Changes in relationships between humic substances and soil structure following different mineral fertilization of Vitis vinif-era L. in Slovakia

The quantity and quality of soil organic matter (SOM) depending on many edaphic and envi-ronmental factors may change in response to agriculture related practices. The SOM humification process can be supported by the application of mineral fertilizers, but in the production vineyards, such information is lacking. NPK fertilizer alters the soil quality, so the aim of this study was to: (1.) assess the extent and dynamics of different NPK (Control—no fertilization; 1st NPK and 3rd NPK levels) rates to the soil with grass sward cover in a productive vineyard on changes in SOM, humic substances (HS) and soil structure, and (2.) identify relationships between SOM, HS and soil structure. Results showed that the share of humic acids in soil organic carbon decreased only in NPK1 compared to Control and NPK3 treatments. The color quotient of humic substances values in NPK1 and NPK3 increased by 4 and 5%, respectively, compared to Control. Over a period of 14 years, the content of soil organic carbon increased by 0.71, 0.69 and 0.53 g kg–1 year–1 in Control, NPK1 and NPK3, respectively. The content of HS increased linearly with slight differences due to NPK application. The vulnerability of the soil structure decreased due to fertilization—more at the higher NPK-level. The rate of formed soil crust was decreasing in the following order: Control > NPK1 > NPK3. In Control treatment, the relations between SOM, HS and soil structure were most abundant and with the greatest significance, while with the increasing level of NPK, these relations lost their significance.