Classification issues of drained organic soils in relation to selected features of soil water regime: A case study from central Poland

Organic soils constitute a major terrestrial carbon reservoir and play a key role in regulating climate, biodiversity, and water balance. In Poland, organic soils—mainly of peat origin—cover approximately 1.3 million hectares (4.3%), of which about 85% have been drained or hydrologically altered. Drainage modifies their water regime, leading to a drop in the groundwater table (GWT), typically oscillating between 0.3 and 1.0 m below the surface, depending on land use and drainage intensity. The lowered GWT enhances aeration and activates a cascade of transformations collectively referred to as the mursh-forming process. This permanent drainage creates practical problems related to the identification of organic material, which, according to the Polish Soils Classification, must meet the criterion of water saturation for more than 30 days per year (on average over a multi-year period), even after drainage. Therefore, the aim of the study was to assess the fulfillment of this criterion in thin murshic soils used for agricultural purposes. The research was conducted on soils within the “Racot” subirrigation facility in central Wielkopolska, Poland. Continuous measurements of GWT depth and soil moisture were carried out in 2019–2020. The murshic horizon developed in these soils had an average thickness of 33–35 cm, underlain by sandy material. Mean GWT depths ranged from 0.80 to 0.83 m below surface, with the most frequent range between 0.5 and 1.1 m. During the study period, the GWT never reached the upper boundary of the murshic horizon; only for short periods (6–9 days) did it rise to 0.3–0.4 m below the surface. The degree of water saturation (f = actual/saturated water content) in the murshic horizon typically ranged between 0.60 and 0.70 m³·m⁻³, never attaining full saturation (f =1). Multiple linear regression revealed that GWT depth was the dominant factor controlling degree of water saturation (f), while precipitation and air temperature had secondary, statistically weaker effects. The results demonstrate that drained murshic horizons are not water-saturated for more than 30 days per year. In light of the research conducted, it seems reasonable to remove the obligatory criterion of water saturation (>30 days) in identifying organic material in the next edition of the Polish Soil Classification.