The study analysed the relationship between the granulometric composition of grassland soils as determined by laser diffraction and their content of mineral forms of nitrogen and organic carbon. The content of mineral forms of nitrogen (NO _3-N and NH _4-N) in soil samples – after their extraction with 1% solution K ₂SO ₄, was determined by flow colourimetry. Soil organic carbon content was determined using the Tyurin method. The study examined soil samples collected from 169 control and measurement sites located in different regions of Poland in terms of conditions for agricultural production. Statistical analyses of the research results showed that the grain size of grassland soils had a significant effect on their ammonium nitrogen content but not on their nitrate nitrogen and organic carbon content. In this respect, it was found that there was a positive correlation between the share of the sand fraction and the content of ammonium nitrogen in soils and an opposite relation between the share of coarse silt, fine silt and clay and the content of the aforementioned component. Results of the analyses differ considerably from the results of studies by other authors on the influence of soil grain size distribution on the content of mineral nitrogen and organic carbon in soils based on classical methods of measurements of soil particle size distribution. There is a need to develop solutions to convert and compare results obtained by laser diffraction and standard methods.

The aim of the study was to recognise the accumulation of organic carbon (SOC) in the soils of Polish grasslands (GL) and to consider the possibility of increasing its sequestration in these soils. The Tiurin method (mineral soils) and the mass loss method (soil of organic origin) were used. It was found that: (i) the average SOC content of mineral soils is 2.44% and of organic soils – 10.42%; (ii) according to the Polish criteria, approximately 84% of GL mineral soils are classified as classes with high and very high SOC content, and over 15% and 1% – in classes with medium and low SOC content, respectively; more than 99% of organic soils belong to two classes with the highest SOC content and less than 1% to the class with an average content; (iii) according to the European Soil Bureau, the share of GL mineral soils with a high SOC content is slightly over 4%, medium – slightly over 47%, and low and very low – around 50%; for organic soils they are 67, 29, and 4%, respectively; (iv) the reserves of organic carbon in the 0–30 cm layer on the entire surface of GL soils amount to 412.7 Tg of SOC. There is considerable scope for increasing the SOC stock in meadow-pasture soils.