Soil colloids

CHEMICAL PROPERTIES OF SOIL: Soil Colloids cat ion Exchange organic matter / Organic carbon Carbon –Nitroge ratio Soil fertility Soil reaction Soil acidity and alkalinity Soil conductivity Soil redox potential Saline Soils Alkali Soils Acid sulphate soils Iron Pyrites Soil reclamation etc

SOIL COLLOIDS Colloids - finely divided particles which cannot be seen with the naked eye Most colloids are smaller than a few micrometers (1-100 mm.) in diameter They do not dissolve in water - remain dispersed Because of their larger to mass ratio, soil colloids settles slowly from suspension Colloidal state - two phase system in which these materials are in a very finely divided and dispersed in a solution

Example: milk and cheese, starch blood proteins and of course the soil Colloidal particles are the most reactive fraction of soil Colloids - electrically charged particles exhibit Brownian movement and interfere with the passage and light by scattering it (Tyndall effect) Due to their very large surface area they can be strongly absorb ions and other dissolved substances and do not pass through ordinary semi permeable membrane The predominant soil colloids are soil clays and humus. Colloids are primarily responsible for the chemical reaction in soils

Clay Colloids - Inorganic colloids The mineral clay colloids are plate like in structure and crystalline in structure Clays - major mineral cat ion exchange particles, they have negatively charged sites in their lattices and attract and hold positively changed ions (cat ions) at the surface If the adsorbed ions on the colloids are mostly Al (OH) 2 + and some H + ions, then the soils are acidic The pH is a measure of acidity or alkalinity Soil pH ranges from about pH 3.5 (extremely acidic) to pH 7.0 (neutral) to about pH 11.0 (extremely basic)

Typical Soil Colloid The minute silicate clay colloid particle is also called as micelles (microcell) A colloidal particle consists of inner ion layer being essentially a huge anion highly ‘negative in change’ Outer ionic layer is made up of swarm of loosely held cat ions

Types of Colloidal clay Layer of silicate clay Iron and aluminium oxide Allophane and amorphous clay

Examples of fine clay fractions Gypsum -CaSO 4 Calcite - CaCO 3 Gibbsite -Al (OH) 3 Hematite-Fe(OH) 3 Quartz - SiO 2 Kaolinite clay-1:1(T: O) Silicate ClaySmectite clay-2:1(T: O: T-Interlayer- T: O: T) Chlorite clay- Silicate clays are crystalline, composed of layers. A layer is made of sheets

Tetrahedral sheet (TS) Basic unit of T.S is of one which magnesium or Aluminum surrounded by six oxygen or hydroxyl group by sharing electric charges O + and OH -

Iron and Aluminum Oxides Highly weathered clay soil, found in tropics and subtropics Contain Iron and Aluminum oxides such as Gibbsite Al (OH) 3 and Geolite (FeoOH) Amorphous Clay The most important amorphous colloid is allophane, (Al 2 O 3 2 Sio 3. H 2 0) Allophane is common in soils developed from volcanic ash

Humus Colloid Humus - partially decomposed product of plant and animal remain and continue to decompose slowly Often referred to as an organic colloid and consists of various chains and loops of linked carbon atoms Important in causing soil aggregation and increasing the ability of soil to hold water The origin of these negative charges is due to dissociated hydroxyl, carboxylic and phenol groups of anions on the surface of the humus

CATION EXCHANGE Cat ions - negatively charged ions and molecules The negatively charged sites attract positively charged ions in the soil water The positively charges ions - cat ions Soil acts as a cat ion exchange Cat ion exchange consists of an interchange between cat ions adsorbed on changed surface and cat ions in the soil solution

To make a room for more Ca 2 + on colloids, proportions of all the other adsorbed cat ions must be released into solution At the new equilibrium concentration of all ions other than Ca 2 ++ decrease on colloids and concentration of all cat ions increase in solution - Cat ion Exchange Cat ion Exchange Capacity (CEC) It is the amount of exchangeable cations per unit weight of dry soil It is measured in centimoles (+) of cat ions per Kg of soil (e mol (+) / Kg) High cat ion exchange is in the silicate clay soils

Anion Exchange Capacity (AEC) The capacity of a soil to adsorb or release anions under normal soil conditions For instance colloids that have appreciable anion exchange have low cat ion exchange Multicharged positive ions such as iron and aluminum have hydroxyl ion (OH -) that can be exchanged with sulphate (SO 4 -- ), phosphate (H 2 PO 4 -- ) or molybdate (MoO 4 -- ) and other anions High anion exchange is in the amorphous silicate soils, iron and aluminum soils etc

Exchange acidity The concentration of acidic cat ions is given in mille equivalents per 100 gm then the soil is called the exchange acidity Eg: Exchangeable aluminum and iron ions in the soils Colloid – Al +++  Al H 2 0  Al (OH) + 3H + Colloid – Fe + ++  Fe H 2 0  Fe (OH) 3 +3H +

The hydrogen (H +) and pH depends on the equilibrium concentration of Al +++ Al +++ in solution which is related to the proportions of adsorbed Al +++ or Al +++ to cat ion exchange of soil All other cat ions are considered to be basic and the proportion or percentage of the Cat ion Exchange Capacity (CEC) occupied by acidic cat ions is known as base saturation Bas unsatuation = Exchange acidity CEC