Silva Balcanica, 15(2)/2014 GENESIS AND CHARACTERISTICS OF REGOSOLS AND CALCISOLS IN THE HILLS OF SOUTH DANUBIAN PLAIN Biser Hristov ISSAPP ‘N. Poushkarov’ - Sofia Abstract In the hilly part of South Danubian plain, Regosols and Calcisols are spread on slopes and eroded terrains in semi-arid areas. They are characterized with a weakly developed process of soil formation, shallow profile with primary AC structure, as well as no diagnostic horizons, except one calcic horizon in Calcisols. Profile devel- opment is minimal as a consequence of young age or slow soil formation. Most of Regosols and Calcisols correlate with the old Bulgarian soil classifications and they are classified as eroded Chеrnozems. In the hilly region of South Danubian Plain, Regosols and Calcisols are widespread in semi-arid areas on slopes and eroded ter- rains. These soil types lay-over regoliths (mainly loess), and they occupy an area of 485 000 ha. The area is increasing, mainly due to improper use and erosion. Key words: soil characteristics, Regosols, Calcisols INTRODUCTION The process of degradation (mainly landslides and erosion) is especially ac- tive in the Lom-Svishtov region, closest to Danube River. In these parts of the hilly- plain, soil formation process is slow. Slope terrain is the major ecological-genetic factor that gives influence over soil structure. Parent rock and relief create specific morph-structural forms. Loess in the plain shows alkalinity. The main components that determine soil reaction are carbonates. South Danubian Plain loess contains be- tween 18-25% carbonates and their distribution in vertical depth is unequal. Regosols and Calcisols are spread in areas where natural vegetation is poor. The remaining natural vegetation, suggests that in the past Danube plain was almost entirely occupied by large forests and steppes in the east. The plain was cultivated since ancient times and the result is a significant impact on the state of soils and en- vironment. Forests and steppes areas were turned into farmlands. Other parts have never been cultivated due to steep terrain. Forest vegetation in the region is declining almost everywhere or it is highly diluted. According to hydrothermal data, the Plain belongs to the semiarid zone. In this zone, the insufficient rain, temperature and potential evapotranspiration are limiting factors for soil development process more than 200 days of the year. It 50 should be noted that the region has a moderate continental climate with hot summers and cold winters and with maximum precipitation in May-June period. The annual amount of precipitation (500 mm) is below the average for Bulgaria. The region is characterized by communities of xerothermic vegetation like steppe grasses, shrubs and perennial vegetation. This research was carried out to determinate characteristics and classification of Regosols and Calcisols developed on loess material in the Hills of South Danu- bian Plain according new WRB(2014). Also to identify environmental conditions, factors of soil formation and their influence on soil properties. MATERIALS AND METHODS According to soil-geographical zoning studied soils belong to the northern forest-steppe zone of Danube Chernozem area, middle Danube province and Lom- Svishtov region (Koinov et al., 1972; Gurov, Artinova, 2001). Lom-Svishtov region covers the northern part from Bulgarian Danubian plain rivers Archar (in the west) to Rusenski Lom (in the east). Typical for the plain is flat terrain with ridges and a hilly relief with a maximum altitude of 250 m. Soil profiles studied are as follows: Profile 11 is located at N 43'37 .580 E 24.51.237 with an altitude of 64 m, near the village of Muselievo, Pleven District. It is in the middle part of the western slope (12 o) and moderate erosion. The parent material is silt loam loess. The vegeta- tion is grassy. Profile 16 is located at N 43'37 .171 E 0 24.43.363 with an altitude of 73 m, near the village of Milkovitsa, Pleven District. It is in the upper part of the northern slope (10o) and moderate erosion. The parent material is sandy loam loess. The vegetation is grassy. Profile 18 is located at N 43'27 .086 E 24'34 .356 with an altitude of 114 m, near the village of Opanets, Pleven District. It is in the middle part southern of the slope (10o) and moderate erosion. The parent material is loam loess. The vegetation is grassy. Profile 19 is located at N 43'26 .952 E 24'34 .849 with an altitude of 132 m, near the village of Opanets, Pleven District. It is in the upper part of the southern slope (10o) and moderate erosion. The parent material is silt loam loess; vegetation is grassy and reforested with black locust. The main diagnostic soil characteristics was obtained using the following methods: pH in H2O; organic matter (Kononova, 1966; Filcheva, Tsadilas, 2002); carbonates content by Sheibler (described by Penkov, 1991), Particle density ISO 11508:1998 with 100 cm3 pycnometers; cation exchange capacity, base saturation, exchangeable calcium and magnesium by Ganev and Arsova 1990, Bulk density ISO 11272:1998, soil texture ( Kachinsky, 1958); primary and clay minerals with x-ray diffraction (Dron2); Chemical composition (Ponomarev, 1951),the soil type is defined using the WRB (2014). 51 RESULTS AND DISCUSSION Physical properties show that studied soils are not texturally differentiated, which indicates a low degree of weathering. Regosols and Calcisols have relatively coarse texture and the clay content is about 15%. Silt and lomay sand fractions are predominant in all profiles. Studied profiles show that Regosols have loamy texture structure, Clacisols have silt texture (Table 1). The particle density varies minimum range - from 2.6 to 2.7, due to the low content of organic matter. Soil horizons have approximately 50% porosity and low bulk density 1.33 g/cm3, these values ​​are close to those of zonal soils with favourable physical properties – Chernozems (Mollisols). The relatively good water stability of soil aggregates in the surface horizons is due to the fact that they are not cultivated and they are covered with grass vegetation. The poor structure in depth could be explained by absence of sufficient organic compounds, and the impact of soft structureless parent rock - loess. Cases with low water stability of soil aggregates in the surface horizons indicate that soils without vegetation are threatened by erosion. Loess in Danubian valley is with alkaline reaction. The pH (in H2O) in different loess types could be from 7.0 to 9.5 but for most loess sediments it is about 7.0. The main components that determine the alkaline reaction are CaCO3 and MgCO3. Loess of Danube valley contains usually 18 to 25% carbonates and their distribution in the vertical depth is unequal (Stoilov, 1984). The main difference between Regosols and Calcisols are carbonates (Table 2). Table 1 Soil physical properties Practical Sand Silt Clay Horizon Bulk Density Porosity density % mm % mm % mm Depth/cm (g/cm3) Р (%) (g/cm) 2 – 0.05 0.05 – 0.002 > 0.002 Profile 16 Eutric Regosols (Loamic) A(h) 0 – 16 1.51 2.6 41.92 62.5 25.15 12.25 С 16 – 35 1.58 2.7 41.48 75.4 12.75 11.85 Profile 18 Calcaric Regosols ( Loamic) А к (h) 0 – 14 1.35 2.6 48.08 53.3 30.5 16.2 Ск 14 – 30 1.31 2.6 49.62 45.4 38 16.6 Profile 11 Haplic Calcisols (Siltic) Ак 0 – 18 1.18 2.6 54.62 22.1 65.5 12.4 Ск 18 – 35 1.23 2.7 54.44 24.9 61.35 13.75 Profile 19 Haplic Calcisols (Siltic) Ак(h) 0 – 15 1.17 2.6 55.00 19.4 55.9 24.7 Ск 15 – 30 1.28 2.6 50.77 21.4 53.65 24.95 52 Table 2 Soil physic-chemical properties pH (H O) Total Car- CEC Ex. Ca Ex. Mg Base Horizon/ 2 bonates mequ.10-2 g satura- Depth/cm % soil tion % Profile 16 Eutric Regosols (Loamic) A(h) 0 – 16 6.0 0.0 15.0 10.5 1.2 78.67 С 16 – 35 6.5 0.0 9.9 8.0 1.0 91.92 Profile 18 Calcaric Regosols (Loamic) А к (h)0–14 8.1 8.36 25.9 22.6 3.3 100 Ск 14 – 30 8.0 3.62 21.9 18.5 3.4 100 Profile 11 Haplic Calcisols (Siltic) Ак0 – 18 7.9 15.2 24.8 21.5 3.3 100 Ск 18 – 35 8.3 18.7 24.7 21.2 3.5 100 Profile 19 Haplic Calcisols (Siltic) Ак(h) 0– 15 8.1 16.96 28.8 25.0 3.8 100 Ск 15 – 30 8.3 13.24 31.6 28.0 3.6 100 According to WBR(2014) Calcisols have one calcic horizon, that means that car- bonates must be more than 15% (Profiles 11 and 19 ). Profile 18 is Clacaric Regosols because carbonates are between 2% – 15%. Studied soils have relatively good cation exchange capacity except Profile 16, where clay content is lower and there are no carbonates. In this case Profile 16 can be classified as Eutric Regosol. Exchangeable calcium dominates in all profiles and the base saturation is high. The presence of carbonates is indicative for low level of soil moisture. The formation of aggregates of colloidal and coarse particles under the influence of car- bonates is part of soil formation. High content of carbonates in the soil is determined by geographical location of the area. Carbonates involved not only directly but also indirectly in soil formation processes influence on vegetation.