Weathering and the soil formation:

 Bulk density  Dry bulk density - It is the ratio of the mass of oven-dried soil (Ms) to total volume (Vt)  of soil. • Units: g/cm3 • Depends upon texture, structure, and OM content • May vary from 1.1 to 1.4 g/cm3 in fine and 1.4 to 1.75 g/cm3 in coarse-textured soils. The bulk density of soil is always smaller than its particle density. The bulk density of sandy soil is about 1.6 g / cm3, whereas that of organic matter is about 0.5. Bulk density normally decreases, as mineral soils become finer in texture. The bulk density varies indirectly with the total pore space present in the soil and gives a good estimate of the porosity of the soil. Bulk density is of greater importance than particle density in understanding the physical behavior of the soil. Generally, soils with low bulk densities have favorable physical conditions.

Soil structure is the arrangement of the primary soil  particles (sand, silt, and clay) and other soil materials into  discrete aggregates . Classification is based upon – 1. Type (shape and arrangement of peds), 2. Class (size of peds) and 3. Grade (durability of peds).                                                                 Peds • Peds consist of primary particles bound together by  cementing agents like organic matter, clay, and hydrous oxides  of iron and aluminum. • They are the structural units that have distinct boundaries  and well-defined planes of weakness. • Based on shapes and arrangements of peds/ aggregates, soil  structure is of 4 principal types:                                                          Ped Shapes                                                        • Plate like                                                        • Prism like                                                        • Block like                                                

 The important physical properties of soils are (1) soil texture (2) soil structure (3) soil consistency (4) soil density (5) soil porosity and (6) soil color

Definition of the soil profile The vertical section through the soil is called as soil profile . The  various layers are known as horizons. Component of soils (volume basis) The soil consists of four major components i.e. mineral matter  (45%), organic matter (5%), soil air (20-30%) and soil water (20- 30%). 1. Mineral matter The minerals are extremely variable in size. Some are as large as  the smaller rock fragments, others, such as colloids clay  particles, are so small that they cannot be seen without the aid  of an electron microscope. 2. Organic matter Soil organic matter represents partially decayed and partially  synthesized plant and animal residues. Such material is  continually being broken down by the action of soil  microorganisms. 3. Soil water Soil water is the major component of the soil in relation to  plant growth. The water is held within the soil pores. If the  moisture content of soil is optimum for plant growth, plants  can readily absorb water. Soil water dissol

 A. Fundamental soil-forming processes (1) Humification (2) Eluviation and illuviation B. Specific soil-forming processes (1) Podsolisation (2) Laterisation (3) Calcification (4) Gleization (5) Salinization (6) Desalinization (7) Solonization (Alkalization) (8) Solidization (dealkalization) (9) Pedoturbation

  A. Passive soil-forming factors               B. Active soil-forming factors (i) Parent material                                            (i) Climate (ii) Relief or topography                                  (ii) Vegetation and organisms (biosphere) (iii) Time                                                                   Soil – the end product of soil-forming factors   A. Passive soil-forming factors 1. Parent material Parent material is that mass (alluvium, colluviums, aeolian/loess,  glacial-till) from which the soil has formed. 2. Relief or topography  The topography refers to the differences in elevation of the land  surface.  On level topographic positions, almost the entire water received  through rain percolates through the soil. Under these  conditions, the soil formed may be considered representative  of the regional climate. They have normal solum with distinct  horizons. The soils on steep slopes are shallow, stony, and have weakly developed  profiles with less dist