Organic Matter & Soil Structure
Organic Matter – Definition & Composition
Derived from living organisms; carbon–based, not mineral.
Typical concentration in Australian soils: organic C (OC).
Relationship: .
Fractions
• Fresh residues (plant/animal)
• Partly decomposed (resistant) residues
• Humus (highly resistant)
• Soil biomass (living micro- & macro-organisms)
Key Roles of Soil Organic Matter (SOM)
Nutrient store: N, P, S held in organic form – released gradually.
Nutrient retention: negative charge provides additional CEC; complexes .
Aggregate binding → improved aeration, infiltration, root penetration.
Energy source for microorganisms (drives nutrient cycling).
Surface protection: mulches reduce crusting, runoff & erosion.
Decomposition & Carbon Loss
Overall reaction: .
Up to of fresh C lost as in first year.
Continuous additions are required to raise SOM levels.
Half-life of SOM Fractions (approx.)
Fresh residues ≈ yr
Soil biomass ≈ yr
Resistant residues ≈ yr
Physically protected ≈ yr
Chemically protected ≈ yr
Mineralisation vs Immobilisation
Mineralisation = release of inorganic from SOM.
Immobilisation = microbial uptake of inorganic nutrients → organic form.
Net effect governed by C:nutrient ratios:
• > → net immobilisation (e.g. straw incorporation).
• < → net mineralisation (e.g. legume green manure).
• Similar thresholds: & ≈ .
Factors Controlling Decomposition Rate
Aeration (O supply)
Temperature: minimal <
Moisture: optimal near field capacity; inhibited when saturated or very dry
pH: optimum ≈ neutral
Toxic compounds, lignin content, high ratios
Cultivation: increases aeration & microbe–substrate contact → faster loss.
Management Effects on SOM
Continuous tillage/cropping → SOM decline; labile fractions lost first.
Continuous vegetative cover (pasture) → SOM maintained/increases, fastest in first 3–4 yr.
Additions (manures, compost, clippings): benefits are short-lived, require regular inputs.
Soil Structure – Definition & Significance
Arrangement of sand, silt & clay into aggregates (peds).
Determines pore size distribution → root growth, water/air movement, trafficability.
Aggregate Formation (Re-working Forces)
Wet–dry & freeze–thaw cycles
Root growth & shrinkage
Soil fauna activity (e.g. earthworms)
Flocculation of clay surfaces via cation effects
Flocculation vs Dispersion
Thin cation layers (high valency , high salt) → flocculation.
Thick layers (dominant , low salt) → dispersion.
Aggregate Size Classes
Macroaggregates > – bound by roots, fungal hyphae, transient polysaccharides.
Microaggregates < – stabilised by clay–organic, Fe/Al oxide & cation bridges.
Causes of Structural Deterioration
Cultivation: excessive frequency, speed, wet/dry extremes, rotary implements.
Water impacts: raindrop/irrigation impact, rapid wetting (slaking), swelling clays, sodic water (dispersion).
Loss of SOM: fewer binding agents, weaker aggregates.
Management to Improve Structure
Maintain/return organic residues; use cover crops & rotations.
Apply gypsum (CaSO) to sodic soils; lime (CaCO) in acidic soils.
Minimise tillage, avoid traffic on wet soils.
Use vegetative or mulch covers to buffer rain impact.
In special cases: add clay to sandy soils (high-value crops).
Describing & Assessing Structure
Field description: type (granular, blocky, platy etc.), size, grade; note structureless single-grain or massive conditions.
Quantitative indicators: bulk density, water-stable aggregates %, infiltration rate, pore distribution, gas diffusion.
Take-Home Points
SOM is small in quantity but central to nutrient cycling, CEC and aggregate stability.
Maintaining SOM requires continual inputs & reduced disturbance.
Aggregate stability hinges on both formation (physical forces, flocculation) and stabilisation (organic & ionic bonds).
Management practices influence both SOM and structure; consider tillage intensity, residue return, cation balance and water quality to sustain soil productivity.
Organic Matter – Definition & Composition
Derived from living organisms, primarily plant and animal residues; it is carbon–based, distinguishing it from mineral components.
Typical concentration in Australian soils: $1\text{–}5\%$ organic C (OC), highly variable depending on climate, vegetation, soil type, and management.
Relationship between Organic Matter (OM) and Organic Carbon (OC): $(\%OM \approx 1.75 \times OC\%)$, indicating that organic matter is approximately carbon.
Fractions, representing different stages of decomposition and stability:
Fresh residues (plant/animal): Undecomposed or recently added organic materials.
Partly decomposed (resistant) residues: Materials undergoing breakdown, still recognizable but altered.
Humus (highly resistant): Stable, amorphous organic material, dark in color, resulting from extensive decomposition.
Soil biomass (living micro- & macro-organisms): The living component of SOM, including bacteria, fungi, protozoa, nematodes, and larger organisms like earthworms.
Key Roles of Soil Organic Matter (SOM)
Nutrient store: Acts as a reservoir for essential plant nutrients like Nitrogen (N), Phosphorus (P), and Sulfur (S), held in organic forms and released gradually through mineralisation.
Nutrient retention: Possesses a high negative charge that provides additional Cation Exchange Capacity (CEC), improving the soil's ability to hold onto positively charged nutrient ions ($NH_4^+$, $K^+$, $Ca^{2+}$). It also complexes micronutrient cations like $Cu^{2+}$, $Zn^{2+}$, and $Fe^{2+}$, making them more available and preventing leaching.
Aggregate binding \rightarrow improved aeration, infiltration, root penetration: SOM glues soil particles (sand, silt, clay) together to form stable aggregates. This creates a more porous soil structure, improving air and water movement, and facilitating root growth.
Energy source for microorganisms (drives nutrient cycling): Provides the necessary carbon and energy for a diverse soil microbial community, which performs vital functions like nutrient mineralisation, organic matter decomposition, and disease suppression.
Surface protection: Surface applied organic materials like mulches reduce the impact of raindrops, preventing surface crusting, decreasing runoff, and minimizing soil erosion.
Decomposition & Carbon Loss
Overall reaction: The general biochemical process during aerobic decomposition is represented as $(CH2O)n + O2 \rightarrow CO2 + H_2O + \text{energy}$. This aerobic respiration releases carbon dioxide to the atmosphere.
Up to $50\%$ of fresh C lost as $CO_2$ in first year: This significant initial loss highlights the rapid turnover of labile carbon fractions.
Continuous additions are required to raise SOM levels: Due to ongoing decomposition, maintaining or increasing SOM requires regular replenishment of organic inputs.
Half-life of SOM Fractions (approx.)
Fresh residues $\approx 0.2$ yr: Very rapid turnover due to high availability of simple carbohydrates and proteins.
Soil biomass $\approx 1.7$ yr: Relatively quick turnover as microbial populations adjust and die off.
Resistant residues $\approx 2.3$ yr: Slower decomposition due to more complex molecules like cellulose and hemicellulose.
Physically protected $\approx 50$ yr: Organic matter trapped within stable aggregates, protected from microbial attack.
Chemically protected $\approx 1980$ yr: Highly recalcitrant organic molecules (e.g., lignin, humic acids) or those strongly adsorbed to mineral surfaces, making them extremely resistant to decomposition.
Mineralisation vs Immobilisation
Mineralisation = release of inorganic nutrients ($NH4^+$, $H2PO4^-$, $SO4^{2-}$) from complex organic forms within SOM by microbial activity, making them plant-available.
Immobilisation = microbial uptake and conversion of inorganic nutrients into organic forms within their biomass, temporarily making them unavailable to plants.
Net effect governed by C:nutrient ratios. Microbes require a balanced diet of carbon (energy) and nutrients. If there's an excess of carbon relative to a nutrient, microbes will draw that nutrient from the soil solution (immobilise) to build their cells:
$C:N$ > $10:1$ (typically $C:N > 25:1$ often cited for net immobilisation) $\rightarrow$ net immobilisation (e.g. high-carbon straw incorporation). Microbes consume available inorganic N to break down high C material.
$C:N$ < $10:1$ (typically $C:N < 25:1$ often cited for net mineralisation) $\rightarrow$ net mineralisation (e.g. low-carbon legume green manure). Excess N is released as microbes break down low C material.
Similar thresholds: $C:P$ & $C:S \approx 100:1$. If the C:P or C:S ratio of the organic input is higher than this, net immobilisation of P or S will occur.
Factors Controlling Decomposition Rate
Aeration ($O_2$ supply): Optimal decomposition occurs under aerobic conditions, as most decomposers are aerobic; anaerobic conditions slow down decomposition significantly, leading to accumulation of organic matter (e.g., peatlands).
Temperature: Microbial activity is temperature-dependent; minimal decomposition occurs below $6^{\circ}\text{C}$, and rates generally double for every $10^{\circ}\text{C}$ increase up to an optimum ($25\text{–}35^{\circ}\text{C}$). Extreme temperatures can inhibit activity.
Moisture: Optimal near field capacity (around $60\%$ water-filled pore space); inhibited when saturated (lack of oxygen) or very dry (microbial dormancy).
pH: Optimum $\approx$ neutral (pH $6\text{–}8$) for most microbial activity. Extremely acidic or alkaline conditions reduce microbial diversity and activity.
Toxic compounds, lignin content, high $C:N$ ratios: Presence of inhibitory substances or complex, resistant organic compounds (like lignin) and imbalanced C:N ratios slow down decomposition.
Cultivation: Increases aeration and microbe–substrate contact, leading to faster loss of SOM due to enhanced decomposition.
Management Effects on SOM
Continuous tillage/cropping: Leads to SOM decline, particularly the labile fractions, due to increased aeration, disruption of aggregates, and enhanced microbial activity that consumes organic matter faster than it is replenished.
Continuous vegetative cover (pasture): SOM is maintained or increases due to continuous root exudates, plant residue inputs, and reduced soil disturbance. Fastest accumulation occurs in the first $3\text{–}4$ yr under pasture conversion.
Additions (manures, compost, clippings): Provide concentrated inputs of organic matter and nutrients. Benefits are often short-lived if inputs are not regular, as the less stable components are rapidly decomposed. Requires consistent inputs for sustainable SOM build-up.
Soil Structure – Definition & Significance
Arrangement of individual sand, silt, and clay particles into stable aggregates (peds) of various shapes and sizes, separated by planes of weakness.
Determines pore size distribution $\rightarrow$ root growth, water/air movement, trafficability. Good structure ensures sufficient macropores for aeration and water infiltration, and micropores for water retention, supporting healthy root development and facilitating farm operations.
Aggregate Formation (Re-working Forces)
Wet–dry & freeze–thaw cycles: Physical forces from wetting/drying or freezing/thawing cause expansion and contraction that break down large clods into smaller aggregates.
Root growth & shrinkage: Roots physically push soil particles together, and their subsequent decay leaves channels. Root exudates also contribute to binding.
Soil fauna activity (e.g. earthworms): Earthworms and other soil organisms create burrows (macropores) and mix soil with their excretions (casts), which are rich in organic glue and enhance aggregate stability.
Flocculation of clay surfaces via cation effects: Clay particles, with their negative charges, attract positively charged cations ($Ca^{2+}, Mg^{2+}, K^+, Na^+$). When polyvalent cations ($Ca^{2+}, Al^{3+}$) are present, they can neutralize charges and bridge between clay particles, causing them to clump together (flocculate) rather than repel (disperse).
Flocculation vs Dispersion
Thin cation layers (high valency $Ca^{2+}, Al^{3+}$, high salt concentration) $\rightarrow$ flocculation: Polyvalent cations effectively neutralize the negative charges on clay surfaces, reducing the repulsive forces between clay particles and allowing them to aggregate.
Thick layers (dominant $Na^+$, low salt concentration) $\rightarrow$ dispersion: $Na^+$ is a monovalent cation that is weakly held to clay surfaces. In the absence of sufficient divalent cations or high salt, the diffuse double layer around clay particles expands, leading to repulsion and dispersion of clay particles, which clogs pores.
Aggregate Size Classes
Macroaggregates > $250\,\mu m$ ($0.25$ mm): Larger aggregates, often visible, primarily bound by roots, fungal hyphae (especially arbuscular mycorrhizal fungi), and transient polysaccharides from microbial activity.
Microaggregates < $250\,\mu m$: Smaller, more stable aggregates, stabilised by strong bonds involving clay–organic matter complexes, iron/aluminum oxide coatings, and cation bridges.
Causes of Structural Deterioration
Cultivation: Excessive frequency, high speed, working soil when too wet or too dry, and use of rotary implements can physically break down aggregates, leading to compaction and reduced porosity.
Water impacts: Raindrop/irrigation impact causes direct physical disruption of surface aggregates. Rapid wetting (slaking) can cause aggregates to break apart due to trapped air and differential swelling. Swelling clays and sodic water (dispersion from strong $Na^+$ presence) also lead to aggregate breakdown and pore collapse.
Loss of SOM: Reduces the amount of binding agents (e.g., glues, fungal hyphae) that stabilise aggregates, resulting in weaker and less stable soil structure.
Management to Improve Structure
Maintain/return organic residues; use cover crops & rotations: Continuous input of organic matter is crucial for supplying binding agents and promoting microbial activity that forms and stabilises aggregates.
Apply gypsum ($CaSO4$) to sodic soils; lime ($CaCO3$) in acidic soils: Gypsum provides $Ca^{2+}$ to replace $Na^+$ on clay particles, promoting flocculation in sodic soils. Lime raises pH in acidic soils, improving nutrient availability and sometimes structure by increasing $Ca^{2+}$ availability.
Minimise tillage, avoid traffic on wet soils: Reducing physical disturbance preserves existing aggregates and prevents compaction. Working wet soil destroys structure by smearing and compacting.
Use vegetative or mulch covers to buffer rain impact: Protects the soil surface from the destructive energy of raindrops, preventing crusting and preserving surface aggregation.
In special cases: add clay to sandy soils (high-value crops): For very sandy soils lacking cohesion, adding clay can improve water and nutrient retention and provide a base for aggregate formation, though this is a significant and costly amendment.
Describing & Assessing Structure
Field description: Qualitative assessment based on visual and tactile properties. Describing aggregate type (e.g., granular, blocky, platy, prismatic, columnar), size (fine, medium, coarse), and grade (strength or distinctness of peds; weak, moderate, strong); noting structureless conditions like single-grain (sands) or massive (compacted clays).
Quantitative indicators: Laboratory or in-situ measurements providing numerical values:
Bulk density: Mass of dry soil per unit volume; lower values indicate better structure/porosity.
Water-stable aggregates %: Proportion of aggregates that retain their integrity when submerged in water, indicating resistance to dispersion.
Infiltration rate: How quickly water enters the soil, reflecting pore connectivity.
Pore distribution: Measurement of the size and continuity of pores, crucial for air and water flow.
Gas diffusion: The rate at which gases (like $O2$ and $CO2$) move through the soil, reflecting aeration.
Take-Home Points
SOM is small in quantity (typically $1\text{–}5\%$ OC) but central to soil health, fulfilling critical roles in nutrient cycling (store and retention), Cation Exchange Capacity (CEC), energy provision for microbes, and aggregate stability.
Maintaining SOM requires continual inputs of organic residues and reduced disturbance (e.g., minimal tillage) to balance decomposition losses.
Aggregate stability hinges on both formation (driven by physical forces like wet–dry cycles, root growth, and flocculation via cation effects) and stabilisation (primarily through organic matter binding and strong ionic bonds).
Management practices profoundly influence both SOM levels and soil structure; consider factors like tillage intensity, continuous residue return, appropriate cation balance ($Ca^{2+}$ vs $Na^+$), and water quality to sustain long-term soil productivity.