Növénytermelés / Volume 67 / Issue 4 (December 2018) / pp. 59-72


Examining the organic matter quality of the soil in the UV-VIS spectrum – the Exponential Fitting Approach (EFA)

Climate change causes problems in global systems, i.e., it has a great impact on soils, too. The measurement of organic matters in the soil (especially their quality, such as the presence and proportions of fulvic acid – FA and humic acid – HA fractions) is a constant task, the method of which was constantly developing in the past century. Some of these methods are the E4/6 [465/665 nm] proportion or the E2/3 [250/365 nm] method. The UV-VIS spectrums of soil extracts (200–900 nm) can be described with an exponential function, from which the proportion derived from the data measured on selected points provides information on the distribution of FA and HA materials. During our examinations, we strived to obtain information using the exponential function. These pieces of information provide data not only about the proportion of FA and HA, but also the distribution of organic matter of the examined soil samples. This approach (Exponential Fitting Approach – EFA) is a promising method for the easy and inexpensive examination of organic matter distribution (and also the analysis of all samples in which the fluid phase contains organic matter or is able to make organic matter released). In order to perform a wide-ranging analysis, the following soil types were selected: sandy soil, chernozem, solognac and slightly acidic brown forest soil. In order to simulate the extreme organic matter distribution of soil, the Na sumate standard was used. Soil extracts were prepared based on a modified Stevenson method (0.5 M NaOH and 0.1 M Na4P2O7). 20 ml extracting substance was added to each soil sample (5.0 g), while in the case of the Na humate standard, 20 ml extracting substance was added to 0.125 g Na humate. Following 4 hours of shaking, it was centrifuged, filtered and diluted 40-fold.

Based on the obtained findings, it can be concluded that, in the case of favourable environment, replications in the E4/6 method show nearly 10% error, while the uncertainty of the EFA method is less than 3% in the case of the examined soils. In the case of examining unfavourable (extreme) circumstances, the original E4/6 method may even produce 30% error. EFA was within 12% even in such cases. It is worth mentioning that the results of EFA were always below the original E4/6 (72–83% of E4/6), but it followed the original values and remained proportional to the values, but produced lower error.

Keywords: SOM, humic acid, fulvic acid, UV-VIS spectrum, EFA

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