Physical properties of soils

soil science

1. Physical properties of soils A.Murugan 2014004055

2. Physical properties Soil texture. Soil structure. Surface area. Soil density. Soil porosity. Soil colour . Soil consistence.

3. Soil texture Soil texture refers to the relative proportion of particles. It is the relative percentage by weight of three soil separates viz., sand, slit, clay. Simply refers to size of soil particles. The soil separates are defined in terms of diameter in millimeter of the particles.

4. Naming soil separates USDA BSI ISSS EUROPEAN SYSTEM. SOIL SEPARATES DIAMETER (mm) Clay <0.002 Silt 0.002-0.02 Fine sand 0.02-0.2 Coarse sand 0.2-2.0

5. Particles less than 2mm is called fine earth , normally considered in chemical and mechanical analysis. The component fine earth : sand, silt, clay.

6. SAND: Consists of quartz but may also contain fragments of feldspar,mica etc., and can be represented as spherical.

7. SILT: Intermediate b/w sand and clay. Small size and more surface area CLAY: Particle size less than 0.002mm, plate like or needle like shape .Belongs to alumino silicate group of minerals.

8. Methods of textural determination Elutriation method - water and air. Pipette method. Decantation method. Test tube shaking method. Feel method- feel by fingers , ball formation , stickiness, ribbon formation.

9. Importance of soil texture Texture has good effect on management and productivity of soil. Sandy soils are poor store house of plant nutrients. Contain low organic matter. Clay particles play a important roles in soil fertility. Loam and silt loam are highly desirable for cultivation.

10. Soil structure The arrangement and organization of primary and secondary particles in a soil known as soil structure. Structure is described under three categories: Type – shape or form and arrangement pattern of peds . Class- size of peds . Grade – degree of distinctness of peds .

11. Types of structure Plate like: The aggregates are arranged in horizontal plates or leaflets. When they are thick – platy, thin – laminar. Prism like : The vertical axis is more developed than horizontal giving a pillar shape. Length 1-10 cm. Commonly occurred in sub horizons of arid and semi-arid regions .

12. When the tops are rounded – columnar. When flat / plane/ level and clear cut - prismatic. Block like: All the dimensions are about the same size. The aggregates are reduced to blocks. when the faces are flat and distinct and the edges are sharp angular- angular blocky. when the faces and the edges are mainly rounded - sub angular blocky. These types confined to sub soil.

13. Spheroidal : All rounded aggregates are placed in this category. The aggregates of this group are usually termed as granular which are relatively less porous. When the granules are very porous – crumb. This is specific to surface soil particularly high in organic matter / grassland soils.

14. Classes of structure Depending upon the size of the individual peds : Very fine or very thin. Fine or thin. Medium. Coarse or thick. Very coarse / very thick The term thick and thin are used for platy types, while the terms fine and coarse are used for other structural types.

15. Grades of structure Grades indicates the degree of the distinctness of the individual peds . Structure less: There is no noticable aggregation. Weak structure: Poorly developed which are not durable. Moderate structure: Moderately well developed peds which are fairly durable ad distinct. Strong structure: Very well formed peds which are quite durable and distinct.

16. Factors affecting soil structure Climate. Organic matter. Tillage. Plant roots and residues. Exchangable cations

17. Soil density Density of a soil is expressed as weight (mass) per unit volume. Soil density expressed in two concepts: Particle density / Absolute specific gravity. Bulk density / Apparent specific gravity. The particle density of normal soils are 2.65g/cc. The bulk density of the normal soils ranges b/w 1.00-1.6g/cc

18. Bulk and particle density TEXTURAL CLASS PARTICLE DENSITY g/cc Coarse sand 2.655 Fine sand 2.659 Silt 2.798 Clay 2.837 TEXTURAL CLASS BULK DENSITY g/cc PORE SPACE % Sandy soil 1.6 40 Loam 1.4 47 Silt loam 1.3 50 clay 1.1 58

19. Soil porosity The pore space of a soil is the space occupied by air and water. In sandy soil the pore space is low. In clay soil pore space is high. Presence of organic matter increases the pore space. calculating pore space: % solid = (bulk density/ particle density )*100

20. % Pore space = 100 – solids % Pore space = 100 – ( bulk density / particle density)*100 Porosity = 1- ( bulk density / particle density)

21. Factors influencing pore space Soil texture. Sandy surface soil : 35 to 50% Medium to fine textured soils: 50 to 60% Compact sub soil : 25 to 30% Vegetation. Biological activity. Size of pores. Macro pores (non capillary pores) : >0.05 mm Micro pores (capillary pores) : < 0.05mm

22. Soil colour Soil colour is due to the reflection of EMR by different soil constituents like minerals organic matter,water , salts etc., The occurrence of two or more patches of colour is called mottling (due to the presence of iron,manganese and copper in soil in patches). Soil colour is inherited from the parent material and that is referred to as lithocromic . Eg : red soils developed from red sand stone

23. Measurement of soil colour Soil colour is measured using Munsell colour chart and expressed in terms of Hue, value and chroma . Hue denotes the dominant spectral colour . Value denotes the intensity of colour . Chroma indicates the purity of colour . 10YR 5/4 means , the Hue value is 10YR, 5 is the value and 4 is the chroma

24. Factors influencing soil colours Parent material. Soil moisture. Organic matter. Mixture of organic matter and iron oxides. Alternate wetting and drying condition

25. Soil consistence The resistance of a soil at various moisture contents to mechanical stresses or manipulations. It combines with both adhesive and cohesive forces which determines the soil can be reshaped or ruptures. The soil consistency is described at three moisture levels : 1.wet , 2.moist , 3.dry soil. Wet soil – consistency is denoted by stickiness and plasticity.

26. Atterberg’s constants He studied plasticity from the point of view of moisture range over which plasticity range is maintained. Shrinkage limit or lower plastic limit. Plastic limit. Liquid limit or upper plastic limit. Plasticity number : The difference b/w the consistency of upper and lower plastic limits

27. Factors affecting A tterberg’s constants Clay content. Nature of clay minerals. Nature of exchangable cations . Organic matter content. Cohesion. Sticky point. Swelling and shrinkage.