24 Soil Horizons

Meenakshi Nandal

epgp books

 

 

25.1 Introduction

 

 

Soils are natural bodies porous in nature consisting of inorganic and organic matter. They are formed by atmospheric and biological influences on the earth crust. These earthen bodies also reflect the integrated effects of those interactions occurred at the earth’s surface. The topography of the landscape with the passage of time keeps on changing with these effects.

 

Soils are structural and functional elements of terrestrial (land-based) ecosystems. Soils are formed by Various geological processes leads the formation of soil through the interaction of various factors like geological, climatic and biotic .And hence soil forming process considered to be very slow normally takes thousands of years to form . For all living beings soil is the fundamental source of life. Soils possess many biological, chemical and physical properties which exert great influence on the distribution and development of vegetation. Each soil having a unique morphology and resulting from grouping of living matter, parent rock materials, climate, relief and time and also soils are considered to be independent natural bodies. The morphology of each soil is given its soil profile for e.g. If soil is dig 2 to 6 vertically downwards into the ground various layers are visible known as soil horizons. The cross-sectional view of the ground (beneath the surface) reveals kind of soils and rocks that make up the soil profile. This cross sectional view is called as the Soil Profile. The profile is made up of different parallel layers called Soil Horizons. These layers of horizon are different from the other layer existing above or below it. Each horizon describe the characteristics of the layers. The layers are divided as top soil layer, sub-soil layer and the bed rock layer.

 

Soil crust and soil horizon generally forms parallel layers but having different physical characteristics among the layers above and beneath. Further three or four horizons are found in each type of soil. Physical features, chiefly color and texture define horizons. These may be in terms of particle size, distribution for texture, for instance and in terms relative to the surrounding material i.e. “coarser” or “sandier” than the horizons above and below. Air, water, solar radiation and plant material, originating at the soil-atmosphere interface results in the differentiation of the soil into distinct horizons. Weathering of the soil takes place first at the surface and then move down therefore the uppermost layers have most of

 

Arabic numerals. The master horizons and layers designate by capital letters and lower case letters are used as suffixes to indicate specific characteristics of master horizons and layers; and Arabic numerals are used both as suffixes to indicate vertical subdivisions within a horizon or layer and as prefixes to indicate discontinuities.

 

The soil profile extends from the soil surface to the parent rock material. There are six master soil horizons O, A, E, B, C, and R commonly recognized and are designated using the capital letters. The following horizons are listed by their position from top to bottom within the soil profile. It is not necessary that every layer is present in every location (Fig-25.2). For instance, P horizons usually form in areas which remains waterlogged for long periods of time.

25.2 O horizon

 

This is the uppermost layer generally above the mineral soil or occurs in an organic soil profile. The “O” stands for organic matter. The large amounts of organic material derived from dead plant and/or animal residues which is in different stages of decomposition found in this layer. The O horizon usually found in forested areas and is commonly referred to as the forest floor whereas it usually absent in grassland. The O horizon should be considered distinct from the layer of leaf litter covering many heavily vegetated areas, which contains no weathered mineral particles and is not part of the soil itself. O horizons is sometime divided into two categories O1 and O2 categories, however O1 horizons includes incomplete decomposed matter (for instance, fragments of rotting leaves), and O2 horizons includes only well-decomposed organic matter, the origin of which is not readily visible. further O horizons may also be divided into three subordinate of O horizons denoted as: Oi, Oe, and Oa.

 

25.3 A horizon

 

The topmost mineral horizon is A horizon, often referred to as the ‘topsoil’ also. This layer lying just below O horizon generally contains enough partially decomposed (humified) organic matter to give the soil a color darker than that of the lower horizons. The A horizons exhibits coarser texture due to losing some of its finer materials by translocation to lower horizons by erosion. This layer is known as the zone in which the most biological activity occurs. A horizon is also referred as the bio mantle as it contain soil organisms such as earthworms, potworms (enchytraeids), arthropods, nematodes, fungi, and many species of bacteria and archaea are also concentrated here which are often in close association with plant roots. Biological activity cannot be used as a chief distinguishing feature of an A horizon as it extends far deeper into the soil.

 

25.4 E horizon

 

E”, horizon normally formed by leaching of clay, iron, and aluminum oxides into this horizon which leaves a concentration of resistant minerals, such as quartz, in the sand and silt sizes. These are present only in older, well-developed soils, and these generally occur between the A and B horizons and are present only in older, well-developed soils. The E horizon often has a pale color that is generally lighter in color than either the horizon above or below it. Soils developed under forests usually contain E horizons, and are rarely found in soils developed under grasslands. In regions if this designated horizon is not employed, than leached layers are classified firstly as an A or B according to other characteristics, and then appended with the designation “e” (see the section below on horizon suffixes). A stone layer commonly forms near or at the base of the E horizon due to the presence of gravels in soil may be due to animal bioturbation.

 

25.5 B horizon

 

The B horizon is also commonly called to as the “subsoil. This horizon either form below an O, A, or E horizon and during soil genesis have also undergone sufficient changes, due to which properties of their original parent material are no longer divisible. In humid regions maximum accumulation of materials such as silicate clays, iron (Fe) occurs in B horizons. The process of accumulation of these material is known as illuviation, wherein the materials gradually wash in from the upper horizons. And hence this layer is also referred to as the “illuviated” horizon or the  “zone of accumulation”. In addition, this horizon has distinctly different structure or uniformity than the horizon(s) above and the horizon(s) below. The B horizon may also have stronger colors (higher chroma) than the A horizon. In arid and semiarid regions, calcium carbonate or calcium sulfate may accumulate in the B horizon.

 

Under the Australian system the B horizon may be divided into B1, B2, and B3 unlike the A horizon type. B1 is a transitional horizon because it contains properties of the B horizons below it and some A-horizon characteristics. B2 horizons show highest soil development within the profile due to the presence of higher concentration of clay, minerals, or organics. B3 horizons are transitional between the overlying B layers and the material beneath it, whether C or D horizon. The A3, B1, and B3 horizons are not well defined, and their functions are generally at the prudence of the individual worker. Plant roots penetrate through this layer, but it has little humus. It is usually brownish or reddish due to residual clay and iron oxides.

 

25.6 C horizon

 

Next to the B Horizon lies C horizon. This layer shows lack of pedagogical development as negligible soil-forming processes occurs in it. In other words, the C horizon is the unconsolidated material underlying the solum (A and B horizons). But not necessarily it may or may not contain the same parent material from which the solum is formed. When R horizon undergoes weathering and further rocks break up into smaller particles the formation of C horizon takes place. The C horizon is lacking the properties of soil formation although it lies below the zones of greatest biological activity. The C horizon in dry regions accumulates carbonates and gypsum and looses enough when dug with a shovel. Some of the structural features of the parent rock or geologic deposits have been retain in C horizon. The A and B layers usually originated from the C horizon. As weathering and erosion continue often upper layers of the C horizon may in time become a part of the solum. In the C Horizon more often lumps or large shelves of unweathered rock are present, rather than being made up solely of small fragments as in the solum. It also contains rocks with crevices and cracks.

 

25.7 R horizon

 

At the base of the soil profile partially weathered bedrock are present in R horizons. Unlike the above layers, R horizons are composed largely of consolidated masses of hard rock that cannot be excavated by hand. The in situ soil so formed will reveal strong similarities to this bedrock layer.

 

25.8 P horizon

 

The P horizons contains ample amount of organic matter, but differ from O horizons in that they appear under waterlogged conditions. The “P” designation for this horizon comes from peats as their common name. As the O Horizons is divided into subordinates P horizon is also divided into P1 and P2. Under the process of illuviation this layer accumulates iron, clay, aluminium and organic compounds.

 

Horizon Boundaries

 

The boundary between any two horizons can vary distinctly like some boundaries are very sharp whereas others may merge gradually into the horizon below it.

 

And these boundaries govern soil development and certain other aspects of soil behavior. Any sudden change in boundary, for example, may be due to the sudden change of one kind of material to another kind of material either by geologic activities or formed by soil development processes. Such abrupt change may cap root penetration, or it may signal a different rate of water movement through the soil.

 

Transitional Horizons

 

In Master horizons this kind of abrupt change is rarely observed from one to another form. These changes occur gradually throughout a zone ranging between 5 to 10 inches in thickness. These zones are termed as transitional horizons. There are three common transitional horizon in soils: AB, BA, and BC. For e.g. if we take AB horizon this transitional horizon is formed between the A and B horizons. It contains some properties of the A and some of the properties of the B in it.

 

 

25.9 Factors affecting soil profile:

 

The development  of  structure in  arable   soil   depends  on  the  following   factors:

 

  1. Climate: the soil structure and degree of aggregation are amply affected by climate. There is very less gathering of primary particles in arid areas. The extent of aggregation is greater in semi arid regions comparatively.
  2. Organic matter: The structure of a sandy soil and clay soil is recuperated by organic matter.In case of a sandy soil, the sticky and slimy material formed by the disintegration of organic matter and the conjoined microorganism bind the sand particles together to form agglomerates. In case of clayey soil, it transfigures the properties of clay by condensing its cohesiveness and this helps in forming of clay more crumby.
  3. Tillage: Large clods are breakdown into smaller fragments due to cultivation. Perfect moisture content in the soil is necessary in getting more granular and crumby structure. High moisture content accelerates the formation of clods on drying and low moisture content will breakdown some aggregates.
  1. Plants, Roots and Residues: Exudates from roots and excretion of gelatinous organic compounds serve as a connecting link. Soil particles cling together due to Root hairs. Pressure exerted by the roots also held the particles together. The soil gets strained due to loss of water and it results into cracks which lead to aggregation. The soil gets protected from extreme and sudden temperature and moisture changes and also from rain drop impedance by plant tops and residues. Plant residues serve as a food to microbes which are the prime aggregate builders.
  1. Animals: Small animals like earthworms, moles and insects etc., are the supreme agents that intrude the aggregation of finer particles.
  1. Microbes: the soil particles are cemented by Algae, fungi, actinomycetes. The fungi help in keeping the soil particles together. Fungi and actinomycetes exert mechanical binding by mycelia, Cementation by the products of decomposition and materials synthesized by bacteria.
  1. Fertilizers: The strength of soil aggregates gets diminished by fertilizers like Sodium and Nitrate.
  1. Wetting and drying: The colloids of dry soil get swelled on absorbing water when it comes in contact with water. Shrinkage due to drying results strains in the soil mass and give rise to cracks, which turns it into clods and granules.

9. Exchangeable cations: Ca, Mg Flocculating Good structure H, Na Deflocculating  Poor structure

  1. Inorganic cements: CaCO3 and Sesquioxides.
  1. Clay- Clays are the active mineral portion of soils dominantly colloidal and crystalline. Definite repeating arrangement of atoms in clays gives it a crystalline nature. Majority are made of planes of O2 atoms with Si and Al atoms holding the O2 together by ionic bonding.
  1. Water – Soils found in shoreline deposits consists of coarser texture and occupy higher landscape positions. In lakebeds, organic substances dominate the sediments, and peats form due to very low influx of sediments. Marine and lacustrine deposits form in low-energy environments under inland seas and lakes. These sediments are typically coarse near the shore and finer toward the middle of the lake or sea. Inland water bodies, including deltas, sand dunes, and beaches can be associated with several shoreline features. Deltas are essentially alluvial fans with their sediments deposited underwater.

 

There are 9 fundamental processes that affect profile differentiation:-

 

 

Humification:- The decomposition of raw organic material into humus under the influence of soil microorganisms is known as humification. During this process, the soluble organic substances regroup themselves into large molecules and become poorly soluble by forming new organic chains (polymers). In the strict sense, polymers so formed are stable that is resistant to the action of microorganisms. The term humification focuses in particular on the phase which follows the decomposition of the organic debris by the processes of synthesis and building up of new molecules through microbial, biochemical, abiotic or physicochemical pathways. Once so formed humus achieve the permanent soil structure, definitely contributing to soil improvement.

 

 

Eluviation and illuviation – The process of amputation of soil material in suspension or in solution by downward movement of water from the upper to the lower layers. This process of mobilization and translocation of mobile constituents (mainly clay) results in changes of textural differentiation, or leaching of soluble elements like salts within the soil profile.

 

Calcification and decalcification –The process like leaching, movement, precipitation and accumulation governs the movement of soluble calcium carbonate in the soil layers. The movement of carbonate follows the general reaction.

 

CaCO3 + H2O + CO2      Ca (HCO3)2

 

Under an active biological activity the reaction proceeds in the right direction in the presence of CO2 and H2O leading in formation of soluble bicarbonate.

 

Podzolization– The process of extreme leaching of bases in an acid environment, relatively poor in weatherable minerals, characteristic of regions with humid boreal or tropical climate. It involves the eluviations of acid and complex forming humus that becomes mobile and gets leached from the upper part of the profile, and their subsequent deposition in the lower horizons. The process is the most active under pine tree forests.

 

Lateritization- The process that removes silica and soluble bases from the upper layers of the soil, creating a relative accumulation and concentration of sesquioxides (Fe and Al-oxides) in the soil. If the alkaline bases are removed from formation their seat, the residual soil becomes acidic in nature. Although substantial eluviations occurs during the process, as the eluviated materials are not re-deposited in the lower layers there is no formation of well define horizonation.

 

Gleization – A process of soil formation under an anaerobic environment soil formation takes place which leads in to the development of a gley horizon with green-blue colors, related to the reduction of soluble ferrous iron under water-logged conditions. Where the groundwater fluctuates considerably with the season, the gley shows distinct mottling of yellow and rusty brown colors caused by alternate oxidation and reduction phenomena.

Salinization– The process of accumulation of salts, such as sulfates or chlorides in the form of a salty horizon. It is active under conditions of highly saline or brackish groundwater, and evaporation being higher than precipitation, so that salts move up by capillary action from the groundwater.

Alkalinization – A process involving the accumulation of sodium ions on the exchange complex of clays, resulting in the formation of a sodic soil. At this moment a high soil pH (>8.5) develops, soil colloids are dispersed and a very poor soil structure develops. The organic matter dissolved under alkaline conditions forms black organo-clay coatings on the ped surfaces giving the soil a dark-colored appearance.

 

Solodization or de-alkalinization– The removal of Na+ from the exchange sites and the dispersion of clay, promoted by the addition of Ca2+ to the formerly alkaline soil, often under the form of easily soluble gypsum.

 

Pedoturbation – The process of mixing the soil due to faunal activity (ants, earthworms, moles, termites, etc.), plant roots, and natural swell-shrink processes or by man-made land management practices. It is very active in boreal areas covered by long-time and usually undisturbed forest vegetation.

 

Summary

 

This module gives an overview regarding the concepts of modern soil sciences. The process of weathering and gradual evolution of a regolith towards a mature soil profile is discussed. Main soil properties and Soil composition as well as their inter-relations with other properties and impact on land use are discussed. Soil science is a multidisciplinary approach as it includes concepts, processes and techniques from other sciences, but with a center point focusing on soils. The main ideas about soil-forming processes include the following points:

 

  1. Illuviation refers to the deposition of weathering products in the horizons and Eluviation represents the removal of soil material in suspension or solution.
  2. Soil horizons are from due to the various processes like additions, losses, transformations and translocation of matter and also result in differentiation of horizons.
  3. Soil forming process is very slow but can approach a dynamic steady state over the time.
  4. Intrinsic thresholds exist in soil systems that explain instability in the absence of environmental change.
  5. Due to soil own threshold value its properties keep on changing by internal processes over time.
  6. Three general kinds of soil-forming and changing processes involves biological, chemical, and physical processes are responsible for the formation of specific soil horizons and soil profile.
  7. Soil-forming processes can also lead to higher categories of soil classification systems.
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