• Surface of sodic soils is usually black due to deposition of dispersed humus and hence the name black alkali.
• A mineral is a naturally occurring inorganic substance with a definite chemical composition and distinct physical characters.
• A mineral that forms the original component of a rock is known as primary mineral.
• Primary mineral is generally anhydrous (without water molecule in its composition) and is originally formed by cooling and solidification of molten mass.
• Feldspar, hornblende, mica etc. are some of the primary minerals.
• Secondary mineral is a mineral that has been formed as a result of subsequent changes in rocks.
• Secondary mineral is hydrous and is formed due to weathering or metamorphosis of primary mineral.
• Examples of secondary minerals are kaolinite, montmorillonite, illite, limonite, gibbsite, dolomite, calcite, apatite, gypsum and pyrites.
• Hydrated aluminosilicate secondary minerals with particle size less than 0.002 mm in diameter are clay minerals.
• Clay minerals together with organic matter constitute the colloidal fraction of the soil which is the active seat of physical and chemical properties of soil.
• Colloids are particles of size less than 0.0002 mm in diameter.
• Individual particle of a colloid is called micelle.
• Most of the clay minerals that are less than 0.0002 mm in diameter and also humus exhibit colloidal properties.
• Humus is an organic colloid.
• Inorganic colloids may be either silicate or non-silicate clay minerals.
• Kaolinite, montmorillonite and illite are silicate clay minerals.
• Depending on the number of silica and alumina layers silicate clay minerals are classified as 1:1 and 2:1 clay minerals.
• Non silicate clay minerals are hydrous oxides of iron and aluminium and allophanes.
• Hydrous oxides of iron and aluminium are colloids present in red and yellow soils of tropical and sub-tropical regions.
• Hydrous oxides of iron and aluminium are oxides of iron and aluminum with different number of water molecules, depending on the clay mineral.
• Gaethite (Fe2O3.H2O) is an iron oxide with one water molecule.
• Gibbsite (Al2O3.3H2O) is a hydrous aluminium oxide with three water molecules in its structure.
• Hydrous oxides of iron and aluminium micelles also carry negative charges, but less than those of kaolinite.
• Most hydrous oxides are not sticky and lesser in plasticity and cohesiveness than silicate clay minerals.
• Soil dominated by hydrous oxides has much better physical condition.
• Amorphous mineral matter in soils is called allophanes which are a combination of silica and sesquioxides.
• Approximate composition of allophanes is Al2O3.2SiO2.H2O.
• Allophanes have high cation and anion exchange capacities.
• Humus is an organic soil colloid.
• Humus is a product of decomposition of plant and animal residues which is fairly stable, amorphous and brown to black in colour.
• On equal weight basis, organic colloids exhibit five to seven times higher adsorption of water and cations than inorganic colloids.
• Charges of humus colloids are pH dependent.
• Under strongly acidic conditions, hydrogen is tightly bound and not easily replaceable by other cations.
• Organic soil colloids exhibit low negative charge.
• With increase in pH, hydrogen from carboxyl group followed by hydrogen from phenolic group, ionize and are replaced by Ca, Mg and other cations in humus.
• Cation exchange capacity is a very important property of the soil from plant nutrition point of view.
• Most clay micelles have a crystalline structure while humus micelles are amorphous.
• Negative charges on clay micelles are due to isomorphic substitution, ionization of hydroxyl groups and exposed carboxyl and hydroxyl groups.
• Isomorphic substitution of silica or alumina atom by an atom of similar geometry, but of low charge results in higher negative charge.
• Negative charges on humus micelles are due to exposed carboxyl (COOH) and hydroxyl (OH) groups.
• Due to the presence of negative charges cations are adsorbed on surface of the micelles.
• Divalent cations are more strongly adsorbed than monovalent cations.
• Cations that are adsorbed on the surface of micelles are capable of exchanging with those in solution.
• Process of exchange of cations between solid and liquid phases is called cation exchange.
• Cations are held over the micelle in two layers and it is known as electrical double layer.
• Quantity of cations is expressed in milli equivalents.
• One gram of hydrogen is one equivalent weight because the atomic weight of hydrogen is considered as one.
• One milli equivalent of hydrogen is one-thousandth of a gram of hydrogen or one milligram equivalent weight, which is shortened to milli equivalent.
• Equivalent weight of an element is its atomic weight divided by its charge.
• Atomic weight of potassium (K+) is 39 and its charge is one.
• 39 grams of potassium (K+) will displace one gram of H+.
• Equivalent weight of potassium (K+) is 39.
• Calcium (Ca2+) has an atomic weight of 40 and charge of 2.
• Equivalent weight of Ca2+ is 40/2 or 20.
• CEC of soils influences the capacity of the soil to hold nutrients such as Ca, Mg, NH4+ etc. and the quantity of a nutrient required to change its relative level in soils.
• Percentage of bases in the total cations (except hydrogen) present on the exchange complex is called base saturation.
• Hydrogen is a cation, but it is acidic and not basic like Ca, Mg, K, Na.
• Per cent base saturation indicates the proportion of basic cations in CEC.
• Availability of nutrients increases with their saturation per cent.
• Anion exchange capacity is less in magnitude compared to CEC.
• When plant absorbs cations, roots release H+ ions in exchange resulting in decrease in soil pH.
• If anions are absorbed, OH- ions are released into the soil solution increasing pH.
• Small increase in H+ or OH- concentration does not reflect on soil pH due to buffering capacity of soil.
• CEC is the principal buffering mechanism in soil.
• Some of the exchangeable cations are basic (Ca2+, Mg2+ etc.) and others are acidic (H+, Al3+).
• Only active portion of the H+ ions are measured in the pH.
• Apatite mineral contains phosphorus.
• Mica and feldspars contain potassium.
• Minerals containing calcium are dolomite, calcite, calcium carbonate, gypsum etc.
• Dolomite is a source of magnesium.
• Manganite and pyrolusite minerals are source of manganese.
• Olivine is a source of molybdenum.
• Sphalerite is a source of zinc.
• On mineralization, organic matter releases ammonium and nitrate.
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