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Soil fertility is largely determined by the content and quality of soil organic matter. SlovakRepublic monitoring system includes evaluation of soil humus content, basic humic parameters and also chemical structure of humic acids (HA). Changes in humic acids structure on five selected key localities during monitoring period (1994&amp;ndash;2000) are presented in this study. Among soil types, essentially between Chernozem and Planosol, in elemental analysis, optical parameter and 13C NMR parameters significant differences determined by Student&amp;rsquo;s test were observed (Tables 2 and 3). During monitoring period small oxidation trend mainly on Calcaric Fluvisol and Planosol HA expressed by increase of oxygen content and O/C, O/H ratio was observed. Oxidation trends during monitoring period confirm also position of Calcaric Fluvisol and Planosol HA on van Krevelen diagram. Changes in optical parameter E 1%  6 and content of carboxylic groups for the period of monitoring were negligible. Some changes throughout monitoring in  13 C NMR parameters were recorded. On all observed HA considerable decrease of carbon in region 106&amp;ndash;143 ppm, characteristic for aromatic and olefinic carbon with C&amp;ndash;C, C&amp;ndash;H bond, was observed. Spectroscopic parameters are in good agreement with elemental analysis and carboxylic groups. Joint relationships between changes in determined parameters are expressed by significant linear correlations. Predominantly between parameters calculated from  13 C NMR spectra and data of elemental analysis and optical parameter E 1%  6 highly significant linear correlations (R &amp;gt; 0.93) are found (Table 4).&amp;nbsp;

Changes of humic acids structure on selected key monitoring localities of arable soils