It is suggested that calcareous soils formed from calcareous gyttja were termed quaternary rendzinas, as indicated by Uggla. Their differentiation is connected with the amount of calcium carbonate and anthropogenic modification of surface horizon. Soils being in the initial phase of organic matter accumulation are suggested to be termed initial quaternary rendzinas, whereas the soils that have humus horizon more than 10 cm thick - proper quaternary rendzinas. Taking into consideration the fact that humus horizon of these soils is developed during siltation with alluvial or deluvial deposits, separation of calcareous post-lacustrine soils in alluvial and deluvial soils should be considered alternatively.
The Mrągowo Lake District (NE Poland) with the area of 182 800 hectares was formed in 1/3 during the Poznań phase (POZ) of the Vistulian glaciation, and in 2/3 in the Pomeranian phase (POM) of that glaciation. A number of fens in the Mrągowo Lake District is 1164, and they occupy 5.9% of its total area. Majority of fens (1018) with an area of 8620 ha is located in the zone of POM phase, and the remaining fens (146) occupying 2157 ha occur within the zone of POZ phase. Peatlands cover 3.5% of area of POZ phase and 7% of area of POM phase. The mean size of fens in the Mrągowo Lake District is 9.3 ha (mean size in POZ phase zone is 14.8 ha and in POM zone – 8.5 ha). Limnic deposits predominate in the bedrock of fens (92.5% of the total number of fens). Originally, lakes occupied 11% of the Mrągowo Lake District, of which 1/2 was transformed into fens. More than 50% of peatlands developed on organic gyttja, 27% on calcareous gyttja, and almost 14% on mineral gyttja. About 7.5% of fens formed as a result of paludification.
The Ełk Lakeland (NE Poland) with the area of 263 100 ha was formed during Vistulian glaciation. More than 66% of this region was shaped during the Leszno phase, 15% during the Poznań phase, and 19% during the Pomeranian phase. There are 1854 fens which cover the area of 7.3 % of the Ełk Lakeland mesoregion. Fens have an area of 10.3 ha on average. About 82.5% of the studied fens is located on gyttja deposits, which suggests post-lacustrine origin of the wetlands. Primary the lakes had covered 11% of the Ełk Lakeland, and 6.3% was transformed into fens. Most of them (60%) was formed on organic gyttja, 16% on calcareous gyttja, and 6% on clay gyttja. About 17.5% of fens was formed as a result of paludification. The types of bedrock underlying peats differ depending on the phases of glaciation.
Several shallow lakes have been drained to make way for additional arable land or pasture in the Mazurian Lakeland (NE Poland) since the 19th century. As a result of these hydrotechnical works, the water level usually decreased by approximately 6 m. Bottom sediments of the former lakes were transformed into surface limnic soils. Part of them, developed from highly calcareous gyttja, may be called limnic rendzinas. However, the present position of these soils in the Polish Soil Classification is unclear. Where the lake has undergone a natural terrestrialisation, the gyttja is covered with peat and mursh. The raw gyttja soils differ in type of organic matter forming the topsoil horizon and are subject to further transformation, the direction of which depends on the sequence of sediments in the profile, mursh formation, mineral admixture and adjoining colluvial phenomena. Common features of all these soils are high content of calcium carbonate in the surface horizons, alkaline reaction, high groundwater level and periodical flooding. The paper presents the variability of limnic rendzinas based on many examples from the Mazurian Lakeland (NE Poland). Finally, new type additions were suggested to the next edition of the Polish Soil Classification.