Soils Lecture

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The properties of soils vary as a result of different combinations of five soil forming factors (climate, parent material, organisms, relief, time) occurring across the landscape

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Soil Processes

ADDITIONS:

Water as precipitation and runoff

Organic matter

Deposited sediments

N₂ from legumes

O₂ and CO₂ from the atmosphere

Salts in groundwater

LOSSES:

Soil by erosion

Water, nutrients, and elements by leaching

Water by transpiration

N₂ by denitrification

Carbon from organic matter

TRANSLOCATIONS:

Clay, organic matter, carbonates, and oxides in leachates

Nutrients circulated by plants

Soluble salts in water

Soil moved by animals

TRANSFORMATIONS:

Mineral decomposition

Reduced particle size by physical weathering

Structure development

Clay and organic matter changes

Humus from organic matter

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Main Soil-Forming Processes:

water infiltration: rain or irrigation waters,

the addition of organic matter at the soil surface,

weathering and transformations of rockforming minerals in the soil parent material,

movement of clays, oxides, and carbonates and their accumulation lower in the soil profile,

leaching and loss of soluble salts from the soil profile.

Inert geological materials are turned into soil profiles by:

weathering: transformations and translocations of mineral matter,

accumulation of organic matter: plant and animal materials in the soil,

live organisms: in the soil converting biotic residues into plant nutrients.

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Soil horizons:

Layers approximately parallel with the earth’s surface.

Interrelated by processes of formation and degradation.

Soil profiles:

Complete vertical succession of soil horizons.

Continue down to: parent material, other consolidated substrate material, a selected depth (~2 m) in unconsolidated materials.

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Soil Horizons:

Topsoil: (A horizon (A1, A2, A3 horizons))

zone of maximum biological activity.

the organically enriched surface horizon(s),

contain the bulk of the plant roots,

is the major entry point into the soil of air and water,

provides the chief nutrient reservoir in the soil.

Subsoil: (B horizon (B1, B2, B21, B22, …, B3, BC horizons))

zone of accumulation of clay, iron, etc.

the horizons in the soil profile below the topsoil, and above the soil parent material (i.e. above rock or deposited sediments).

Deep subsoil: (C horizon)

the weathered soil parent material.

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Properties of good soils:

Topsoil depth: 100 – 200 + mm

Soil profile depth: 1 - 2 m

Plant nutrient supply: organic and mineral

Beneficial (non-toxic) soil chemical properties

Good surface soil structure

Large pores: water entry, transmission, and drainage

Fine pores: water retention and storage for plant use

Approximately equal amounts of sand, silt, and clay: loam texture

Little gravel: few particles larger than 2 mm

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Soil Components:

equal parts of solid matter and holes.

Nearly equal proportions of air and water are best for plant growth.

Fluctuations in water and air contents occur as the soil dries or moistens.

Organic matter provides most of the naturally-occurring plant nutrients: living tissues and dead remains, Living soil microbes, Organic substances, such as “humus”

Soil organic carbon (SOC) x 1.72 = soil organic matter (SOM)

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Soil colours indicate:

soil moisture status: dark soil indicates soil is wet, OR, has a high soil organic matter content

organic matter content (i.e. soil fertility),

oxidation state of iron (i.e. profile drainage),

seasonal waterlogging

Red soils: contain hematite [Fe₂O₃], an oxidised iron oxide – good drainage and aeration,

Yellow soils: contain goethite [FeO(OH)], a hydrated iron oxyhydroxide – generally poorly drained,

Grey soils: iron leached out of the soil matrix, may be deposited as ironstone gravel in the soil – very poor drainage and/or seasonal waterlogging,

Mottles: strong iron redistribution – prolonged waterlogging,

Bleached A2 horizons: impeded drainage in subsoil.

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Soil Textures:

The sandy (light), weakly cohesive feel, and very short ribbon of a sandy soil.

Smooth, coherent feel, and slightly longer ribbon of a loamy soil.

Smooth, shiny, plastic, and long, flexible, self-supporting ribbon of a clayey soil (heavy).

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Characteristics of the Red, Yellow, and Grey Kandosols

sandstone and sandy sedimentary parent material;

non-calcareous profiles (i.e. well leached soils);

weakly differentiated soil horizons;

usually gradational texture changes down soil profiles; stable, massive structure and earthy fabric;

acid - neutral pH,

low cation exchange capacity,

low natural fertility.

Vertesols form cracks when dry and top soil falls through, gets mixed when wet (self turning soil)

Best fertility

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17 Elements essential for plant growth:

Macronutrients needed in large amounts:

H, C, O, N, P, K, Ca, Mg, S

Micronutrients (trace elements) needed in minor amounts:

Cl, Fe, B, Mn, Zn, Cu, Ni, Mo

Important to some plants:

Beneficial: Na, Co, Si

Undesirable: Al, Heavy metals (Cd, Pb, Hg, As, Cr, …)

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Yield-Law of minimum