Determination of organic matter in soil using near-infrared spectroscopy and partial least squares regression

An alternative approach for the determination of organic matter content in soil is proposed, using near infrared spectroscopy and partial least squares regression for data modeling. This method was applied to two different types of soils: Oxisol and Ultisol. One hundred soil samples, collected at different depth intervals. with organic matter content from 0.40 to 4.83 mg g(-1) were obtained from the Instituto Agronomico (Campinas-Brazil). For test the reference data were obtained by titration of the excess of Cr(VI) with standard Fe(II) solution, after sample digestion with potassium dichromate in an acid medium in digestion tubes. The organic matter contents determined by the so called digestion tube method were also correlated with those obtained by near infrared spectroscopy, using partial least squares regression. A root mean squares error of prediction equal to 0,20 mg g(-1) was obtained, The digestion tube method was also compared with the Walkley-Black method for twelve soil samples and no difference in results for the organic matter determination was found.