Soil Ecology: Composition, Formation & Components

Soil Ecology

• The study notes here will cover soil composition, soil formation, factors affecting soil formation, soil texture, structure, profile and organic matter

The Soil

• Soil makes life on Earth possible
• Soil is a thin covering over land
• The outermost layer of the planet is soil
• Soil contains living, breathing and dying beneficial organisms
• Soil is a regenerative living system
• Soil contains Abiotic components, like minerals, air, and water
• Soil contains Biotic components like living organisms and organic matter, supporting plant growth

Components of Soil

• The proportions of soil components varies with soil types
• Good agricultural soil contains 45% mineral, 25% air, 25% water, and 5% organic matter
• Soil components are needed for good aeration and drainage
• The nature and composition of soil determines which crops can be grown

Minerals and Organic Matter

• Minerals found in soil come from weathering rock fragments
• Organic matter in soil comprises living organisms
• Microorganisms and dead plant material are examples of living organisms found in soil
• Colloidal humus is formed by microorganisms acting on plant litter

Soil and Soil Quality

• Soil organisms are involved in many aspects of soil quality.
• Soil organisms impact structure/aggregation, humification, nitrate leaching and nutrient cycling

Soil Types and Composition

• All soils contain a mixture of soils
• Sandy soil contains 70% sand
• Silty soil contains 80% silt and 15% clay
• Clay soil contains 35%-40% clay

Soil Formation

• Soil formation varies depending on how and when soils were formed
• Some soils form from the weathering of underlying rocks
• These soils end up with the same general chemistry as the original rocks
• Chemical weathering changes the chemical composition of rock
• This makes the rock more likely to fragment or dissolve

Oxidation and Hydrolysis

• Oxidation of exposed rock fragments in the atmosphere results in them being oxidised by combining of oxygen with metals; this makes them susceptible to weathering
• Hydrolysis is when water combines with rocks, making them more readily soluble
• Rain can carry soil fragments/rain is slightly acidic

Erosion and Glaciers

• Wind can move sand
• Dust wears away rock by sandblasting surfaces
• Glaciers move large rocks
• Rocks rub against each other
• This cause surfaces to fall off

Factors affecting soil formation

• Temperature, climate, and moisture causes weathering and leaching
• Wind redistributes sand in arid areas
• The chemical nature of rock influences the rate of weathering
• The size and chemical nature of particles affects the nature of soil

Humus

• Humus consists of organic materials from decayed plants and animals
• Humus is crucial to soil formation
• Humus becomes mixed up with top layers of mineral particles

Soil Bacteria

• Soil bacteria, fungi, burrowing animals and roots of plants are crucial for soil formation
• They bring nutrients from deeper layers of soil to plants roots
• Decomposers improve soil quality

Soil Texture

• Soil texture is usually the first thing determined when a soil is examined
• It is determined in the field by feeling sand particles and estimating silt and clay content
• Flexibility and stickiness of the soil are texture indicators
• Soil texture is related to weathering and parent material, and is determined by the size of mineral particles
• The structural characteristic of the soil relates to soil texture
• Soil texture is the most important physical property of the soil
• The texture affects plant growth
• Soil texture is a classification instrument used in the field by feel, and in the lab
• Identifying soil texture is key to understanding soil behaviour, and how it can be managed
• Soil texture provides information about the fertility potential, capacity to hold water, water flow potential and suitability agriculture or urban uses

Soil Components and Texture

• Soil texture refers to the proportion of sand, silt and clay sizes
• Soil consists of different sized soil particles (combinations)
• It is determined by fractions of each separate soil (sand, silt & clay)
• Particles make up the mineral fraction of the soil / % sand, clay, silt & clay in soil
• Light soil is high in sand relative to clay
• Heavy soil is largely made up of clay

Clay and Particle Size

• Clay determines most soil characteristics
• Clay has the greatest surface area of all soil particles
• One gram of clay is the equivalent of two basketball courts
• Soil particle size influences physical properties of soil
• Particle size greater than 2 mm is also excluded from soil texture
• Particles greater than 2 mm are not soil particles because they give no value to plants

Sand, Silt and Clay

• Sand has the largest soil particles. Sand particles measure 0.05mm - 2mm in diameter
• Sand particles are large enough to be seen easily with the eye
• Silt particles measures 0.002 - 0.05 mm in diameter
• Silt particles are not visible to the naked eye and are about the size of flour particles
• Clay has the smallest particles

Clayey Texture

• Clayey Texture have small particles & large surface area
• Clayey Texture has a high water-holding capacity
• With Clayey Texture, large surface area allows for minerals & water to be maintained in soil
• Clayey Texture has small spaces create small pores for holding water

Clay Particles

• Clay particles have negative electric charges on outer surfaces
• They can reversibly bind cations, which are positively charged
• Potassium (K) and Magnesium are examples of nutrients
• Cation adhesion to soil particles are important for soil fertility

Clay Detriments

• Clay can be undesirable for plants because of poor drainage
• Clay soils often don't have enough oxygen
• Therefore compact clay soils reduce number of soil spaces that can be filled by water & air
• Clay feels sticky when wet & clumpy and individual particles aren't visible
• Clay soils are flat & easily packed together
• Hence movement of water through is reduced
• The most common soil texture in western Northern Cape is Clay

Loamy and Sandy Soil

• Loamy texture is equal parts of sand, clay & silt
• Loamy soil is smooth / silky and greasy if there is plenty of organic matter
• Ideal soil texture for most non-container outside plants
• Ideal agricultural soil
• With good aeration & drainage from large particles
• With high nutrient retention & water-holding capacity
• Has appropriate combination of different soil particle sizes
• Larger particles provide structural support and aeration
• Smaller particles bind into aggregates or clumps
• Sandy texture has large particles with many small spaces in between
• This allows water and air to flow through
• But it drains rapidly, crucial nutrients to lower soil layers which is not within reach by the roots
• It has a Low moisture-holding capacity so is prone to droughts
• Empty free spaces between particles result in free drainage & entry of air

Soil Composition

• Sandy soil feels gritty
• Sandy soil is not cohesive
• Sandy soil don't stick together unless when wet
• Sandy soil has less nutrients for plants than smaller particles
• Soil with larger proportions not good for must plants
• It usually doesn't hold water & mineral ions
• This means the plants are susceptible to drought & mineral deficiencies
• Most common soil composition in eastern Northern Cape

Classifying Soil

• A Textural Triangle classifies the texture class of a soil
• The sides of soil texture triangle are scaled for percentages of sand, silt & clay
• Clay % is read from lower left to upper right
• Silt is read from upper right to lower right
• Sand is right to left
• There are 12 classes of soil texture

Soil Structure

• Soil structure is different from soil texture
• Soil structure is defined by the arrangement of individual particles of sand, silt & clay
• These arrangements are in well defined patterns
• Sandy soil does not attach to one another/granular structure
• Clay soils particles stick to one another to form large aggregates
• Clay sticks together when wet making it difficult to break

Agricultural Soil

• A soil good for agricultural use crumbles/breaks easily
• Need space for water & air
• Air/pore spaces determine water & air content
• 1/2 - 2/3 of spaces contain air after excess water has drained
• Air provides source of oxygen for root cells & soil organisms
• But if most spaces are filled with water & has less oxygen will cause roots to die from oxygen shortage & drown in too much water
• If the soil doesn't have enough moisture, plants will wilt and die from lack of water

Types of Soil Structure

• Granular has a crumb-like appearance
• Particles have a diameter less than 1-10mm. Granular structures also occur in surface layers in A-horizon with high organic material
• Granular structures are more porous and ideal for agriculture
• Blocky structure has irregular in shape and imperfect cubes
• Common where clay has accumulated & there is poor penetration of water and has a diameter size of 5-50mm
• Columnar Soil Structure has Vertical columns with rounded tops.They are 10-100mm wide and is common in soils in dry arid climates where clay has accumulated & high sodium levels
• Prismatic has Vertical columns with flat tops, it is commonin lower horizons

Platy and Single Grained Soil

• Platy Soil Structure has thin and flat plates placed horizontally on top of each other, and is 1-10mm thick
• Platy is common in compacted soils of clay soils
• Single Grained Soil structure is soil broken consisting individual particles not sticking together with loose consistence
• This structure is only mostly common in sandy soils

Soil Profile

• The soil profile is a series of horizontal layers in the soil showing variation in: soil structure, particle size, physical properties like colour, texture temp, also chemical composition pH, cations & anions
• And Amount of organic matter
• Each recognizable layer is the horizon

Soil Layers - O Horizon

• The O Horizon is on the surface and is made up of a layer of litter found in forest soils
• The O Horizon is lacking in agricultural soils because soil is worked to incorporate surface crop residue
• When organic matter decomposes it is incorporated into the A horizon
• O Horizon is made of litter/undecomposed/partially decomposed organic matter

A Horizon

• The A Horizon is the first layer/Topsoil
• It is most fertile with more nutrients & organic matter
• Contains small mineral particles/nutrients mixed with organic matter
• Colour is dark
• Made up of humus
• However, as water moves through A horizon it carries dissolved organic matter & minerals to lower layers, this is leaching
• Its thickness can vary from less than a centimeter on steep mountain slopes to over a meter in the rich grasslands

E Horizon

• The E horizon is below A horizon
• Colour is lighter
• Due to dark materials such as iron compounds leaching
• Contains few nutrients & little organic matter
• Water flows down through the soil dissolving and transporting nutrients to the B horizon layer
• Not found in all soils

B Horizon

• The B Horizon is a Subsoil, containing less organic materials, less organisms like clay minerals from topsoil & more nutrients
• A valuable source of plant nutrients as it is a developed root system
• Amount of leaching is dependent on rainfall
• Examples of grasslands with little rainfall produce poorly developed B horizon
• Soils in woodlands high rainfall produces develop good B horizons

C Horizon

• The C Horizon is below the subsoil & rests on the bedrock
• Consists of weathered parent material with no organic materials
• If parent material is limestone it will neutralize acids, but not if it is granite rock
• Chemical composition of minerals determines soil pH

R Horizon

• The R Horizon os the bedrock

Soil Organic Matter

• Soil organic matter includes includes living organisms and tissues, materials produced by soil organisms, plant/animal residues, fresh residue and well-decomposed material
• Soil organic matter refers to substances containing carbon
• Soil organic matter consists mainly of living organisms/tissues in the soil, materials produced by soil organisms, plants/animals residues at different stages of decomposition and fresh or well-decomposed residue
• The residues include plant and animal material

Fertile organic matter

• Productive agricultural soils have 3-6% organic matter
• Contributes to soil productivity & helps provide structure to all the smaller particles
• Prevents compaction & can increase in root growth
• Enables faster water absorption, more water retention resisting erosion
• Attracts & holds many soluble available plant nutrients
• Hence nutrients are retained & are available to plants
• Leads to an increase in numbers & types of microorganisms which are catalysts for nutrient-releasing processes
• Primary source of carbon, providing energy/nutrients
• Improves activity of microorganisms in the soil and enhance biodiversity

Sources of organic matter

• Fresh organic matter may include weed seeds and available forms of nutrients (leaching & decaying)
• Old, decayed plant materials that can not be identified becomes humus
• Active breaking down over short time
• Component breaking down over a long time

Factos increasing degradation

• Soil moisture means faster degradation
• And Enhances microbial activity for degradation
• Temperature increase decomposition
• Organic matter decomposes slowly at lower temperature (under 2°)
• High microbial processes
• Soil temperature between 2°C-38°C increases organic matter

Soil Texture and oxygen

• Aeration enables respiration
• Soil texture also impacts this
• Breakdown speeds up when it is sandy, breaks down more

Farming Practices

• Some farming practices can lead to rapid loss of organic matter such as fertilizers
• Therefore, only apply when needed to have a limited impact
• Some soil microorganisms can be stimulated by oxygen so be weary if too many
• Incorporating excessive amounts of fresh organic matter may over stimulate the soil food web & decreasing organic matter
• If farmers grow the same crop all the time can decrease bacteria

Measuring Soil

• Measuring soil organic matter consists off
• Loss on ignition and burning it by 400°C
• Entails measuring weight lost by an oven-dried (105°C) soil sample
• Soil Organic Matter that has burnt away

PH

• Is a measurement alkalinity
• Can be meausred using in Scale from, is mostly acidic
• Most organic matter is acidic
• Ideal pH for most ornamental plants / lawn / grasses is 5.5 to 7.0
• Therefore pH is important because : affects plants and nutrients avaliability
• and changes solutbity
• 6%-65 can be toxic to plants sometimes

Composting and Vermicomposting

• Composting is process of materials
• This is organic materials for humans and plants can be good or bad depending pathogens
• At high temp. (above 50)
• temperatures break down materials such as potienis so orgnaism can not function
• Vermitinsg has earthworms
• Food scarps Earthworm waste is full of nutrients and organic ways 60%