Edaphic factors

• The factors which relate to structure and composition of

soil are called edaphic factors.

• Soil is a very complex medium. A good fertile soil contains

mineral matter (40%), organic matter (10%), water (15%)

and air (25%).

• Mineral matter in the soil occurs in the form of particles.

Soil can be studied under Physical and chemical

properties.

Soil profileVertical section of earth crust (~6 feet) – soil profile.• Structure, thickness, consistency, texture, porosity, colour, chemical composition.• Soil profile vary from place to place• Depends on climate, vegetation and parent rock• Top soil thicker in forest

Layers of soil is Horizons- 6 horizons.1) O-horizon (organic horizon or litter zone)2) A-horizon (top soil)3) B-horizon (sub soil)4) C-horizon (weathering rock)5) D-horizon (weathering rock)6) R-horizon (bed rock)

O-horizon: litter zonea) Fresh or partially decomposed organic matterb) Rich in saprophytic, fungi, bacteria, protozoac) Undergo changes soil temp, moistured) Present in forest, absent in desert, grassland &

cultivated land. A-horizon: Its a top soil- Zone of eluviation (leaching)A1-horizon: • rich in humus, dark coloured.• rich in organic material mixed with mineral soil• rich in bacteria and fungiA2-horizon: • zone of maximum leaching• Less humus, light coloured• Silicates, clays, iron oxide, aluminium oxide are

removedA3 horizon: it is transitional zone to the subjacent B-

horizon

B-horizon: zone of illuviation (collection of materials)

• Consists of B1, B2, B3 – precipitation of transported materials

• Firm zone, granular or prismatic structure,

• deep coloured with aluminium, iron and organic colloids

• Rich in clay, roots of shrubs and trees reach up to this horizon.

C-horizon: thick and contains large masses of

weathered mineral materialsZone G- water logged, low oxygen, grey layer Zone Cc- rich with calcium carbonateZone Cs- rich with calcium sulphate,

Cc-s zones Characteristic of grasslandD-horizon: rocks in active weathering state.

R-horizon: (bed rock): lowermost layer parental rock from which layers of soil are derived

Soil profile

Soil texture • Various combination of different size

of mineral particles form soil texture. • Coarse textured soil - light soil • Fine textured soil - heavy soil.

Different textures based on their particle sizes.• Fine textured soil (clay soil)• Moderately fine textured soil (loamy soil) • Medium textured soil (loamy soil)• Coarse textured soil (sandy soil)

Importance: Soil texture plays crucial role for nutrient supplies, aeration, root development, moisture content.

Nutrient supplies: The fine-textured soils are high in nutrient status; but sandy soils are low in fertility.Aeration: Coarse-textured soils are better aerated than clayey soils.Root development: percentage of soil particles affect root development.Moisture: Coarse-textured soils are easily drained; fine-textured soils are poorly drained and hold much water on the large surface area.

Names of particles Size (mm)

Clay <0.002 mm

Silt 0.002 - 0.02 mm

Fine sand 0.02 - 0.2 mm

Coarse sand 0.2 - 2.0 mm

Fine gravel 2.0 – 5 mm

Gravel > 5 mm

Soil structure• Arrangement of soil particles in earth crust is called soil

structure.• Soil structure is influenced by texture, air, moisture, organic

matter. Types of aggregates-• Granular – small , spherical, non porous (< 0.5 cm), found

in horizons, roots growth • Platy – thin, flat plates that lie horizontally; usually found in

compacted soil• Blocky – irregular blocks, may be aggregated (1.5 – 5.0 cm) • Columnar – vertical columns of soil having salt ‘caps’ at the

top; found in soils of arid climates• Prismatic – vertical columns of soil; usually found in lower

horizons • Single grained – soil is broken into individual particles that

do not stick together; loose consistency; commonly found in sandy soil

1. Density: depends on extent of weathering and particle arrangement.

2. Porosity: is the space between soil particles in a given volume.– The pore-spaces are necessary for holding water, and for the free gaseous exchange of oxygen

and carbon dioxide between the plant roots and the soil surface– Coarse texture has more air spaces, Organic matter increases the porosity

3. Permeability- determines the movement of water through pore spaces. It is higher in loose soil and lower in compact soil.

4. Soil temperature: is influenced by soil colour, texture and water and also by altitude and slope.– Affected by climate and vegetation type. – Black soil absorb more heat– Sandy soil absorb more heat and radiate it out quickly at night than clay and loamy soil.

5. Soil atmosphere: contains 3 main gases Oxygen, carbon dioxide and nitrogen.– Different from atmospheric air with more carbon dioxide, moisture and less oxygen.– Influenced by temperature, wind, rainfall and pressure

Physical properties of soil Density, Porosity, Permeability, Soil temperature, Soil atmosphere, Soil water

Soil water• All the water available in the soil is called

soil water.• Soil water act as solvent for many minerals

and organic materials.• It maintains the texture of soil (particle

arrangement)• Supports soil organisms

Types of soil water-• Gravitational water: free water moving

down due to gravity• Hygroscopic water: water held tightly by soil

particles• Capillary water: water held by capillary

forces• Combined water: water chemically bound

to soil surface• Water vapour: un-combined water as

moisture in soil

Field capacity: the amount of water soil can hold after gravitational drain off.

• Field capacity = capillary + hygroscopic + combined + water vapour.

Wilting coefficient: percentage of water left at the time when plant permanently wilts.

• The total water present in soil- holard• Water absorbed by plant- chresard• Remaining water - echard

1. Chemical composition: consists both organic and inorganic compounds. Inorganic: mainly Ca, Mg, Fe, Al, Si, Na, K and traces Mn, Zn, Co, I, Cu. It influences the pH of soilOrganic: proteins, amino acids, aromatic compounds, purine, pyrimidines, sugar, alchohol, fats, oils, resins, waxes and lignin. Organic matter and humus forms the organic compounds. Chemical nature of soil greatly influences the plant growth and type of vegetation.

2. Soil pH: Soil pH ranges between 2.2 – 9.6. some soils are acidic and some alkaline.- Soil pH is influenced by mineral content, climate, weathering and rainfall- Soil pH determines the vegetation type of an area (Sal- 4.5 to 5.5; Teak-6.5 to 7.6)- Warm and dry climate soil- Strongly basic - Acidic soils occur in high rainfall regions (6.6 at Western ghats, Kerala, Assam),- pH affects the availability of nutrients and minerals.- Increase in pH – increases calcium availability- Nitrogen available at @ 6 to 8 pH

Chemical properties of soil Composition, Soil pH, Soil enzymes, Soil humus,

Soil enzymesSoil Enzymes: There are 50 enzymes in various types of soil. • The main source of soil enzymes are microorganisms, soil animals and plant roots. • Catalyze biological reactions in the soil• Maintain soil fertility and support plant life.• Common enzymes are- • Amylases (wheat roots), catalases, invertases, dehydrogenases, phenol oxidases,

glycerophosphatases and urease (earthworm) etc.• Saline soil have high activity of ureases• Dehydrogenase is highest in forest soil and absent in alkali soils.

Humus is a complex organic substance resulting from the breakdown of plant material and other organic compounds in a process called humification. Naturally or from composting.

Types of humus: Depending on the level of decomposition, humus can be classified into Mor, moder, mull

1. Mor: is the least decomposed humus. low-biological activity in soil. (coniferous forest soils) litter of large thickness. C/N ratio more than 20, and pH is acidic.mineral-organic complex is slow and create layers

2. Moder: is a transitional stage of decomposition (mountain grassland soils) litter of low thickness (2-3 cm)medium humified humus. C/N ratio equal 15-25mineral-organic complexes are weakly bound

3. Mull: is a fully decomposed organic matter, high biological activity (evergreen thick forest soil)dark humus, C/N ratio ~10 and neutral pH, It creates a stable mineral-organic complexes.

Soil humus

Soil organisms• Plants, animals and microorganisms are

also important component of the soil• Soil organisms play a major role in

decomposition, fertility and porosity of the soil.

Classification of soil organisms:• Soil microflora: bacteria, fungi, algae

viruses• Soil macroflora: oxylophytes (acid),

Halophytes (saline), Psammophytes (sand), Lithophytes (rock) and Chasmophytes (crevices)

• Soil microfauna: protozoa, nematodes, • Soil mesofauna: micro-arthropods, insect

larvae• Soil macrofauna: insects, annelids,

grasshoppers, ants, earthworm, snakes,

• Microbial population- 6 – 12 inches of soil.

• Bacteria constitute the highest biomass of soil organisms.

• Bacteria and actinomycetes- Decomposition, break down cellulose and chitin.

• Nitrogen fixation (free living and root nodule- rhizobium)

• Bacteria exude a sticky substance that helps bind soil particles- water retention, texture.

• Fungi: Decomposition, degrade and assimilate cellulose

• mycorrhizal fungi- symbiotic, P absorption.

• Fungi- nematode, insect controlling• Pathogenic fungi- affect plant growth

Soil organisms• Rhizosphere: soil environment under the influence of plant

roots.

• Soil microfauna: protozoa – feed on bacteria and actinomycetes- maintain balance.

• Nematodes – feed on dead n decay- bacteria, pathogenic – root knot, gall

• Soil mesofauna: collembell- feed on bacteria, nematode, fungi- pathogenic, pests

• Soil macrofauna: insects, annelids, grasshoppers, ants, earthworm, snakes.

• Feed on dead and decay. Excreta are richer in bacteria and mineral nutrients. Cycling of nutrients. Soil structure, texture and porosity.