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Mineral colloids are usually refine clay particles. Under a microscope, they display thin, plate-like bodies. When these particles are well mixed in water, they remain suspended indefinitely, turning the water murky. Organic colloids are tiny bits of organic matter that are resistant to decay. What are Biogeochemical Cycles? Types of Soil Colloids. Non-crystalline silicate clays [Dominantly amorphous clays allophane and imogolite ].
Iron and aluminium oxides [Dominantly gibbsite Al-oxide and goethite Fe-oxide ]. Organic humus colloids [Non-crystalline colloids dominated by long C-chain molecules]. Properties of Soil Colloids.
Colloidal particles are always in motion because of charge particles. Colloidal particles are transformed from a liquid into a soft semisolid or solid mass by adding an opposite charged ion. For this activity, assume the soil samples have been extracted and the filtrates are now available for analysis.
Calculate the CEC and record your data in Table Here is an example of how to calculate the CEC, assuming 2. The reaction occurring during titration is. The solution of 0. Therefore 2. CEC calculations. Label the axes and plot all data. Draw a line through the points on the graph for the Norfolk E, Norfolk Bt, and Cecil Bt, and answer the following questions:. Of the four major factors that affect soil CEC amount of clay, type of clay, amount of humus, pH , which is responsible for this linear increase in CEC?
The slope of the line represents the change in CEC divided by the change in clay content. The calculated slope is the CEC on a clay basis. The line you have drawn does not pass through the origin, indicating that a soil with no clay would still have CEC. How could this be possible? The Cecil Ap horizon sample falls just above the line you drew in question 1, yet both Cecil samples were taken from the same profile.
How can you explain this difference in CEC? Explain what may be different about the colloids in the White Store Bt soil. Assume a soil has a CEC of 1. Assume a bulk density of 1. Because Ca has a valence of 2, one mole of Ca has 2 moles of charge:. The soil can adsorb 1. The amount of Ca required to supply this amount of charge is. The atomic weight of Ca is The quantity of Ca adsorbed in the total soil volume is.
Because the valence of Al is 3, one mole of Al has 3 moles of charge. The atomic weight of Al is An alkaline soil found in a semi-arid region might have the following cation exchange characteristics per kg of soil :. If these cations are the only ones on the exchange sites of this soil, what is the percent base saturation?
In lower pH soils, iron and aluminum oxides carry a net positive charge and attract negatively charged ions which partly contribute to the increased phosphate fixation in the soil colloids in acidic soils while at higher pH values, the particles carry a small negative charge that is balanced by adsorbed cations[2,9].
Organic colloids: Organic colloids also called humus which is the dark-colored, stable colloidal organic decomposed plant and animal remains [1,3]. Humus is a mixture of various complex compounds like lipoproteins, polysaccharides, and polyuronides [5,18,21].
Organic colloids exhibit adsorptive capacity higher than colloids [18,21]. They are negatively charged colloids like clay colloids. When organic colloids are added to the sandy soils, increases provisionally its moisture and nutrient holding capacity will be increased.
During hydration, each particle of humus forms a micelle and due to the presence of Because of high negative charges available on the surface of the humus which can attract different positively charged organic and inorganic constituents. Organic colloids have high CEC as compared to clay and they can store and slowly release essential nutrients for plant growth [11]. An organic colloid is normally amorphous or shapeless. This type of colloids is mostly pH-dependent because the CEC process depends on the replacement of hydrogen and CEC normally increases at higher pH values.
The organic colloids consist of convoluted chains i. Humus particles are often among the smallest of soil colloids and exhibit a very high capacity to absorb water but almost no plasticity or stickiness.
They have a net negative charge per unit mass because of partly dissolved hydroxyl, carboxyl, and phenolic content but the charge is pH-dependent and becomes extremely high in neutral to alkaline soils [2]. Properties of soil colloids: Certain characteristics also assume considerable importance such as shape, surface area, plasticity, cohesion, swelling, shrinkage, dispersion, and flocculation.
The properties of soil colloids are all surface phenomena and their strength depending on the amount and nature of the interaction hand over by the colloids [9]. Size: The organic and inorganic soil colloids are extremely smaller in size less than 0. These particles cannot be viewed using an ordinary light microscope but can be visualized only with an electron microscope. The soil colloidal particles never pass through a semi-permeable membrane [9].
Colloids in natural systems are characterized by a continuous particle size distribution of complexity and diversity. The allocation of shapes, densities, surface chemical properties, and chemical composition might different extensively with size [18]. Shape: The shape of colloidal clay particles is seen as spherical, crystalline, and or amorphous.
Clay particles that are composed of plates of layers and have an internal structure are crystalline [9]. Colloidal shapes mainly depend on their mineralogical arrangement [18]. Surface area: The total surface area of soil colloids is the sum of both internal and external surfaces [12]. For instance, the surface area of fine-grained mica is about 10 5 m 2 kg -1 on average, whereas the vermiculite and smectite surface area can approximate 8x10 5 m 2 kg -1 concerning the number of structured layers in an aggregate [17].
All clay particles have a high external surface area because of their small size. The external surface area of 1 g of colloidal clay minerals is expected times greater than that of 1 g of coarse sand [9]. Surface charge: Soil colloidal particles are always in movement due to their charged particles.
Colloids are reactive because of their total surface area and increased reactivity related to rough surfaces and highly energetic sites, and the effect of electrostatic charges[18,22]. Surfaces of clay colloids have positive or negative charges on their surfaces to attract charged ions, but negative charges predominate [5]. Soil colloidal particles can absorb different phases from their suspension.
Humus and clay mineral cations and anions are adsorbed on their surfaces because of permanent negative charges of clays formed by isomorphous substitution [9]. The charge on the mineral surface is calculated as the difference between the moles of charge donated per unit mass of mineral by the positive and negative anions adsorbed from an electrolyte solution of known pH.
The positive and negative charge adsorbed give rise to a cation exchange capacity CEC and anion exchange capacity AEC in centi mol charge per kg. Clay minerals such as kaolinite, and the surfaces of Fe and Al oxides have pH-dependent charges [12]. Permanent charges of silicate layers resulted from isomorphous substitutions [19]. Plasticity: Plasticity is the capacity of clay particles to be easily molded when it moist or wet. This property is may be due to the plastic-like nature of the clay particles.
Plasticity is exhibited when soils are wet or moist [9]. Cohesion: When the water content of too wet clay is decreased, the attraction of colloidal particles will seem to increase. This situation of the clay particles to aggregate probably is due to the attraction of the clay particles for the available water held between the clay particles.
Soil colloidal particles have cohesion and adhesion properties [9]. Swelling and shrinkage: Swelling is the process of expansion of crystal layers that occurs because of water moving between crystal layers. Swelling is considered by water attracted to ions and adsorbed by clays and water entered in soil pore spaces soils [9].
Swelling occurs when the soil has small particle sizes like smectite e. However, Kaolinite, chlorite, fine mica, do not show this characteristic to any extent, and also vermiculite is in between.
The swelling property of certain clays can be change volume by adsorbing water into their structure [23]. Dispersion and flocculation: The term flocculation refers to the action by which individual fine soil particles are come to sticking together to form flocculates [9]. A principal manner of a dilute colloidal suspension in water is complete dispersion means that the particles repel one with the other [9]. Soil colloids are the most active site of the soil which includes organic and inorganic clay.
Silica is the major structural element contained in many mineral soils. Soil colloids can be identified one from the other based on their composition, tetrahedral arrangement, and octahedral sheets of silica and also alumina. Soil colloids are very vital for nutrient reserving which contributes to most essential plant nutrients not be removed by the percolating water.
This review paper will contribute as a baseline for soil science scholars and related fields. Further research on soil colloids and their properties is essential for more understanding in the practical management of soils. The review paper is an original work carried out by ourselves. The matter embodied in this review work has not been submitted earlier for publication in other journals.
Order for reprints. Toggle navigation. Author and article information. DOI : Open J Bioinform Biostat 5 1 :
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