Thursday, March 28, 2024
General Agriculture

Colloidal Materials of Soils and Cation Exchange Reactions

Soils colloids are soil particles (organic and inorganic) that carry charges on their surfaces. They are also referred to as surface-active materials in the soil. The soil colloids are therefore able to attract either cations or anions to themselves depending on whether they carry negative or positive charges.

Ion exchange between the solid and liquid phase of the soil is one of the important properties of soils. Cation exchange capacity of the soil measures the quantity of cations that a given amount of soil can hold or exchange which in turn depends on the type of colloidal materials that constitute the soil.

Colloidal Material of Soils

The most important and active portions of the soil are those in colloidal stems and they are the seats of ion exchange comprising of the organic and mineral fractions of the soil.

The organic fraction consists of the plant and animal residues in all stages of decomposition with humus as its stable phase. Both the primary and secondary minerals of the soil form the inorganic (mineral) fraction of soil colloids.

Cation Exchange Reactions

The colloidal constituents of any soil are all the clay fractions, part of the silt and the humus. In all cases, they usually have effective diameter less than 2 micro meters (microns). By virtue of their large negative charge, the colloidal constituents are capable of attracting (adsorbing) positively charged ions (cations) such as K+, Ca2+, Na+, Mg2++ or NH+.

Cations are clustered around the surfaces of the clay and humus particles and are usually readily displaced by other cations added to soil or released into soil solution when they may be taken up by plant root.

These displacement reactions are called cation exchange reactions which could also be defined as the interchange between a cation in the soil solution and another cation on the surface of any surface-active material (colloidal constituent).

The most numerous cations on exchange sites (surface – active materials) in soils are calcium Ca++, Magnesium Mg++, hydrogen ion H+, sodium Na+, potassium K+ and aluminium A13+.

An example of cation exchange reaction is the mechanism involves in liming of acid soils. In acid soils, some hydrogen ion (H+) are present in solution but most of them are adsorbed on the surfaces of soils colloids.

When calcium hydroxide (lime) is added to soil, much of the hydrogen (H+) and other cations (e.g. H+, Mg2+, Na+) are displaced by the calcium Ca2+) in lime. The hydrogen displaced is then neutralized with hydroxyl ions (OH) in lime to form water.

In this process, the concentration of hydrogen ions in soil is reduced and the soil becomes less acid (i.e. pH value rises) as lime is added.

Lime also neutralizes these H+ ions in soil solution, but the quantity of ions is solution is very small compared to the quantity absorbed by the soil particles (Buffer effect).

Read Also: Factors Affecting Mineralization and Nitrification Processes

Cation Exchange Capacity

In the soil cations are attracted to neutralize negative charges that develop on the surfaces of organic and mineral colloids. Cation exchange capacity (CEC) of any soil is the sum total of all the exchangeable cations that the soil can absorb or that could be attracted to the surfaces of the colloids in the soil, CEC is the amount of exchangeable cations per unit weight of dry soil.

The quantity of the cations is expressed in milliequivalent per 100g of oven dry soil. Milliequivalent is the amount of an atom or an ion that will react with or combine with one milligram of hydrogen (1.0mg H+), or eight milligram of oxygen (8.0mg ½ 02).

The “equivalent” unit has been changed to the new metric system (Miller and Donahue, 1990) being represented by moles (+) or mole, which indicates a monovalent ion portion. For example, 6meq/100g is now written as either:

6 cmo1 (+) Kg-1 of soil        (= centimoles) 60 mmo1 (+) Kg-1 “ (= millimoles)

60 mmo1 ( ½ Ca2+) Kg-1 of soil (if Ca is used). 60 mmole Kg-1 of soil (where c + one charge).

Characteristic of Cation Exchange Reactions

1. Bond Strength

The bond strength attracting cations to the surfaces of negatively charged soil colloids is strong enough so that the cations are not easily leached by water of infiltration, and at the same time, the bond is not too strong for the plant to absorbed the cations.

The cations are held on varying degrees of tenacity, depending on their charges and their hydrated and undydrated radii. Generally, ions with 2 or 3 valence are held more tightly than monovalent cation; whereas the greater the degree to which the ions is hydrated, the less tight it will be held on the colloidal surface. The approximate adsorption order from strongest to the weakest is H+ = A13+ > Ca2+ = Mg2+> K+ = NH+4 > Na+.

Cation Exchange Reactions are completely reversible

Implying that they can proceed in either directions. For example the reaction describing the displacement of calcium by potassium from exchange site.

The reaction can go the either direction depending on the relative concentration of Ca2+ or K+ ions in soil solution. Most ions adsorbed on colloidal surfaces are readily exchangeable with other cations depending on the bond strength and whether the quantities are large enough to cause displacement by mass actions.

Specificity

This means that not all exchange sites have equal capacity to attract all cations: some bonding sites are specific for certain cations. An example was the reaction of humic acid, montmorillonite, Kaolinite and muscovite with Ca2+ and NH+4 ions.

That is, humic acid has preference for Ca2+ ions as oppose to muscovite which prefers NH4+ ions. Other examples of specific adsorption are known, and they are very important in agriculture.

The most important are the fixations of K+ and NH4+ by vermiculite: and the fixation of PO43- by positively charged sites on koalinite, or Fe and A1 hydroxides.

The fixation of K+ and NH4+ occurs because of their hydration energies, and their ionic radii which is small enough to fit into the “hexagona holes” in adjacent silica tetrahedral sheets. Fixation of PO43- is believed to involve the replacement on hydroxide surfaces of – OH2+ by phosphate anions (Evans et al 1982).

Exchange reactions are always stoichio metric meaning that they take place on equivalent weight basis. That is, one milliequivalent weight of a cation will displace or replace one milliequivalent weight of another cation.

Calculating Exchangeable Cations in Soils

Problem: In laboratory analysis, the following values were obtained for exchangeable K+, calculate whether or not potassium fertilizer will be needed for a maize crop in each soil. If the critical value of K in maize is 400kg K+/ha – 30cm.

Read Also: Ranching and Domestication as a form of Wildlife Exploitation

Laboratory Determination of Cation Exchange Capacity

Several methods of determining C.E.C of a given soil exist but the most widely used method in routine analysis is described here.

Ammonium Saturation Method: The soil is treated with N NH4 OAC at pH7.0 in order to saturate the colloidal complex with NH + ion. The excess salt is removed with ethanol, containing 5% water.

Then the NH4+ ion is displaced with K+ by treating it with 10% KCL at pH2.5. Finally, NH4 is measured in the displacing solution either by distillation and titration or other method. The amount of displacement is measured and expressed in term of meq per 100g oven-dry soil.

However, the new unit (metric system) expressed weights of displaced cations in cmo1 (+) per kg of soil or mmo1 (+) per kg of soil.

Exchangeable Ca, Mg, K, Na, H+ and A1 can be determined in the NH4 OAC extract and the sum of exchange cations and exchange acidity will give a good estimate of C.E.C. Other C.E.C methods are sodium saturation method (Na – Acetate at pH 8.2) and Barium saturation method (0.5N Barium Chloride pH 8.0).

Exchangeable Cations: The extracts prepared during the saturation process in CEC analysis, contain the exchangeable cations of the soil – Ca, Mg, K and Na. Sodium is in large amounts from saline soils while H and A1 are high in acid soils. When Li and NH4 saturation is used, the extracts are suitable for determination of Ca, Mg, K and Na. Manganese (Mn) can be determined also in neutral NH4 OAC extracts.

Exchange reactions are rapid: they take place almost instantaneously and attain equilibrium rapidly.

Cation exchange capacity is affected by surface area of soil colloids and are both related to colloidal structures: Organic colloids have extremely complicated structures formed by the microbial decomposition of organic compounds. They have large surface areas and large cation exchange capacity that is pH dependent.

Expandable clay minerals such as smectite or     montmorillonite, have large surface areas; whereas non- expanding clay minerals, such as Kaolinite, have small. The ranges of cation exchange capacity and specific area of some soil colloids responsible for most soil Cation Exchange Capacity are outlined in the table below.

Table: CEC of some soil colloids

Soil ColloidCEC by N NH40AC, pH 7.0Specific surface Area
 cmo1 kg-1 of soilM2/g
Humus (organic-100 – 300
matter Kaolinite  3 – 15  3 – 30
Smectite60 – 100700 – 800
(montmorillonite) Vermiculite  80 – 150  300 – 500
Illite10 – 40100 – 200
Geotite15 – 100
Hematite15 – 200
Hydrous mica25 – 40

Read Also: Classification of Some Exotic/Foreign Fin Fish Species

Effect of pH on Cation Exchange Capacity

Organic matter, hydroxides of Fe and A1, Kaolinite and chlorite clay minerals have CEC that are pH – dependent.

This pH dependent cation emanates from the edge charges in Kaolinite and chlorite protonation of Fe and A1 hydroxide surfaces, and the ionizable H in organic colloids. Anion exchange capacity may occur in acid solution and cation exchange capacities increase as the pH increases.

Table:     Effect of pH on soil CEC

Cation Exchange Capacity (cmo1 kg-1 soil)

Soil typeSoil pH H20E.C.E.C.N. NH4Bac12
Alfisol rich in Kaolinite5.02.673.204.64
Ultisol rich in hematite and4.72.144.1325.64
gibbsite    
Oxisol rich in hemiatite and5.14.0015.6516.99
goethite    
Volcanic ash soil5.113.0820.8030.52
Acid peat soil4.899.00184.0
    0

Agric4Profits

Benadine Nonye is an agricultural consultant and a writer with over 12 years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education... Visit My Websites On: 1. Agric4Profits.com - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. WealthinWastes.com - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

Leave a Reply

Your email address will not be published. Required fields are marked *

error

Enjoy this post? Please spread the word :)

Discover more from Agric4Profits

Subscribe now to keep reading and get access to the full archive.

Continue reading

0
YOUR CART
  • No products in the cart.