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The paper aimed to determine humus quality in Terra rossa soils under olive groves with different soil management types. A total of 10 top-soil samples (0-20 cm) were collected in olive groves of Middle Dalmatia, Croatia, out of which 5 were from the traditional low-input olive groves (TOG) and 5 in the intensive olive groves (IOG). The soil samples were analyzed for basic soil properties, soil organic carbon (SOC), and fractional composition of humic substances. Spectroscopic characterization of humic substances was carried out by measuring absorbance in the VIS spectral range (400-700 nm). Optical index E4/E6 (ratio of optical absorbance at 465 to 665 nm for humic substances in solution) was calculated. The mean value of SOC in soils under TOG (3.06%) was lower than in soils under IOG (3.88%). Higher variations of carbon in humic (CHA) and fulvic acid (CFA) were observed in the soils under IOG. The mean CHA/CFA ratio in soils under TOG was higher than in soils of IOG (1.78 and 1.26, respectively). The soils under TOG had fulvic-humic to humic types, while the soils of IOG had humic-fulvic to humic types of humus. A lower mean E4/E6 index of soils under TOG than IOG (3.78 and 4.36, respectively) confirmed the results of the classical analytical method. Our findings reveal higher variation and lower humus quality in soils under intensive olive cultivation.

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The study aimed to evaluate the humus quality of anthropogenic soils derived from flysch deposits. A total of 10 samples were collected in the top-soil horizon (0-20 cm) of olive groves in Kaštela Bay in Middle Dalmatia, Croatia. Five samples were taken from traditional low-input olive groves (TOG), and the other five samples were from intensive ones (IOG). The soil samples were analysed for basic soil properties: particle size distribution, pH, carbonate content, P2O5, K2O, and soil organic carbon (SOC) content. Humic substance isolation was done following the procedure given by Schnitzer (1982). Spectroscopic characterization of humic substances was carried out by measuring absorbances in VIS spectral range (400-700 nm). Optical index E4/E6 (ratio of optical absorbance at 465 and 665 nm for humic substances in solution) was calculated. The mean value of SOC content in soils of TOG was lower than in soils under IOG (1.20 and 1.60%, respectively). The absorption spectrum of the studied soils showed a monotonous decrease of absorbance from 400 to 700 nm. The E4/E6 indices for soils under TOG varied from 4.60 to 5.00, while for soils under IOG were in the range of 4.83 - 6.89. The E4/E6 indices > 4 revealed low humus quality in studied soils. However, soils under TOG have lower mean values E4/E6 index in comparison to soils under IOG (4.76 and 5.70, respectively), indicating slightly higher humus quality. [ABSTRACT FROM AUTHOR]