Determination of Soil Nitrate and Water Content Using Attenuated Total Reflectance Spectroscopy A. BORENSTEIN, R. LINKER,* I. SHMULEVICH, and A. SHAVIV Division of Environmental, Water and Agricultural Engineering; Faculty of Civil and Environmental Engineering; Technion-Israel Institute of Technology, Haifa 32000, Israel Direct determination of nitrate and soil moisture can significantly improve N-application management and thus reduce N-derived environmental pollution related to agriculture. Several studies have shown that Fourier transform infrared attenuated total reflectance (FT-IR/ATR) spectroscopy could be used to estimate the nitrate content of standardized soil pastes. Paste standardization appeared to be the main obstacle to in situ application of this approach, and the present study shows how FT-IR/ATR can be used to estimate both water content and nitrate concentration of field soil samples. Water content and nitrate concentration are determined sequentially using two subsamples of the initial soil sample. An a priori determined amount of highly concentrated nitrate solution is added to the first subsample and the ATR spectrum of this paste is used to estimate the sample water content. It is then possible to calculate the amount of water that should be added to the second subsample so that the resulting paste is very close to the ideal standard paste. Nitrate concentration, mg [N]/kg [dry soil], is estimated using the FT-IR/ATR spectrum of this second paste. Results are presented for a laboratory experiment with four agricultural soils, as well as for a field trial with a calcareous soil. For water content, the determination errors range from 0.01 to 0.02 g [water]/g [dry soil]. For nitrate concentration, the errors for three of the soils range from 5.9 to 8.4 mg [N]/kg [dry soil], while for the fourth, calcareous clay soil, the determination error is 13.6 mg [N]/kg [dry soil]. The determination errors obtained for the field trial are similar to the ones obtained for a similar soil under laboratory conditions, which shows the potential usefulness of the approach for improving N-application management and reducing environmental pollution. Index Headings: Attenuated total reflectance; ATR; Mid-infrared; Nitrogen management; Partial least squares; PLS; Precision fertilization. INTRODUCTION Over-fertilization causes significant damage to the environ- ment and increases farming costs. The so-called precision fertilization concept states that the use of fertilizers could be greatly reduced by matching locally the application rate with the crop requirements. 1,2 Successful application of this concept would require knowledge about the crop needs, soil properties, and nutrients (e.g., N) status. The lack of sensors for in situ determination of critical soil characteristics constitutes a major bottleneck to the successful large-scale application of this concept. 2,3 This study focuses on direct in situ determination of soil nitrate, which due to the high mobility of this ion is of crucial importance for two main reasons: (1) nitrate that is not used by the crop rapidly leaches to the ground water or can form greenhouse gases (e.g., N 2 O), thus contaminating the environment, and (2) nitrate concentration within a given field may greatly vary due to the intensive transformation of nitrogen in soil. 4 Techniques that could potentially lead to the development of an in situ nitrate sensor include ion selective electrodes, 5 ion sensitive field effect transistors, 6,7 and mid-infrared spectros- copy, which is the focus of the present study. Ehsani et al. 8 showed that mid-infrared spectroscopy could be used to estimate nitrate content of dry soil samples, and Shaviv et al. 9 extended this approach to saturated samples using the attenuated total reflectance (ATR) configuration. This approach was further improved by Linker et al., 10–12 , who reported determination errors ranging from 5 to 25 mg [N]/kg [dry soil], depending on soil type. All the previous Fourier transform infrared (FT-IR)/ATR studies were conducted with standard- ized soil pastes prepared from sieved and dried soils. Sample standardization was obtained by preparing saturated pastes of known water content, which were obtained by slowly adding water to the initially dry sample. Such careful sample preparation would not be suitable for in situ measurements, and the first objective of this study is to devise a determination procedure that is suitable for field samples that are of unknown water content. The second objective of this work stems from the fact that farmers’ decisions regarding nitrate fertilization are based on the estimation of nitrate content on a dry soil basis (or on soil volume basis). Since for wet samples all the nitrate is in the solution phase, estimates of both soil water content and nitrate concentration are required. Although several methods for rapid determination of soil water content are available (e.g., electrical resistance, 13 electrical impedance, 14 time-domain reflectometry (TDR), 15 near-infrared (NIR) spectroscopy 16–18 ), each method has drawbacks such as sensitivity to clay content, salinity, or particles’ size. In addition, the use of either of these methods would require additional equipment and would not be cost effective. Accordingly, the goal of the present study is to develop a determination method that is based solely on mid- infrared measurements and that allows determination of water content and nitrate concentration with minimal treatment of the samples. MATERIALS AND METHODS Description of the Method. Due to the very high solubility and mobility of nitrate, all the nitrate present in wet soil samples is found in the solution phase of the sample. Therefore, quantitative determination of nitrate on a dry soil basis requires that the water content of the sample be known. Attenuated total reflectance-based spectroscopy requires excellent contact between the ATR crystal and the sample, which for soil samples is achieved by working with soil pastes that are close to saturation. Accordingly, direct determination of the sample water content using the sample ‘‘as is’’ is not possible, and a paste must be prepared. The water contained in that paste consists of the water initially present in the sample plus the water added, which can be directly measured. Determination of the sample water content and nitrate Received 16 May 2006; accepted 8 August 2006. * Author to whom correspondence should be sent. E-mail: linkerr@tx. technion.ac.il Volume 60, Number 11, 2006 APPLIED SPECTROSCOPY 1267 0003-7028/06/6011-1267$2.00/0 Ó 2006 Society for Applied Spectroscopy