INTRODUCTORY SOILS

Subject Outline Introduction What is Soil Functions of Soils Composition of Soils Soil Formation

Why Soil Science? Soil is an essential part, and some would argue, the most important component of the terrestrial ecosystem

Beyond that, Soil is an environmental interface Lithosphere -rocks Atmosphere -air Hydrosphere -water Biosphere –living organisms

What is soil Concepts of soil differ greatly among users of soil Mining engineer Civil engineer Home owner Farmer

Definition … natural product formed from weathered rock by the action of climate and living organisms modified by topography over a period of time.

The Soil Profile Alabama –Bama Soil

How different can they look like? Tanzania Quebec Sri Lanka Brazil Virginia Montana

Functions of soils

Functions of Soils Medium for plant growth Regulator of water supplies Recycler of raw materials Habitat for soil organisms Engineering medium

Soil as Medium for plant growth Physical support Air Water Temperature moderation Nutrient elements Protection

Soil as Regulator of Water Supplies Soil regulates both the quality and quantity of water in rivers, lakes, and underground aquifers Quantity of water supplies Some of the water may be stored in the soil and used by trees and other plants Quality of water supplies Water is purified and cleansed as it soaks through the upper layers of soil.

Soil as Recycler of Raw Materials Soils play a role in geochemical cycles Assimilate organic waste Turn it into beneficial humus Convert the mineral nutrients into plant and animal usable forms Returning carbon to the atmosphere to be used for photosynthesis again

Soil as Habitat for soil organisms Handful of soil is home to billions of organisms in thousands of species How does this happen? Micro-environment differences Pore spaces Moisture Temperature Organic matter

Soil as Engineering Medium Soil is firm and solid Good base to build structures Soils differ in stability Designs for structures are different for soils Physical properties influence engineering uses

Composition of Soils Soil consists of three major phases OR four components: Solid phase (Soil Minerals) Liquid phase (Soil Water) Gas phase (Soil Air)

Components of Soil

Soil Solid Phase (two components) Inorganic minerals Soil minerals are either primary or secondary minerals Primary minerals were formed in the original igneous rocks. Secondary minerals formed in soil by weathering of the primary minerals. Organic matter (humus). Humus is the product of the decay of organic residues such as wood, leaves, and other biological materials.

Soil water (soil solution) Water is vital to the ecological functioning of the soil. Soil solution which contains water, dissolved ions, molecules and gases. Soil water is different from drinking water in 2 ways: Soil water is held by many types of forces within the pores of the soil. Soil water is never pure but contains hundreds of dissolved organic and inorganic compounds.

Soil Atmosphere contains similar gases as found in the atmosphere above the soil But often in very different proportions. Usually higher in carbon dioxide and lower in oxygen than the atmosphere.

Interaction of the Components The components interact to determine the nature of a soil e.g., soil moisture controls air and nutrient supply mineral particles control water movement Organic matter controls arrangement of minerals which influence pores that determine water and air relationships

Soil Formation

Soil Formation Five factors of soil formation Weathering of rocks and minerals Processes of soil formation

Five factors of soil formation s = f(cl, o, r, p, t…) Where: s = any soil property cl = climate (rainfall & temperature) o = organisms (biota) p = parent material r = relief (slope aspect and position) t = time (relative age of soil formation)

1. Parent Material Nature of parent material influences the characteristics of soils E.g., texture of sandy soils is determined by parent material movement of water is controlled by texture of the parent material Influences the chemistry of the soil Influences the type of clay minerals present in soil.

Weathering of Rocks and Minerals What is weathering? .. the modification or breakdown and destruction of the physical and chemical characteristics of rocks and minerals and carrying away the soluble products. ..the nature of the breakdown depends on the type of material

Types of Rocks Igneous rocks Sedimentary rocks Metamorphic rocks Form from molten magma Granite and diorite Sedimentary rocks Compacted or cemented weathering products from preexisting rocks Sandstone and shale Metamorphic rocks Formed by change in the form of other rocks Gneiss, marble, and slate

Two main types of weathering a) Physical (Mechanical) weathering .. Causes rocks to disintegrate into smaller pieces without affecting their composition Factors: Temperature Abrasion by water, ice and wind Plants and animals

b) Chemical weathering .. Degradation of rocks and minerals by the chemical activities of water, oxygen, and microbial action Factors of Chemical weathering: Hydration Hydrolysis Dissolution Carbonation Oxidation-reduction Complexation

Hydration Process of binding of water molecules to a mineral Hydrolysis Splitting of water molecules into its components. Split components in turn attack the minerals. Dissolution Process of hydrating of ions until they become dissociated

Carbonation Oxidation-Reduction Complexation Carbon dioxide dissolves in water to form carbonic acid which accelerates chemical breakdown of materials Oxidation-Reduction Minerals that contain Fe, Mn,or sulfur are susceptible to this reaction when exposed to environments different from the ones in which they formed. This destabilizes the mineral. Complexation Biological processes produce organic acids that can form complexes with elements within the structure of a mineral thereby pooling the element from the mineral and destabilizing it.

Classification of Parent Materials Organic Deposits Weathering of rocks in place Deposition of weathered rock materials from elsewhere Parent materials are classified by their mode of placement at their current location.

Types of Parent Material Gravity colluvium Ice transport glacial till, moraine, outwash Wind transport Eolian (dune sand, loss, dust) Water transport Lakes -lacustrine Streams –alluvium (floodplain, alluvial fans, delta) Oceans –marine Volcanic ash

2. Climate May be the most influential of the four factors acting on the parent material Determines the nature and intensity of weathering (precipitation and temperature) Both affect the physical, chemical and biological processes Climate also exerts influence indirectly through a second soil forming factor, the living organisms (natural vegetation). Climate is so important in soil formation that certain evidence of climatic change could be found in the soil

Precipitation Water is essential for all the major chemical weathering reactions. The deeper water penetrates the parent material, the more effective it is in soil development. Water percolating through the soil profile transports soluble and suspended material from the upper to the lower layers. Thus percolating water stimulates weathering reactions and helps differentiate soil horizons.

Temperature Every 10 deg C, the rate of chemical reaction doubles If warm temperatures and abundant water are present in soil at the same time, the processes of weathering, leaching, and plant growth will be maximum and lead to deep soil profiles. Compare this to very modest soil profile development processes that are obtained in cold areas

3. Organisms (Biota) a) Role of natural vegetation Soil organisms, both the animals (fauna) and the plants (flora) physically churn the soil and help stabilize the soil structure a) Role of natural vegetation Organic matter accumulation Cation cycling by trees Heterogeneous rangelands

Heterogeneous Rangelands (Plants & soil formation)

b) Role of animals Animals such as gophers, moles, prairie dogs bore into lower soil horizons and bring materials to the surface –tunnels. Earthworms and termites Bring about considerable soil mixing Human influence destruction of natural vegetation Soil tillage for crop production Irrigation Fertilizer application

4. Topography Relates to the configuration of the land surface It is described in terms of differences in elevation, slope and landscape position Steep slopes encourage soil loss by erosion and allow less rainfall to enter the soil Thus prevents formation of soils from getting ahead of soil destruction In the depressions where runoff tends to concentrate, the soil is usually more deep

Role of Topography in Soil Formation

5. Time Time that materials have been subjected to weathering is important because soil forming processes take time to show their effects. Clock of soil formation starts when e.g. Landslide exposes a new rock Flooding river deposits sediment on floodplain Glacier melts and dumps load of debris Bulldozer cuts and fills landscape, etc. Rates of weathering and soil development This is a function of the interaction of time and the other factors of soil formation.

Role of Time in Soil Formation

Processes of Soil formation Processes that are involved in soil formation can be placed in four main groups Additions Transformations Transfers or Translocations Losses

Additions Additions entail the inputs of materials to the developing soil profile from outside sources. E.g. Addition of organic matter from Plant leaves and sloughed-off roots Addition of water by precipitation Addition of dust particles that fall on the soil surface Addition of salts and silica dissolved in groundwater and deposited near or at soil surface

Transformations Transformations entail disintegration and altering of composition and form of primary minerals Physical weathering to smaller particles Decomposition of organic residues Recombination of decomposition products to form new minerals such as silicate clays and oxides Synthesis of organic acids, humus and other products Aggregation of mineral particles

Translocations Translocations involve the movement of organic and inorganic materials laterally within a horizon or vertically from one horizon to the another. Movement of water Movement of dispersed fine clay particles Movement of dissolved organic substances The most common translocation agents are water and soil organisms.

Losses Materials are lost from the soil by the following: Drainage and leaching to groundwater Erosion of surface materials Evaporation Plant uptake Microbial decomposition Animals and humans

Combination of Processes Losses Translocations Additions Transformations

Example of the soil forming processes in action Addition of litter and root residues Transformation of plant substances into humus Accumulation of humus enhances water and nutrient capacity (feedback for plant growth) Microorganisms attracted by humus buildup accelerate transformations and cause translocations

Soil Horizon Development A-Horizon development Accumulation of organic matter Clumping of individual soil particles Distinct from parent material and other layers B and C horizon development Carbonic and organic acids are carried by water into soil where dissolve various minerals (transformations) Soluble materials (ions –Ca2+, CO32-, SO42-, etc) are carried by water and precipitate in the soil from upper to lower horizons (translocation) Weathering of primary minerals into secondary minerals Wetting and drying cracks soils and makes structures.

Soil Horizon Development

Soil-Landscape-Ecosystem Relationships The above processes of soil genesis, operating under the influence of the five environmental factors discussed previously gives us a framework for understanding the relationships between particular soils and the landscapes and ecosystems in which the soils exist.