A Simple UV- Visible Spectroscopy Method To Estimate SOIL Organic Matter Content.

Soil organic matter content is considered a soil quality indicator. There are many methods for measuring SOM, of different degrees of complexity, requiring various pieces of equipment. The goal of this research was to develop and evaluate a quick and simple method based on UV-visible spectroscopy to estimate SOM content, in different soil types of Chile and carbon-amended soil samples.  For this, 95 Andisol, 79 Inceptisol and 64 C-amended Inceptisol soils samples were selected. Samples were analyzed for SOM through the standard Walkley-Black (WB) method. The UV-visible spectroscopy method consisted on extracting the soil with 0.5M NaOH in a 1:10 soil to extractant relation by 30 minutes, centrifuged by 5 minutes at 9000 rpm, and reading the supernatant at 465 nm and 665 nm. Regression analyses were performed to determine the performance of the method against the standard one. There was a significant and positive relationship between SOM by the standard method and the absorbance at 465 nm and 665 nm (R²=52% and R²=50% in Andisol soils and R²=38% and R²=22% in Inceptisol soils, respectively). On the other hand, SOM, as determined by WB method, did not increase with C rate at amended soils (R²=3%); however this increase was observed when using the UV-visible spectroscopy method (R²= 80% at 665 nm and R²= 29% at 445 nm). The developed method is very suitable for mapping SOM for variable rate nitrogen application; the spectroscopic index would be used as follows: A composite sample is analyzed for SOM by WB method as well as by the UV-visible spectroscopy one; individual samples are analyzed by the UV-visible spectroscopy method; readings are transformed to SOM values by using the quotient between the numbers of WB and those of the UV-visible spectroscopy method. The spectroscopy method is a rapid and useful tool to estimate SOM content in Andisols and Inceptisols soils.