The potential of near infrared reflectance spectroscopy as a tool for the chemical characterisation of agricultural soils

The feasibility of near infrared (NIR) reflectance spectroscopy in determin ing various soil constituents such as total organic carbon, total nitrogen, exchangeable potassium and available phosphorus has been investigated, to monitor their concentration during a long-term agronomic trial. Soil sample s previously analysed by conventional chemical methods were scanned using a NIRSystems 5000 monochromator and spectra were treated using several algor ithms. The first derivative of each NIR spectrum was used for all statistic al analyses. Step-up, stepwise and modified partial least squares (MPLS) re gression methods were applied to develop reliable calibration models betwee n the NIR spectral data and the results of wet analyses. MPLS almost always gave the most successful calibrations. The results demonstrated that NIR r eflectance spectroscopy can be used to determine accurately two important s oil constituents, namely total nitrogen and carbon content. This technique could be employed as a routine testing method in estimating, rapidly and no n-destructively, these constituents in soil samples, demonstrating soil var iations within a long-term field experiment. For other determinations, such as exchangeable potassium and available phosphorus content, our results we re less successful but may be useful for separation of samples into groups.