Soil has a mollic horizon mixed with skeletal part overlapped a calcareous rock; ... Rendzinas (humus-calcareous) soils have formed under different vegetation ...
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14. RENDZINAS (HUMUS-CALCAREOUS) SOILS Abstract. Ecological conditions and in particular soil forming factors responsible for the development of rendzinas (humus-calcareous) soils are describedin this chapter. The contemporary profiles status is shown in relation to different soil forming factors. Scientific information and data provide horizons, soil texture, structure, and properties as well as the national classification, FAO-UNESCO-ISRIC (1990), WRB (2006) and Soil Taxonomy (2010). Keywords Bulgaria, Soil, Rendzina, (Humus-calcareous) soils, Type, Classification, Morphology, Texture, Humus, Carbonates, Revised Legend FAO-UNESCO, WRB, Soil Taxonomy, Properties. The term rendzic derived from the Polish “ren-dzic rze-dzic” and is a composed syllable meaning the sound of an implement being scraped across a stone. Rendzinas are shallow dark colored soils developed on consolidated calcareous rock.
Fig. 14.1 Stoniness is common on the surface of cultivated land 14.1 Concept of the type Main concepts 13.1.1. Soil has a mollic horizon mixed with skeletal part overlapped a calcareous rock; 13.1.2. Well drained mineral soil that develops in many environments.
2 Rendzinas (or humus-calcareous) soils show evidence of regular organic substances accumulation at the bottom limited by a calcareous bedrock. 14.2 Ecology of soil formation 14.2.1. Distribution Mostly during the Tertiary period, tectonic movements uplifted the terrains dominated by basic and ultra basic rocks. Anticline and syncline activity lead to the formation of intermediate valleys and coastal plains. Rendzinas (humus-calcareous) soils are distributed in many environments throughout the county (Fig. 14.1). Rendzinas occupy about 265, 841 ha or 2.4% of the entire territory. Shallow soil varieties prevail. Climatic conditions The term intrazonal soil is means a formal independence from climatic factor. Rendzinas are intrazonal soils that occurr in different landscapes with various type of climate. Usually there is a yearly drought period. Topography Relief determines conditions for the intensive lateral and sub-soil geochemical flood migration. Common are old landscapes that vary from flat to sloping. Karsts are typical phenomenon in the areas of redzinas occurrence. Parent materials Parent materials always contain high rates of calcium and/or magnesium carbonate. Consolidated rocks such as limestone and chalk are common. Consolidation of parent materials has a marked influence on the soil development. In the southern part of the country, rendzinas have developed on the Precambrian Age marbles, dolomitic marbles and schists. In the western part, rendzinas have developed on the Upper Triassic Age variously colored terrigenous and terrigenous-carbonate rocks; and on the Lover-Upper Triassic Age limestones, dolomitic limestones, dolomites and less shales. In the northern part, rendzinas have developed on the Lover-Upper Triassic Age limestones, dolomitic limestones, and dolomites; and on the Cretaceous Age (ConiacianMaastrichtian) terrigenous-carbonate rocks in different combinations in different localities. In the Balkan Mountains, rendzinas have developed on the Lover Cretaceous (BerriasianBarremian) Age limestones, clayey limestones and marls; and on the Upper Cretaceous (Maastrichtian) Age limestones. Vegetation Rendzinas (humus-calcareous) soils have formed under different vegetation and in almost all types of environment. Vegetation can be coniferous or broadleaf forest as well as grassy or mixed associations belonging to the different landforms. The type of vegetation is in compliance with the moisture and temperature regimes. The shallow depth and shortage of moisture is a limiting factor for plant growth. Time Soils are of Quaternary or Pleistocene age but some may to be much older. Human influence
3 The hilly lands are suitable for grazing. The mat of dense roots at the surface limits the pronouncement of erosion. The removal of woodland exposed a loose litter layer and provoked rapid soil degradation. Due to agricultural use, the physical properties of these soils have deteriorated. 14.2.2. Genesis Due to the high humus and carbonate content these soils are called humus-calcareous or rendzinas. The high carbonate content slows the decomposition of organic matter which means that mineralization is slow and they accumulate a large amount of humic substances. During soil evolution a dark clayey layer is formed. The mineral part has inherited insoluble residues of primary rock minerals. Rendzinas formed on dolomites can have an enrichment of Mg. The main soil forming process is the continuous solution and removal of inherited bedrock carbonate in the drainage water. The degree of humification depends on many factors. Rendzinas (or humus-calcareous) soils have formed “mull” type of humus. Dominated humic acids are bound with calcium, thus forming a strong granular structure. Soils are very well drained. The high value of total porosity resalts in rapid moisture percolation. 14.3 Soil diagnostic and classification
Fig. 14.2 Rendzina or humus-calcareous soil, Bulgarian classification (1976), Rendzic Leptosol, WRB (2006), Typic Haprendolls, Soil Taxonomy (2010), Rendzic Leptosol, Revised Legend FAO-UNESCO (1990) 14.3.1. Morphology Most rendzinas have AC(k)/Rk horizon sequence (Fig. 14.2). Soils are shallow, and vary in thickness from 10 to 60 cm. The soil profile is uniform in texture, very skeletal and often with large rock fragments. The classes of percentage of surface rock outcrops and stoniness according to the FAO (1990) are as follows: very few 0-2%, few 2-5%, common 5-15%, many 15-40%, abundant 40-80%, dominant ≥ 80%. Rendzinas have mollic horizon and no other diagnostic horizons. The thickness of the mollic horizon is not lesser 20 cm. The enrichment with clay is fixed from the surface.
4 Below, lying on the calcareous rock , there is a thin mineral layer penetrated by mixed organic matter and partly weathered in situ. Rendzinas show the following morphological features. Ah(k) - a thin sod or loose litter layer, the fine earth is of very dark color, between the roots there are strong structural aggregates of fine granular structure (spheroids or polyhedrons), with size of 1-2 mm diameter; beneath there is gradual transition to the lower ACk - a mineral humus accumulative horizon, the fine earth is homogeneous, color can vary from dark brown or black to lighter. Structure is granular (spheroids or polyhedrons), with medium size (2-5 mm diameter), strong pedality grade, and consistence is firm (moist) and slightly hard (dry). Abundance of fine roots with very fine (< 0.5 mm) and fine size (0.5-2.0 mm). Carbonate is present throughout. Many parent rock fragments of different size are in the soil mass; beneath is clear change to CRk - a thin half-weathered mineral horizon, transitional to the parent rock, light colored compared to the upper horizon. Structure is massive (structureless), many rock fragments of different size occur. There is sharp transition into Rk - calcareous continuous hard rock. 14.3.2. Classification Since 1986 the soil group of Rendzic Leptosols after the Revised Legend of the World Soils (FAO-UNESCO-ISRIC 1990) has included the former soil unit rendzinas. A defining characteristic is carbonate materials with equivalent content over 40% calcium carbonate. In WRB (2006) this is the sub-group of Rendzic Leptosols based on the diagnostic mollic horizon. The Bulgarian soil classification (Koinov et al. 1964) defined calcareous and typical rendzinas (humus-calcareous) soils. The classification of Yolevsky et al.(1976) subdivided the type of rendzina into eroded and eroded subtypes. The classification of Yolevsky et al. (1983) renamed subtypes as haplic rendzina and pararendzinas (formed on the unconsolidated calcareous materials). Principal variation within the type is soil thickness (< 20 cm, 20-40 and > 40 cm depth) and degree of erosion (non eroded, slightly, moderately, and severely eroded). Classification after WRB (2006) required Mollic diagnostic horizon in the reference group of Rendzic Leptosols. The Mollic horizon has base saturation over 50%. Soil Taxonomy (2010) required Mollic epipedon for Rendolls sub-order classification in the order of Mollisols. Soil Taxonomy requires subsurface horizons: calcic is allowed. 14.3.3. Rendzina (Humus-Calcareous) Soil Type in Bulgaria 14.3.3.1. Rendzina (Humus-Calcareous) soil Non eroded and Eroded Non eroded and eroded (subtype) rendzina (type) is named after the Bulgarian classification of Yolevsky and Hadzhiyanakiev 1976. Rendzic Leptosol is named according to WRB (2006) and Typic Haprendolls according to Keys to Soil Taxonomy (2010). Rendzic Leptosol: (Revised Legend of the World Soils (FAO-UNESCO-ISRIC 1990). The thickness of the soil is defined by topography, where bottom is hard calcareous rock. Texture is homogeneous at the depth. The clay content is uniformly distributed. Particle-size distribution shows that the clay fraction (< 0.001 mm) dominates (Table 14.1 and 14.2).
5 The average particle density is 2.50-2.55 g/cm3 at the surface and 2.65 g/cm3 downward. Bulk density is about 1.2 g/cm3 at the surface and 1.25-1.30 downward. Water stable aggregates with size >1 mm are about 55-65 %. Prevailing water stable aggregates with size over 0.25 mm are 80-85 %. In rendzinas near 15-20% of pore space is represented by macropores or the pores of aeration (with diameter >300 µm); and 20-25% is mesopores or pores with rapid moisture drainage (> 30 µm diameter). Micropore space or water-filled is about 15-20%, represented by slow drainage pores (diameter 0.2-30.0 µm) and 15% of micropores is with none remaining available for roots moisture (diameter < 0.2 µm) (Dilkova 1985, Dilkova at al. 1998). Total porosity is 55% (at field capacity) at the surface and gradually decreases with depth. Soils have good water permeability. Table 14.1 Variables of the particle-size distribution in referenced profiles of rendzinas in Bulgaria Profile count
Horizon
Upper Lower Loss depth depth HCl
11 Mean
>1 mm
Particle-size distribution (mm),% (according method of Kachinskiy revised) 1- 0.25- 0.05- 0.01- 0.0050.25 0.05 0.01 0.005 0.001
