Composition of soil and earth crust

Impact of soil component on soil properties

1. Soil - A three phase system

2. Ideal composition (in volume basis) SOIL Sand In- oganic (45 %) Organic (5 %) Solid (50 %) Gaseous (25 %) Liquid (25 %) Silt Clay Humus Primary minerals (Quarts, mica, Feldspar) Secondary minerals (Clay minerals, hydrous oxides)

3. Solid Mineral phase The mineral phase (Dixon and Schulze, 2002) is commonly separated into three particle-size fractions : clay (less than 0.002 mm particle diameter), silt (0.002–0.05 mm), and sand (greater than 0.05 mm). The clay fraction , because of its large surface area and its surface charge and adsorption properties , is the site of many important chemical reactions occurring in soils. The principal clay-size inorganic components are the phyllosilicate minerals ( kaolinite , smectite , vermiculite, illite and chlorite) and the oxides of iron (goethite, hematite and ferrihydrite ), manganese ( bimessite ), aluminum (gibbsite) and silicon ( allophane and immogolite ; these minerals also contain Al). Important minerals in the sand and silt fractions include quartz, feldspar, mica, calcite and gypsum . The individual particles within the soil interact with each other as well as with ions and molecules in the soil solution due to their charge and adsorption properties is called particle – particle interaction .

4. Solid Organic phase Organic matter in the soil originates from natural plant, animal and microbial biomass and from man-made chemicals such as pesticides, hydrocarbons, plastics and industrial by-products. The components of soil organic matter range from living biomass to simple organic molecules (such as organic acids, amino acids and carbohydrates) to complex polymers ( humic and fulvic acids) resulting from the decomposition of plant and animal materials in the soil (Stevenson, 1994). Organic matter also is the major storehouse of nitrogen in the soil and therefore plays a major role in the availability of this essential nutrient to the plant. Even though organic matter is usually present in relatively low concentrations (1 to 5% in most mineral soils), it has a strong influence on soil properties due to its high surface area and high concentration of reactive sites .

5. Soil organic matter, especially the humic and fulvic components , has a high concentration of carboxyl (COOH) and phenolic hydroxyl groups . These groups are important because of their pH-dependent negative-charge character, which influences cation -exchange reactions in the soil, and their ability to specifically bind certain metal cations . The ability of organic matter to bind metal cations decreases according to the following approximate order (Stevenson, 1994): Cu > Ni > Co > Zn > Fe > Mn > Ca > Mg . Organic matter also contains positive charge sites , attributable predominantly to NH 2 and aromatic NH groups. Soil humic acid also has a significant hydrophobic character , which is attributable to aliphatic hydrocarbon chains in the humic structure. These hydrophobic binding sites strongly influence the retention of pesticides, hydrocarbons and organic industrial by-products in the soil ( Sawhney and Brown, 1989).

6. Liquid phase The liquid and gas phases occupy the pore space not occupied by the solid phase. The relative proportion of liquid to gas is variable and is dependent on environmental factors such as rainfall / irrigation practices. Soil liquid phase is generally called as soil solution . The liquid phase contains dissolved organic (e.g., simple organic acids, simple carbohydrates, plant exudates, fulvic acid) and inorganic components (e.g., Ca, Mg, Na, K, Cl , SO 4 , NO 3 ). The concentrations of dissolved organic components, and inorganic components that are complexed by the organic components, may be considerably high at the soil-root interface than in the bulk soil, due to the exudation of organic compounds by the plant root. The composition of the soil solution is very different in the vicinity of the plant root (the rhizosphere) than in the bulk soil. Solid – Solution interaction / bonding is important especially in retention, mobility and bioavailability of nutrients in many soils.

7. In all cases, the solution phase is in dynamic equilibrium with solid and adsorbed phases . There is considerable interest in the composition of the soil solution phase, since the individual species within this phase are highly mobile and available for uptake by plants and microorganisms. Also, there is considerable interest in the development of models, which describe the equilibrium relations between solid and solution phases. Samples of the bulk soil solution are usually obtained by either vacuum or pressure extraction or displacement with an immiscible solvent. The solution may then be analyzed by procedures such as those detailed by Sparks et al. (1996): Atomic absorption and flame emission spectometry , inductively coupled plasma emission spectometry , neutron activation analysis, X-ray fluorescence spactroscopy , liquid chromatography, differential pulse voltametry , infrared and Raman spectroscopy, electron spin resonance spectroscopy, X-ray photoelectron spectroscopy and X-ray absorption fine structure spectroscopy; as well as by traditional wet chemical procedures (Weaver, 1994). The analysis of rhizosphere solution concentrations remains a very difficult problem.

8. Gaseous phase Since the respiration of plant roots and soil microorganisms results in the consumption of O 2 and the production of CO 2 , Oxygen concentration in soil air is lower and CO 2 concentration is higher than that of the above ground atmosphere. The depletion of oxygen within the soil will result in a lowering of soil redox potentia l , which controls the form and concentrations of multi- valent chemical species, such as Fe 3+ / Fe 2+ and Mn 4+ / Mn 2+ ( Sposito , 1981; Sposito , 1989). For example, low redox potentials can result in the transformation of Mn 4+ to Mn 2+ and an increased solubility of Mn to a concentration that may be toxic to plants . The rate of replacement of depleted oxygen to the soil is strongly influenced by soil water relations, since diffusion of CO 2 out of the soil and O 2 into the soil are considerably more rapid in the gas phase than in the liquid phase. Low redox potentials are usually associated with flooded and waterlogged soils.

9. Composition of Earth

10. Structure of the Earth S. No Parts of Earth Thickness Composition Density 1 Crust 5-56 KM Rocks 2.6 – 3.0 2 Mantle 2900 KM Mixed metals, silicates and ultra basic rocks 3.0 – 4.5 3 Core 3500 KM Metals - Nickle and iron 9.0 – 12.0 Density of earth as a whole = 5.5 Mg/m 3 ; Density of rock = 2.6 to 2.7 Mg/m 3 Major Divisions of Earth Lithosphere : Solid zone, surface and interior of the earth, consists of rocks and minerals. It accounts 93.06 % of earth mass. Hydrosphere : Incomplete covering of water (sea and oceans) Atmosphere : Gaseous envelope over earth surface Hydrosphere : Sphere of water covering the earth (sea and oceans) which contains absorbed air and rock fragments as sediments. Water covers almost 3/4 th of the earth surface . Eg . Oceans, basin, rivers, ponds, lakes and ground water. EC of sea water is 60,000 dS /m. Atmosphere : Gaseous envelope over earth surface (lithosphere and hydrosphere). It contains water particles and dust , which act as nuclei for the condensation of water vapour to form clouds or fog . Nitrogen – 78.084 %; Oxygen – 20.946 %; Argon – 0.934 % and CO 2 – 0.033 %. In addition the inner gases neon, helium, krypton and xenon are present.

11. Dynamics of Earth Crust

12. Composition of the Earth crust

13. Mineralogical composition of upper crust by volume

14. Soil minerals

15. Phyllosilicates The 2:1 phyllosilicates ( Gieseking , 1975; Greenland and Hayes, 1978; Dixon and Schulze, 2002), smectite and vermiculite , are characterized by their permanent negative charge and their property of shrinking and swelling . The chemistry of these minerals in the soil (e.g., cation retention and swelling properties) is strongly influenced by the following structural variables (Bohn et al ., 2001): (1) layer charge density, (2) site of charge deficit (octahedral versus tetrahedral layer), (3) di -octahedral versus trioctahedral mineral, and (4) the iron content of the octahedral layer. The latter property is important since a change in redox potential of the environment may influence the oxidation state of the iron and hence the layer charge density of the clay ( Stucki , 1988). The negative charge of the phyllosilicate (which is usually expressed as cation exchange capacity) is balanced by exchangeable cations , which exist predominantly between the internal swelling layers but also on the external surfaces and edges of the clay particles.

16. Phyllosilicates