SOIL SCIENCEFPT 2093

Lecture Week 6: Factors & Processes of Soil Formation

En Mohd Fauzie JusohLecturer

Agrotechnology ProgrammeFaculty of Agro-Based Industry

Universiti Malaysia Kelantan (Jeli Campus) Locked Bag No.100, 17600 Jeli, Kelantan.

014-2903025/fauzie.j@umk.edu.my/

Soil Forming Processes

Processes of Horizon Formation in the Soil Profile

• AdditionWater from rainfall.O2 & CO2 from the atmosphere.OM from decomposition of plants & animals

residues.Eroded materials from sedimentsNutrients or ions from rain water, atmosphere,

etc.Energy from sunlight

• LossesWater through evapotranspiration.Nitrogen by denitrification & volatilization.O2 & CO2 through oxidation of OM.Soil by erosionWater & nutrients by leaching.Energy by radiation.

• Translocation (from one location to another)Salts that are dissolved in water.Nutrients that cycle in plants.Soil by animals, eg. Erthworms

• TransformationPhysical weathering that change size of rocks.Chemical weathering that change or produce

various salts or nutrients.Formation of humus by decomposition of OM.Reactions of clay with OM.

Soil Forming Factors

• Soil are considered to be independent natural bodies, each with a unique morphology resulting from a unique combination of climate, living matter, earthy parent materials, relief (topography) and age of land form. These factors which result in the development of soils are called soil-forming factors.

• The type of soil formed at any place is the result of interactions between various materials and processes. The formation of soils is a long-term process involving physical, chemical and biological activity on the parent material.

Five principle factors are involved in affecting soil formation are:-

Parent material (local rock or transported rock debris, plant debris) Climate (temperature and moisture) Topography (shape and position) Living matter(though mainly microscopic and macroscopic plants, it also

include animals that depend on the vegetation or the residue) Time (period during which the interactions take place)

A change or difference in any of the weathering processes leans todifferent kinds of soils. For example, the interaction of similar weathering processes but with

different kinds of parent materials can result in different kinds of soils. Likewise, the development of the same parent material under different

climatic regimes and vegetation covers can result in different soils.

Parent Materials• Different parent materials will give rise to different soil properties even though

in time similar soils can be formed. Parent materials determine the rates of soil formation as well as its nutrient content and particle size distribution. Soil parent materials are either organic or inorganic.

• Organic parent materials give rise to organic soils whereas inorganic parent materials form mineral soils. Organic materials are the remains of vegetation under varying degree of decomposition accumulated in water-logged environment.

• The inorganic or mineral parent materials are most varied. Soils can be formed on rock in-situ. The weathered rocks serve as the parent materials and they are called Residum.

• Soils can also developed in material that has been eroded from high land and deposited at valley bottoms. This material is called Colluvium.

• Other soils can be formed in material that has been transported by water and deposited in river levees, floodplains, estuarine areas and beaches. These materials are Alluviums.

• Regardless of the types of parent materials, they are invariably derived from solid rocks. Rocks can be divided into three broad groups which are:- Igneous rock Sedimentary rock Metamorphic rock

Major Rock Types, Their Origin and PropertiesRock Type Origin Examples Properties Igneous Sedimentary Metamorphic

Cooling magma Deposition and compaction Change in igneous or sedimentary

Granite Basalt Shale Limestone SlateMarble

Light colored, coarse grainedDark colored, fine grained Any color, fine grainedLight colored, shells or CaCO3

Any color, hardened shaleAny color, changed limestone

Igneous Rocks• Igneous rocks are formed from the cooling of magma

(molten solid) which results in solidification and crystallization. The magma below the earth crust can be of different composition. The mineral and textural composition of the rock derived from a magma depends on the chemical composition of the magma as well as the position it is formed in the earth. If solidification and crystallization occur below the earth’s surface, large crystals are formed and the resultant rocks are called Intrusive rocks.

• Where a volcano throws out the magma and crystallization and solidification occur on the surface of the earth, only fine crystals develop and the rocks are called Extrusive or Volcanic. Intrusive igneous rocks are generally crystalline and coarse-grained, whilst the extrusive igneous rocks are semi-or non-crystalline and fine-grained or glassy.

• Where crystallization of the magma occurs near earth’s surface, in dykes (vertical fissures) and sills (horizontal fissures) the rocks formed are medium-grained.

• Igneous rocks are further divided on their silica content which are:- Acid igneous rocks are high in silica Basic igneous rocks are low in silica Intermediate igneous rocks have intermediate

amounts of silica

The main rock-forming minerals are listed below:-

• Quartz: this is one of the most common mineral in acid igneous rocks. It is white, very resistant to weathering and does not contribute any chemical fertility to the soil.

• Alkali feldspars: alkali feldspars consist of silica, aluminum and a combination of potassium and sodium. Potassium feldspars of orthoclase and microcline are most common rock-forming minerals. They are milky white in color and are less resistant to weathering than quartz.

• Plagioclase: this is another feldspar with calcium in its structure. It is similar to alkali feldspar in appearance but us slightly more susceptible to weathering especially if the calcium component is high.

• Mica: two important minerals, biotite and muscovite are found in this group. Potassium, magnesium, iron aluminum and silica are the main constituents of biotite. However, magnesium and iron are absent in muscovite. Both biotite and muscovite are platy minerals.

• Amphiboles: the major elements in amphiboles are silica, aluminum iron and magnesium. Calcium and sodium are also present in some minerals of this group. Amphiboles can be brown, black or green. Amphiboles are more susceptible to weathering than the mica. They are generally absent in Sarawak soils

• Pyroxenes: phyroxenes are one of the earliest minerals crystallized from magma. Under surface conditions they are very susceptible to weathering and are absent in our soil. Phyroxenes are commonly black or green in color.

Sedimentary Rocks• The mineral composition of sedimentary rocks is much simpler than igneous rocks as

they easily weatherable minerals have been lost into solution during weathering and transport.

• The main sedimentary rocks in the order of decreasing size are conglomerate,

sandstone, siltstone and shale.

Conglomerate: it is composed of rounded gravel size fragment (2mm in diameter) of different rock types.

Sandstone: it consists of at least 50% of sand size (0.02 – 2.0mm) grains. The main mineral is quartz but alkali feldspars, plagioclase and muscovite may be present in small amounts.

Siltstone: it is made up mainly of silt-size sediment. The common minerals present are quartz muscovite, biotite, clay minerals and iron oxide.

Shale: these rocks consist mainly of clay-size particles. Clay minerals, quartz and muscovite are the main minerals in shale.

Limestone: the other group of sedimentary rocks is formed from chemical precipitation. It is formed from chemical precipitation of carbonate and/or from the accumulation of fragments of shells and hard parts of sea animals consisting of calcium carbonate. Limestone is generally fine in texture and appears light grey to black depending on the organic matter impurities present.

Metamorphic Rocks• Both igneous and sedimentary rocks may undergo changes

because of heat and/or pressure generated by earth movements where the rocks are displaced from their original position in the earth crust. These altered rocks are termed metamorphic. Examples are gneiss from granites, marble from limestone, slate from shale, quartzite from sandstone and schist from micas.

Climate• The most important climatic factors in soil formation are rainfall,

temperature and evaporation. Rain provides water needed for the chemical processes as well leaching of elements and soil erosion. Indirectly, rain determines the types of vegetation which in turn determines the amount of organic matter accumulation, and hence the types of soil formed.

• For example, soils in the humid tropics formed under rain forest are different from soils formed under grassland vegetation with drier climate. Similarly different temperature will result in different vegetation and different microorganisms which will give rise to different types of soil.

• Generally, it is the intense climate of high rainfall and temperature that develop acidic soils of the tropics that are low in nutrients, low in cation exchange capacity, low in organic matter content but highly acidic and high in exchangeable aluminum. Areas with low rainfall will have alkaline soils with high base as well as high organic matter content.

Topography• Topography is an important factor which influence soil development

through its effect on soil erosion and drainage. Topography exerts it influence on soil formation as it determines the ease of penetration of water or run-off and the drainage of water from a specific area.

• Erosion occurs during heavy rain on steep land. Surface water run-off can remove the surface layer of soil in the form of sheet erosion. On steep slopes surface erosion very often surpasses soil formation and hence one would expect to reach the parent material at much shallower depths in areas of strong relief than in areas of gentler terrain.

• Soil drainage is partly a function of topography. Soils on a river floodplain tend to be wet for long period of the year while soils on the hill slopes tend to be reasonably dry for most of the time.

• The amount of soil water has a great influence on the properties of the soils. The soil water takes up air space in the soil and insufficient air generally prevents root development of plants. In aerobic conditions, organisms break down leaf litter to produce plant nutrients while under anaerobic conditions, anaerobic bacteria produce reduced forms of chemical compounds which are often toxic to plants.

Living Matter (Organisms)

Organisms include microorganisms, macro-organisms and man.

Microorganisms: are beneficial in that they breakdown litter into simple organic elements or compounds that can be used as nutrients by high plants.

Macro organisms: can improve the soil structure by forming burrows and holes.

Man: can play an important part in modifying the soils to cause erosion by cultivation hill land and depleting nutrients by removing the crops.

Time (Age)• Soils change with age. The length of time required

for any soil to be mature depends on the other soil forming factors particularly topography and parent material. However it is an exceedingly slow process. It may be noted that a mature soil is not necessary more fertile than a young alluvial soil and the length of time a soil has undergone development is not the only factor to measure its maturity. A sandy soil on a level landscape can reach maturity in decades due mainly to erosion of the soil material formed.

Soil-Forming Factors• Interactions of Soil Forming Factors– 5 soil-forming processes must work in combination– Deficiencies/unfavorable conditions greatly affect the

ability of the soil to develop

Parent material

Topography

ClimateBiota

Time

Soil-Forming Factors

Conditions that retard soil development:• Low rainfall• Low relative humidity• Sandy parent materials• High clay content• Resistant bedrock materials• Very steep slopes• Presence of substances toxic to plants• High water tables• Cold temps• Ongoing accumulation of soil material by decomposition• Severe erosion

Questions… Thank You