Ion exchange in soil
Ion exchange in soil system refers to exchange of equilavent amounts of ions between two phases in equilibrium in contact in reversible process.
When cations are involved , the process is termed as cation exchange, while for anions, it is referred to as anion exchange. Cation exchange reaction is considered as the second most important reaction next to photosynthesis.
The exchanges may take place between soil solid phase and the soil solution phase, or less commonly between the soil solid phases in contact or soil solid phase and growing plant in contact ( contact exchange).
Cation Exchange:
The Cation Exchange phenomenon was first identified by Harry Stephen Thompson in England during 1850. When soil was leached (washed) with ammonium sulphate, and upon filtration,calcium, and to lesser extent magnesium, potassium ions were detected in the leachate. The total amount of calcium and other cations so released is equivalent to that of ammonium retained.
(NH4)2SO4 + Soil Ca = Soil (NH4)2 + CaSO4
The various cations adsorbed by negatively charged colloids are subject to replacement by other cations through a process called cation exchange. The cation exchange reactions takes place reversibly, and the interchange is chemically equilavent.
- Cation Exchange Capacity (CEC)
- The CEC is the capacity of soil is defined as the capacity of soil to adsorb and exchange cations.
- The cation exchange capacity is the sum total of the exchangeable cations that a soil can adsorb. The higher the CEC of soil the more cations it can retain. Soils differ in their capacities to hold exchangeable cations.
Unit of expression
CEC is expressed as milliequivalents of cations per 100 grams of soil (meq /100g soil). After 1982, in the metric system the term equivalent is not used but moles are the accepted chemical unit. The recent unit of expression of CEC is centi moles of protons per kilo gram soil [cmol (p+) kg-1 soil]. One meq/100 g is equal to one cmol (p+) kg-1 soil.
- Soil texture: The negatively charged clay colloids attracts positively charged cations and holds them. Therefore, the cation exchange capacity of soils increases with increase in per centage of clay content .
- Clay soils with high CEC can retain large amounts of cations and reduce the loss of cations by leaching. Sandy soils, with low CEC, retain smaller quantities of cations and therefore cations are removed from soil by leaching.
- Soil organic matter: High organic matter content increases CEC. The CEC of clay minerals range from 10 to 150 [cmol (p+) kg-1] and that of organic matter ranges from 200 to 400 [cmol (p+) kg-1].
- Nature of clay minerals: The CEC and specific area of the clay minerals are in the order : smectite>fine mica>kaolinite. Hence the CEC of a soil dominated by smectite type of clay minerals is much higher than kaolinite type dominated soils
- Soil Reaction: As the pH is raised, the hydrogen held by the organic colloids and silicate clays (Kaolinite) becomes ionized and replaceable. The net result is an increase in the negative charge on the colloids and in turn an increase in CEC.
Importance of Cation Exchange
Cation exchange is an important reaction in soil fertility, in causing and correcting soil acidity and basicity, in changes altering soil physical properties, and as a mechanism in purifying or altering percolating waters.
The plant nutrients like calcium, magnesium, and potassium are supplied to plants in large measure from exchangeable forms.
The exchangeable K is a major source of plant K. The exchangeable Mg is often a major source of plant Mg. The amount of lime required to raise the pH of an acidic soil is greater as the CEC is greater.
Cation exchange sites hold Ca+, Mg+, K+, Na+, and NH4+ ions and slow down their losses by leaching. Cation exchange sites hold fertilizer K+ and NH4+ and greatly reduce their mobility in soils.
Cation exchange sites adsorb many metals (Cd2+ , Zn2+, Ni2+, and Pb2+) that might be present in wastewater adsorption removes them from the percolating water, thereby cleansing the water that drains into groundwater.
Anion Exchange
Adsorption of negative ion (anions) e.g. Cl-, NO3-, SO42-, and H2PO4- on positively charged sites of clay and organic matter is known as anion adsorption. These anions are subject to replacement by other anions through a process known as anion excange.
Clay NO3- + solution Cl- = Clay Cl -+ Solution NO3-
Source of positive charge:
- 1. Isomorphous substitution: Low valence cations replaced by high valence cations.
- 2. Surface and exposed broken bonds of clay lattice: OH group in certain acid soils.
- 3. Complex aluminium and iron hydroxy ions in acid soils.
- 4. pH dependent charges are important for anion exchange of organic matter
The basic principles of cation exchange apply also to anion exchange, except that the charges on the colloids are positive and the exchange is among negatively charged anions.
Anion exchange is an important mechanism for interactions in the soil and between the soil and plant. Together with cation exchange it largely determines the ability of soil to provide nutrients to plants.
Anion exchange capacity
“The sum total of exchangeable anions held exchangebly by a unit mass of soil , termed as its anion exchange capacity( AEC.)”. It is expressed as cmol / kg or m.eq./ 100 g soil. The AEC is much less than CEC of the soil.
Kaolinitic minerals have a greater anion adsorbing and exchange capacity than montmorillonitic and illitic clays because the exchange is located at only a few broken bonds. The capacity for holding anions increases with the increase in acidity.
Some anions such as H2PO4 are adsorbed very readily at all pH values in the acid as well as alkaline range. Cl and SO4 ions are adsorbed slightly at low pH but none at neutrality, while NO3 ions are not adsorbed at all. The affinity for adsorption of some of the anions commonly present in soil is of the order: NO3 < Cl < SO4 < PO4.
Hence at the pH commonly prevailing in cultivated soils, nitrate, chloride and sulphate ions are easily lost by leaching.
Importance of anion exchange
The phenomenon of anion exchange assumes importance in relation to phosphate ions and their fixation.
The adsorption of phosphate ions by clay particles from soil solution reduces its availability to plants. This is known as phosphate fixation. As the reaction is reversible, the phosphate ions again become available when they are replaced by OH ions released by substances like lime applied to soil to correct soil acidity.
Percent Base Saturation.
The extent to which the adsorption complex of a soil is saturated with exchangeable basic cations is termed as base saturation. It is expressed as a percentage of the total cation exchange capacity.
% base saturation = Excaneable bases(cmol/kg) / CEC(cmol/kg) x 100
Percent base saturation tells what percent of the exchange sites are occupied by the basic cations. If the percetage base saturation is 50, half of the exchane capacity is satisfied by bases, the other by hydrogen and aluminium.
Soil PH
The term pH is from the French “pouvoir hydrogen” or hydrogen power. Soil pH or soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH units. The pH scale goes from 0 to 14 with pH 7 as the neutral point. As the amount of hydrogen ions in the soil increases, the soil pH decreases, thus becoming more acidic. From pH 7 to 0, the soil is increasingly more acidic, and from pH 7 to 14, the soil is increasingly more alkaline or basic.
Sorenson (1909) defined the pH and gives the pH scale. Using a strict chemical definition, pH is the negative log of hydrogen ion (H+) activity in an aqueous solution in moles/ L. The point to remember from the chemical definition is that pH values are reported on a negative log scale. So, a 1 unit change in the pH value signifies a 10-fold change in the actual activity of H+, and the activity increases as the pH value decreases.
To put this into perspective, a soil pH of 6 has 10 times more hydrogen ions than a soil with a pH of 7, and a soil with a pH of 5 has 100 times more hydrogen ions than a soil with a pH of 7.Activity increases as the pH value decreases.
- pH= -log10 (H+) | Where: (H+) is the activity of hydrogen ions in moles/lt.
Pure water is weakly dissociated in to H+ and OH’ ions according to following equations
According to law of dissociation
Where;
H+ etc are the concentration and K is the dissociation constant. Since concentration of undissociated water remains practically the same because of very little ionization of H2O molecules the above relationship becomes:
[H+] x [OH’] = Kw =10-14 at 200C Kw=Ion product constant of water
At neutrality H+ = OH’ and H+ = 10-7or pH=7. Pure water has a pH value of 7. As the hydrogen activity increases the pH value will decrease while it will go up with rise in hydroxyl ion activity.
Importance of Soil pH:
The pH of of soilis an important physico-chemical characteristics because it influences:
- Sutability of soil for crop production
- Availability of soil nutrients to plants
- Microbial activity in the soil
- Lime and gypsum requirement of soil
- Physical properties of soil like structure, permeability etc.
Factors affecting soil pH:
- Percent base saturation:
- A low percent base saturation means acidity, whereas a percent base saturation of 50-90 will result in to neutrality or alkanity.
- Nature of soil colloids:
- The colloidal particles of the soil influence soil reaction to a very greatest extent.
- Different types of colloids vary in their pH at the same percent base saturation . This is due to the difference in the ability of different colloids to release H+ ions to the soil solution. For example at the same percent base saturation the smectite has much lower pH than kaolinite.
- When hydrogen (H+) ion forms the predominant adsorbed cations on clay colloids, the soil reaction becomes acid.
- Soil solution:
- The more dilute the solution, the higher the pH value. Hence the pH tends to drop as the soil gets progressively dry. Soil reaction is also influenced by the presence of CO2 in soil air.
- As the CO2 concentration increases, the soil pH falls and increases the availability of the nutrients. Under field conditions, plant roots and micro-organism liberate enough CO2, which results in lowering the pH appreciably.
- Adsorbed basic cations:
- The comparative quantity of exchangeable Ca, Mg, Na and K adsorbed on the colloids will determine the pH. The dominance of Na+ will raise pH much higher than other basic cations.
- Climate:
- Rainfall plays important role in determining the reaction of soil. In general, soils formed in regions of high rainfall are acidic (low pH value), while those formed in regions of low rainfall are alkaline (high pH value).
- Native Vegetation:
- Soils become more acidic when develops under conifer ecosystem
- Soils often become more acid when crops are harvested because of removal of bases.
- Type of crop determines the relative amounts of removal. For example, legumes generally contain higher levels of bases than do grasses.
- Nitrogen fertilization:
- Nitrogen from fertilizer, organic matter, manure and legume N fixation produces acidity.
- Nitrogen fertilization speeds up the rate at which acidity develops. At lower N rates, acidification rate is slow, but is accelerated as N fertilizer rates increase.
- Flooding:
- The overall effect of submergence is an increase of pH in acid soils and a decrease in basic soils.
- Regardless of their original pH values, most soils reach pH of 6.5 to 7.2 within one month after flooding and remain at the level until dried.
Nutrient Availability:
The availibility of plant nutrients are more at a pH range of 6-7 except Mo
- Nitrogen
- One of the key soil nutrients is nitrogen (N). Plants can take up N in the ammonium (NH4+) or nitrate (N03-) form.
- At pH near neutral (pH 7), the microbial conversion of NH4+ to nitrate (nitrification) is rapid, and crops generally take up nitrate. In acid soils (pH < 6), nitrification is slow, and plants with the ability to take up NH4+ may have an advantage
- Phosphorus
- The form and availability of soil phosphorus is highly pH dependent.
- When the soil is neutral to slightly alkaline , the HPO4-- ion is the most common form. As the pH is lowered both the HPO4-- and H2PO4 - ion prevail. At higher acidities H2PO4 - ions tends to dominate. The most plants absorb phosphorus in HPO4-- .
- Between pH 6-7, phosphorus fixation is at minimum and availability to higher plants is maximum.
- Potassium:
- The fixation of potassium (K) and entrapment at specific sites between clay layers tends to be lower under acid conditions. This situation is thought to be due to the presence of soluble aluminum that occupies the binding sites.
- Calcium, Magnesium and Sulphur:
- The availibilty of Ca and Mg is more above pH 7.0.
- Sulphate (S042-) sulphur, the plant available form of S, is little affected by soil pH.
- Micronutrients
- The availability of the micronutrients manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), and boron (B) tend to decrease as soil pH increases.
The exact mechanisms responsible for reducing availability differ for each nutrient, but can include formation of low solubility compounds, greater retention by soil colloids (clays and organic matter) and conversion of soluble forms to ions that plants cannot absorb.
Molybdenum (Mo) behaves counter to the trend described above. Plant availability is lower under acid conditions.
Soil pH and soil organisms:
- Growth of many bacteria and actinomycetes is inhibited as soil pH drops below 6
- Fungi grow well across a wide range of soil pH
- Therefore fungi are dominant under acid conditions
- Less competition from bacteria and actinomycetes
- Earthworms do best when soil pH >6.5
- Nitrification greatly inhibited at pH <5.5
- N fixation greatly restricted a pH <6
- Decomposition of plant residues and OM may be slow in acid conditions (pH <5.5)
Soil Buffering Capacity
The ability to resist a change in pH refers to buffering capacity of the soil. The buffering capacity increases as the cation exchange capacity increases. Thus, heavier the texture and the greater the organic matter content of a soil, the greater is the amount of acid or alkaline material required to change its pH.
The colloidal complex acts as a powerful buffer in the soil and does not allow rapid and sudden changes in soil reaction. Buffering depends upon the amount of colloidal material present in soil. Clay soils rich in organic matter are more highly buffered than sandy soils.
Importance of buffering in agriculture
The stabilization of soil pH through buffering act as a effective guard against deficiency of certain plant nutrients and excess availability of others in toxic amounts which would seriously upset the nutritional balance in the soil.
Soil Organic Matter
Organic materials are intrinsic and essential component of all soils. Whereas the body of the soil is constituted by the inorganic materials, one may look upon the organic matter as its life.
Organic matter makes the soil a living, dynamic system that supports all life on this planet.
Soil Organic Matter (SOM) comprises an accumulation of
- Partially disintegrated and decomposed plant and animal residues
- Other organic compounds synthesized by the soil microbes upon decay.
- SOM is frequently said to consist of humic substances and nonhumic substances.
- The term SOM is generally used to represent the organic constituents in the soil, including undecayed plant and animal tissues, their partial decomposition products, and the soil biomass.
Importance of organic matter
It is the food source for soil microorganisms and soil fauna. If there is no organic matter the soil would be almost sterile and consequently, extremly infertile . Organic matter also supplies hormones (Auxin ,Gibberellins , IAA) and antibiotics for plant growth.
Organic matter is an index of the productivity of the soil since it is a store house of essential plant nutrients for plant growth. It functions as a reservoir of nitrogen, phosphorus and sulphur and thereby contribute significantly to the supply of these nutrients to higher plants.
Humus (a highly decomposed organic matter) provides a storehouse for the exchangeable and available cations.
Soil organic matter contributes to nutrient release from soil minerals by weathering reactions and thus helps in nutrient availability in soils. Organic acids released from decomposing organic matter help to reduce alkalinity in soils; organic acids along with released CO2 dissolve minerals and make them more available.
It acts as a buffering agent which checks rapid chemical changes in pH and soil reaction. Organic matter creates a granular condition of soil which maintains favorable condition of aeration and permeability. Water holding capacity of soil is increased and surface runoff, erosion etc., are reduced as there is good infiltration due to the addition of organic matter. Surface mulching with coarse organic matter lowers wind erosion and lowers soil temperatures in the summer and keeps the soil warmer in winter. The organic substances influence various soil processes leading to soil formation It is the prime decider of soil health and soil quality.
Sources of Soil Organic Matter
- Primary sources
- Plant, animal and microbial materials are the primary source of organic matter. Plant tissues and microbial cells contains approximately 40 to 50 per cent carbon on dry weight basis.
- Secondary sources
- On –farm sources: Crop residues,roots,root exduates, organic manures,composts and green manure crops contribute significantly towards build up of soil organic matter.
- Off- farm sources: Biodegradable wastes like agro-industrial wastes and muncipal wastes
Decomposition of Soil Organic Matter
The organic materials (plant and animal residues) incorporated in the soil are attacked by a variety of microbes, worms and insects in the soil if the soil is moist.
Some of the constituents are decomposed very rapidly, some less readily, and others very slowly .The constituents in terms of ease of decomposition are:
- Sugars, starches and simple proteins = easy to decompose
- Crude proteins
- Hemicelluloses
- Cellulose
- Fats, waxes, resins
- Lignins = Very difficult to decompose
The organic matter is also classified on the basis of their rate of decomposition
- Rapidly decomposed : Sugars, starches, proteins etc.
- Less rapidly decomposed : Hemicelluloses, celluloses etc.
- Very slowly decomposed : Fats, waxes, resins, lignins etc.
The organic/ humic substances are produced when plant residue and other organic debries are broken down or chemically altered. Fungi dominate over others in the initial stages while bacteria are the important agents of decomposition during the later stages.
At first, the easily decomposable substances like sugars, starches and water soluble proteins are acted upon by the microorganisms and decomposition and digestion is rapid. Crude protein is next in order, followed by hemicelluloses.
Cellulose, which is more resistant to microbial attack than hemicellulose decomposes much more rapidly than oils, fats, waxes, resins etc. Lignin decomposes very slowly and continues to dominate soil organic matter when the decay process slows down.
The sugars, starches, hemicelluloses and celluloses are ultimately decomposed to carbon dioxide and water and energy is liberated which is utilized by microorganisms. Some oils, fats, waxes and resins are also slowly decomposed to carbon dioxide and water and some energy liberated for use by microorganisms.
A small portion of lignin may be slowly decomposed to form aromatic compounds. Other portions may be chemically altered. Some other portions may chemically unite with protein to form part of the soil humus.
Proteins are gradually decomposed to amino acids and amides which are further decomposed to ammonium compounds by microorganisms. Ammonium compounds are oxidised to nitrites by Nitrosomonas bacteria. Nitrites are further oxidized to Nitrates by Nitrobacter bacteria.
Phosphorus is present in the organic matter as phytin, nucleic acid and phospholipids, which are decomposed to liberate the phosphorus present in them as orthophosphate ions, H2PO4-.
Similarly, sulphur containing amino acids like methionine, cysteine etc. Are decomposed by microorganisms to liberate the sulphur contained them as sulphate. When organic matter decomposes, other complex organic forms of nutrients are converted to simple ionic forms like K+, Ca++, Mg++ , etc.
This process of conversion of complex organic forms of nutrients to simple inorganic forms by microorganisms is called the mineralisation of nutrients.
A portion of the nutrients thus mineralized is assimilated by the microorganisms themselves for synthesis of their cell protoplasm. Thus the simple inorganic form of the nutrient is recovered to the complex organic form of nutrients. This process of conversion of the simple inorganic form of nutrients to the complex organic form of nutrients is called immobilization of nutrients.
During the earlier stages of decomposition of organic matter all the simple inorganic forms of nutrients are assimilated by microorganisms which multiply rapidly and continue to decompose the organic matter.
When almost all the carbon compounds have been decomposed, the microorganisms die due to the lack of sufficient a mounts of energy giving carbon compounds. Their bodies decay when the complex organic forms of the nutrients are reconverted to simple inorganic forms.
Some proteins combine with organic compounds like lignins, tannins, humic acids etc. Some proteins are absorbed by the clay, especially the expanding ones. All these reactions protect the proteins from microbial decomposition. At this stage, almost all the original organic material has been converted to dark heterogeneous mass called humus.
Humus is a resistant complex mixture of dark brown to black coloured colloidal and amorphous substances synthesized or modified from the original organic materials by various microorganisms.
Simple decomposition products under aerobic decomposition are CO2,NH4, NO3, H2PO4, SO4 and H2O. Simple decomposition products under anaerobic decomposition are CH4, H2S, dimethyl sulphide, ethylene, ammonium ions, amine residues and organic acids
C: N ratio
A close relationship exists between organic matter and nitogen content of the soil. The ratio of organic carbon to nitrogen in soils is known as Carbon: Nitrogen ratio or C/N ratio of the soils.
The C / N ratio of residue affects the rate of decomposition of organic matter. The organisms that decompose residues need N (and other essential elements) as well as C, if there is little N in the residue, decomposition is slow
Also, if there is little N in the residue, microorganisms will utilize inorganic N in the soil to satisfy their N requirement, thereby competing with plants for N and reducing the amount of soil N available for plant growth.
The C / N ratio in soil is relatively constant and = 12. In plant residues, it is highly variable and increases with maturity. The C / N is lower in microorganisms and = 8. Since microbes incorporate only about 1/3 of the C metabolized into biomass, the substrate material must have C / N = 24 to satisfy the N requirement of microbes.
Generally, when organic substances with C/N ratios greater than 30:1 are added to soil , there is immobilization . For ratios between 20 and 30 , there may be neither immobilization nor release of mineral N. If the C / N ratio of residue is < 20:1 , there is usually release of mineral n.soil N is consumed by microbes and plant- available N decreases
