fertilizer application rates in corn production. Use of the technology could benefit agricultural producers economically, and reduce the adverse effect of commercial fertilizers on the environment.

Technical Abstract: Ion-selective field effect transistors (ISFETs) have inherent features such as small dimensions, low output impedance, high signal-to-noise ratio low sample volumes and the potential for mass production, which are required for a real time soil sensor. However, ISFETs have the disadvantage of long-term drift, which is diminished by the use of a Flow Injection Analysis (FIA) system. In fact, FIA and ISFETs are complementary since the small sample volumes and rapid response of ISFETs allow the miniaturization of the FIA system, dramatically decreasing the sample dispersion and thereby increasing both sample resolution and sample frequency. A multi-ISFET nitrate sensor was developed and integrated into a flow injection system to measure soil nitrates. The multi-ISFET sensor was successful in measuring soil nitrates in manual soil extracts (r2 > 0.9). The precision and accuracy of the system were highly dependent on maintaining precise, repetitive injection times and maintaining constant flow parameters during the testing cycle. During tests almost all of the errors in prediction can be traced to these two factors. The rapid response of the system allowed samples to be analyzed within 1.25 s, which is satisfactory for real-time soil sensing. The development of an automated soil extraction system was not particularly successful and requires considerable improvement. However, the concept of using ISFETs for real-time analysis of soil nitrates is sound. The extremely rapid response and low sample volumes required by the multi-sensor ISFET/FIA system make it a strong candidate for use in real-time soil nutrient sensing.