Impact of soil component on soil properties

2. Soil - A three phase
system

3. 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)

4. 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.

5. 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.

6. • 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 NH2 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).

7. 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, SO4, NO3).
• 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.

8. • 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.

9. Gaseous phase
• Since the respiration of plant roots and soil microorganisms results in the
consumption of O2 and the production of CO2, Oxygen concentration in soil air
is lower and CO2 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
potential, which controls the form and concentrations of multi-valent chemical
species, such as Fe3+ / Fe2+ and Mn4+/ Mn2+ (Sposito, 1981; Sposito, 1989).
• For example, low redox potentials can result in the transformation of Mn4+ to
Mn2+ 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 CO2 out of the soil and O2 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.

10. Composition of Earth

11. 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/m3; Density of rock = 2.6 to 2.7 Mg/m3
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 3/4th 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 CO2 – 0.033 %. In
addition the inner gases neon, helium, krypton and xenon are present.

12. Dynamics of Earth Crust

13. Composition of the Earth crust

16. Mineralogical composition of upper
crust by volume

17. Soil minerals

22. 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.

23. Phyllosilicates