CLAY HUMUS INTERACTION

Clay - Humus Interaction

Clay and Humus in Soil

Clay:
- Properties: Clay particles are the smallest soil particles, measuring less than 0.002 mm in diameter. They have a large surface area relative to their volume, which gives them a high capacity to retain water and nutrients.
- Types: Common clay minerals include kaolinite, montmorillonite, and illite.
- Charge: Clay particles carry a negative charge, which allows them to attract and hold onto positively charged ions (cations) such as potassium, calcium, and magnesium.
Humus:
- Properties: Humus is the organic component of soil, formed by the decomposition of plant and animal material. It is dark, rich, and contains essential nutrients for plant growth.
- Composition: Humus is rich in carbon and contains complex organic compounds like fulvic and humic acids.
- Role: Humus improves soil structure, enhances nutrient availability, increases water retention, and promotes microbial activity.

Interaction between Clay and Humus

Cation Exchange Capacity (CEC):
- Both clay and humus have high CEC, which means they can attract and hold onto nutrient ions, making these nutrients available to plants over time. This is crucial for maintaining soil fertility.
- The negative charge on clay particles and humus attracts positively charged ions (cations) like calcium, magnesium, potassium, and ammonium.
Soil Structure and Aggregation:
- Humus binds soil particles, including clay, into aggregates. This improves soil structure, making it more friable and easier to work with.
- Good soil structure enhances root penetration, water infiltration, and air movement within the soil.
Water Retention and Drainage:
- Clay particles alone can lead to poor drainage and compacted soil. However, when combined with humus, the soil becomes more porous, improving water retention and drainage.
- Humus increases the water-holding capacity of soil, which is particularly beneficial in sandy soils that drain quickly.
Nutrient Availability:
- Humus releases nutrients slowly as it decomposes, providing a steady supply of nutrients to plants.
- Clay and humus together enhance the availability of nutrients by preventing leaching and by retaining nutrients in the root zone.
pH Buffering:
- Both clay and humus have buffering capacities that help to stabilize soil pH, creating a more favorable environment for plant growth and microbial activity.

Clay humus and organo metallic complexes

Clay and humus (organic matter) are the two important components of soil colloids, which act as the active sites and the seat of soil fertility, where many soil reactions takes place.

Both clay and organic particles have net negative charges, such that a clay-humus particle acts as a large highly charged anion, known in chemistry as a micelle. Attracted to the negative charge are numerous cations in a diffuse cloud about the micelle. This cloud is known as the Gouy layer.

The interaction of organic complexes with clay mineral is known as the Clay - humus complex. This Clay - humus complex is an important factor in the ability of recycle the nutrients that are temporarily tied up. Whenever the organic molecules are brought in contact with the surface of clay mineral, a major interaction takes place between the functional group on the organic molecule and active sites on the surface of clay. Apart from this, due to the clay and humus, change in free energy, involving the heat of adsorption.

A number of organic compounds including humic and fulvic acids are capable of farming complexes with metal ions. Depending on the stability of the complexes, they can be soluble or insoluble in water.

M2++ 2HA <---------------> MA 2 + 2H+

M- metal ion, HA- Humic acid and MA2- metal humic acid complex.

Humification of leaf litter and formation of clay-humus complexes

Humus is a complex organic substance resulting from the breakdown of plant material in a process called humification. This process can occur naturally in soil, or in the production of compost. Humus is extremely important to the fertility of soils in both a physical and chemical sense (see below). Physically it helps the soil retain moisture and encourages the formation of good soil structure. Chemically, it has many active sites which bind to ions of plant nutrients, making them more available. Humus is often described as the 'life-force' of the soil. Yet it is difficult to define humus in precise terms; it is a highly complex substance, the full nature of which is still not fully understood. Physically humus can be differentiated from organic matter in that the latter is rough looking material, with coarse plant remains still visible, whilst once fully humified it become more uniform in appearance (a dark, spongy, jelly-like substance) and amorphous in structure. That is, it has no determinate shape, structure or character.

Plant remains (including those that have passed through an animal and are excreted as manure) contain organic compounds: sugars, starches, proteins, carbohydrates, lignins, waxes, resins and organic acids. The process of organic matter decay in the soil begins with the decomposition of sugars and starches from carbohydrates which break down easily as saprophytes initially invade the dead plant, whilst the remaining cellulose breaks down more slowly. Proteins decompose into amino acids at a rate depending on Carbon:Nitrogen ratios. Organic acids break down rapidly, while fats, waxes, resins and lignins remain relatively unchanged for longer periods of time. The humus that is the end product of this process is thus a mixture of compounds and complex life chemicals of plant, animal or microbial origin which has many functions and benefits in the soil as outlined below

The mineralisation process that converts raw organic matter to the relatively stable substance that is humus feeds the soil population of micro-organisms and other creatures thus maintaining high and healthy levels of soil life.
Effective and stable humus (see below) are further sources of nutrients to microbes, the former providing a readily available supply whilst the latter acts as a more long term storage reservoir.
Humification of dead plant material causes complex organic compounds to break down into simpler forms which are then made available to growing plants for uptake through their root systems.
Humus is a colloidal substance, and increases the soil's Cation Exchange Capacity, hence its ability to store nutrients on clay particles, thus whilst these nutrient cations are accessible to plants they are held in the soil safe from leaching away by rain or irrigation.
Humus can hold the equivalent of 80-90% of its weight in moisture, thus increases the soil's capacity to withstand drought conditions.
The biochemical structure of humus enables it to moderate- or buffer- excessive acid or alkaline soil conditions.
During the Humification process microbes secrete sticky gums- these contribute to the crumb structure of the soil by holding particles together, allowing greater aeration of the soil. Toxic substances such as heavy metals, as well as excess nutrients, can be chelated (that is, bound to the complex organic molecules of humus) and prevented from entering the wider ecosystem.
The dark colour of humus (usually black or dark brown) helps to warm up cold soils in the spring

HUMIFICATION OF LEAF LITTER AND FORMATION OF LEAF LITTER COMPLEXES

Humus which is readily capable of further decomposition is referred to as effective or active humus. It is principally derived from sugars, starches and proteins and consists of simple organic (fulvic) acids. It is an excellent source of plant nutrients, but of little value regarding long term soil structure and tilth. Stable (or passive) humus consisting of humic acids, or humins, on the other hand, are so highly insoluble (or tightly bound to clay particles that they cannot be penetrated by microbes) that they are greatly resistant to further decomposition. Thus they add few readily available nutrients to the soil, but play an essential part in providing its physical structure. Some very stable humus complexes have survived for thousands of years. Stable humus tends to originate from woodier plant materials, eg, cellulose and lignins. Humus should not be thought of as 'dead'- rather it is the 'raw matter' of life- the transition stage between one life form and another. It is a part of a constant process of change and organic cycling, thus must be constantly replenished- for when we are removing prunings and crops for the kitchen we are depriving nature's cycle of potential humus. This is why we need to substitute compost and other sources of organic matter to maintain the fertility of our productive land.

Forces involved in clay humus complex formation.

1. Coulombic force

This is due to electrical charges. It takes place between negatively charged clay surface and positively charged organic molecules.

2. Vanderwalls force

This force is a result of short-range dipole-dipole interaction. it is of important only at close distances. i.e. the distance between clay surface.

Bonding involved in Clay - humus complex

Complex formation is the reaction of a metal ion and legends through electron pair sharing. The metal ion is the electron pair acceptor and the legend is the electron pair donor. Clays can adsorb organic compounds through a number of mechanism and they are

1. Cation Bridge

The organic cations have organic molecules of various sizes and hydrophobicities that contribute to the adsorption energy via, non-Coulombic interactions. So, both the electro static and Vander walls interaction are important in cation exchange reactions involving organic cations.

In a simple way in cation exchange reactions, inorganic cation act as a link between the clay and organic matter. Cations that are small enough to penetrate the inner sphere region of siloxene surface may attract an organic anion. the metal then forms bridge between the organic compound and clay surface.

A study conducted by Ahmed et al., 2005 stated that , the Clay-humus complexes, isolated from 5 soils (Entisol, Alfisol, Vertisol, two Mollisols), were extracted with 0.1 n citrate, EDTA, and oxalate at pH 7.0-10.0; amounts of Ca2+, Mg2+, Fe3+/2+, Al3+, as well as humic acid (HA) in the extract were determined.

- HA extracted increased with pH and varied with nature of ligand; largest amounts were extracted by EDTA at high pH.

-In the Entisol clay-humus extract, Ca2+ is dominant.

-In Alfisol sample, Ca2+ and Mg2+ have little role in clay-HA bonding; apart from monovalent cations, bonding is mainly through Fe3+/2+ and Al3+, which are well correlated to HA extracted.

-In Vertisol sample contains little Fe3+/2+ or Al3+ and major bonding is through Ca2+.

-In Mollisol I and II, Ca2+, Mg2+, Fe3+/2+, and Al3+ are all involved in bonding and are highly correlated to extracted HA.

Difference in mineralogy determines the difference in bonding strength between Alfisol and Vertisol complexes. DTA indicates dual bonding modes. A major fraction of HA (in clay-humus complexes) shows thermal destabilisation due to multiple attachments on the clay surface; a small fraction is also thermally stabilised by ionic bonding with Ca2+/Mg2+ and absence of ring strain in the complex. Only the Alfisol HA does not show thermal stabilisation in the complex.

2.Cation exchange

Many organic cations are positively charged because of protonation of amino groups as in the case of alkyl amines and amino acids. These organic compounds having attained positive charges may replace inorganic cations on exchange positions or in interlayer surfaces of clays. Major organic functional group involved in cation exchange reactions are, amines and heterocyclic N compounds.

Claym+ + RNH3+ <----------------> Clay - RNH3+ + M+

A study conducted by Muneer and Oades 2005 state the addition of CaSO4.2H2O and CaCO3 led to increases in water-stable aggregates (Clay - humus complex) 50-250 µm diameter, and decreased the amount of dispersible clay. In the presence of calcium and glucose, the stabilization of aggregates >1000µm occurred and they persisted for a longer time than when no additions of calcium were made.

3. Protonation of organic molecules at clay surfaces

It refers to the formation of complex by the reaction of organic functional group which has a surface proton, either is itself in inner or outer sphere surface complex or in an acidic water molecule solvating a metal cation in an outer sphere complex.

Morland (1970) suggested that after adsorption by clay, the organic compounds become positively charged by accepting protons as follows.

H- saturated clays may donate the proton.

R- NH2 + H- Clays <---------------- > R- NH3 + Clay

Water associated with metal cations at the exchange sites or water polarised by the cation can donate a proton to the organic compound. By the presence of a protonated species that donates a proton to the organic molecule. These organic compounds which are positively charged may replace inorganic cations present on the exchange complex.

The system in which proton transfer observed was given in the following table. The degree of transfer is variable but generally it is in accordance with the relative basicities or the interacting species and the concentration of the interacting species.

Exchangeable cations on clay Molecules accept a proton

NH+4 Pyridino , methylamine, 3 aminotriazole Pyridinium NH3, methylamine,3 aminotriazole

Ethyl and methyl ammonium NH3, Pyridine, 3 aminotriazole

(NH2) COH+ (urea ) NH3, 3 aminotriazole, Pyridine,

4. Anionic exchange

Mechanism of organic anion transfer through Cation Bridge. Organic anions are normally repelled from the surfaces of negatively charged clay particles. But if polyvalent materials, eg, cellulose and lignins. Humus should not be thought of as 'dead'- rather it is the 'raw matter' of life- the transition stage between one life form and another. It is a part of a constant process of change and organic cycling, thus must be constantly replenished- for when we are removing prunings and crops for the kitchen we are depriving nature's cycle of potential humus. This is why we need to substitute compost and other sources of organic matter to maintain the fertility of our productive land.

Exchangeable cations are present there will be some adsorption of organic anions. For this two major types of bridging interactions have been proposed by Stevenson (1994), they are

In the first type water in the hydration shell of polyvalent cation is not displaced and reacts with the organic anion by H bonding.
In the second type water is displaced from the hydration shell of the polyvalent cation and the organic becomes directly co-ordinated to the cation. Main organic group involved in anion exchange is carboxyl group.

5. Ion dipole and coordination reaction

This reaction is common for adsorption of polar but non ionic organic molecules by clay minerals. In this the interaction takes between the hydrogen bonding of oxygen atom or hydroxyl groups of silicate surface and the functional groups of organic molecules.

This reaction occurs for a wide group of other polar molecules, i.e. NH3, ketone, urea and amides, pyridine, nitrobenzene, amino acids, amines etc.

6. Hydrogen bonding

In this hydrogen bonding, hydrogen atoms act as the connecting linkage. This atom bonding takes place between functional group of organic compound and oxygen on the clay surfaces( siloxene group- Si-OH group)

This reaction is more important in organic compounds having functional groups such as N-H, NH2, OH and COOH groups.

This is an extremely important bonding process in many clay organic complexes. Even though, it is less energetic than colelombic interaction, it becomes significant particularly in large molecules and polymers where additive bonds of this type a relatively stable complex.

7. Water bridging

This bonding mechanism involves the linkage of polar organic molecule to an exchangeable metal cation through a water molecule in the primary hydration shell.

Type of organic molecule Bonding mechanism

Organic anion Positive and negative adsorption, Metal bridging, Water bridging, Vander walls

Organic cations Protanation, Cation exchange

amphoteric organic compounds Protanation, Cation exchange

Non ionic organic compound Water bridging, Vander walls attraction, Hydrophobic bonding.

Types of Linkage

There are two types of linkage take place during the interaction of clay with humic acids.

1.Unstable: Humic acid is arranged on the external micelle surface of the clay, this humic acid is easily extracted by alkali.

2. Stable: Humic acid is not extracted by alkali. It is formed from the interaction of clay, humic acid and the exchangeable cations occurring in the lattice of the clay mineral, the –OH and –COOH groups in the humic acid take part in the linkage.

Significance and impact on soil properties.

Clay humus complex play an important role in improving the soil fertility and health. It affects the physical, chemical and biological properties of the soil by both directly and indirectly. Plant growth and the recycling of its residual vegetative matter is an essential part of the overall biological associations in such a Native System, and requires the intimate involvement of indigenous soil microbes, as illustrated in Figure 3 below; with the microbes using the energy made available to allow their digestion of the raw organic matter, the release of elements from basal mineral & humus, and their complexion of essential organic compounds for support of the plant and creation of soil aggregates, and ultimately the all important clay-humus complex in the soil profile.

CLAY HUMUS INTERACTION

Physical properties

Soil organic matter is one of the most important factors for improving the physical conditions of the soil and there by the plant growth. The complex formation between clay mineral stability of soil organic matter against decomposition and its consequent accumulation in soils.

Adequate organic matter content is essential for development and maintaining an excellent soil structure and proper bulk density. This favourable soil structure and proper bulk density will in turn improve soil porosity, drainage and aeration.

A study conducted by Jan Hassink (1997) in grasslands of Newzeland stated that the adsorption of organics to clay and silt particles is an important determinant of the stability of organic matter in soils, no attempts have been made to quantify the amounts of C and N that can be preserved in this way in different soils. His hypothesis is that the amounts of C and N that can be associated with clay and silt particles is limited. This study quantifies the relationships between soil texture and the maximum amounts of C and N that can be preserved in the soil by their association with clay and silt particles.

The interaction of polysaccharides with clay particles results in the formation of stable soil structure, due to this enhanced aggregation of soil particle, granular or crumb structure is formed, which is essential for the better crop growth.

Chemical properties

Clay, humus and sesquioxides are all intimately associated in clay humus complex. This determines the chemistry of the soil. In solution, acids, alkalis and their salts dissociate to positive and negative ions (cations and anions). The negatively charged clay-humus complex (micelle) acts like an anion and is surrounded by adsorbed cations. The cation dominant at the exchange positions around micelles determines the physical and chemical characteristics of the soil. Monoj kumar et al.,(2002) stated that the CEC of the clay humus complex was increased with increase in content of humic acid irrespective of the interlayer cations investigated.

The clay humus complex formation accelerate the decomposition of soil minerals. It reduces the harmful effect of fixation of P and K. The humic acid and fulvic acid increase the release of K fixed in the intermicellar spaces of clays.

Manjaiah and Ghosh ( 1995) stated that the organic component of complex imparted higher CEC, higher surface area, lower phosphate and potassium fixation capacity. The crstaline clay mineral relatively higher K fixation capacity( 18.4 to 29.8 g kg-1) compared to P fixation i.e.4.2 to 6.2 g kg-1. association of crystalline clay minerals with allophane and free oxides or hydroxides and organic matter drastically reduced K fixation capacity (10.3 to 16.7 g kg-1).

The complex formation may also make the insoluble inorganic phophorus in to more soluble. The solubility of AlPO4, Fe PO4 or Ca3 (PO4) 2 is increased considerably by complex formation with humic matter or other organic compounds.

In acid soils, the toxicities of micronutrient like Fe, Cu, Zn and Mn may occurs due to their abundant presence, complex formation reduces the toxicity of these nutrients, by its adsorption with humic compounds.

The complexion of heavy metal ion by humic compounds may temporarily reduce toxic hazards for human beings, plants and animals. Some exchange studies have been made by Mukherjee et al 1993 to determine the exchange behaviour of trace element e.g. Co, Ni, Mn clay humus complex systems and the results are compared with those obtained with clay and humic acid individually. The order of adsorption as with humic acid is Ni> Co> Mn and the order of release in terms of percentage of symmetry concentration is Mn>Co>Ni.

The interaction of pesticides and their degradation products with soil organic matter form stable complexes with soil organic compound.

Biological properties

It acts as the energy source for most of the heterotropic organism, which are essential for nutrient cycling. Enzymes bound to humic acid are considered to remain stable and active and most resistant to degradation by other enzymes. These enzymes are important in the organic and nutrient cycling. Nayak et al. 1990, investigated the biodegradability of clay humus complex with Pseudomonas florescence. They stated that the decomposition of humic substances reduces significantly on complexation with clay minerals.

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