Table of Contents
Soil is more than just “dirt” under our feet. For organisms, it is a complex, changing environment with specific physical, chemical, and biological properties. These soil factors strongly influence where plants and soil-dwelling animals can live, how well they grow, and how entire ecosystems function.
In this chapter, the focus is on the abiotic (non-living) soil properties and how they affect organisms. Living components of soil (roots, fungi, bacteria, animals) are important too, but they are treated as biotic factors elsewhere.
Formation and Basic Properties of Soil
Soil develops from rocks and organic material over long time spans. Weathering, organisms, climate, and relief (slope, exposure) together create different soil types. For organisms, several properties of the resulting soil are particularly important:
- Grain size and structure (sand, silt, clay)
- Soil texture and porosity (pore size and volume)
- Water-holding capacity and air content
- Temperature behavior
- Chemical composition (nutrients, salts, toxic substances)
- Soil pH
- Organic matter (humus) content
These factors do not act in isolation; they combine to form a complex “soil environment” to which organisms must be adapted.
Physical Soil Factors
Soil Texture and Structure
Soil texture describes the proportions of sand, silt, and clay:
- Sandy soils
- Large particles, large pores
- Water and dissolved nutrients drain quickly (low water- and nutrient-holding capacity)
- Warm up and dry out fast
- Favor plants and animals that tolerate drought and nutrient-poor conditions (e.g., heaths, some grasses, pioneer plants, many burrowing insects)
- Clay soils
- Very small particles, many tiny pores
- Store water and nutrients well
- Poor aeration; can become waterlogged
- Warm up slowly
- Favor plants adapted to periodically wet, oxygen-poor conditions; soil animals that tolerate low oxygen or live in surface layers
- Loam soils
- Balanced mix of sand, silt, and clay
- Good water and nutrient storage, yet reasonably well-aerated
- Often particularly productive, supporting high plant biomass and diverse soil communities
Soil structure refers to how particles aggregate into crumbs or clumps:
- Well-structured, crumbly soils contain stable aggregates with many interconnected pores
- They are easier to root through and to inhabit for soil animals (earthworms, arthropods)
- Poor structure (compacted, cloddy, crusted) reduces pore space, impedes roots, and restricts soil fauna to surface or cracking zones
Human activities such as plowing, heavy machinery, or overgrazing can degrade soil structure, which, in turn, changes habitat conditions for many organisms.
Soil Porosity, Air Content, and Compaction
Pores in soil are essential for gas and water movement:
- Large pores (macropores): allow rapid drainage and air exchange
- Small pores (micropores): hold water against gravity
The ratio and continuity of these pores affect:
- Soil aeration: Roots and many soil organisms require oxygen and must excrete carbon dioxide. Poorly aerated soils lead to:
- Reduced root growth and function
- Shift from aerobic to anaerobic microorganisms
- Accumulation of toxic by-products (e.g., reduced iron or manganese compounds, hydrogen sulfide)
- Soil compaction:
- Heavy machinery, trampling, or repeated tillage can compress soil, destroying large pores
- Reduced aeration and increased bulk density make it harder for roots to penetrate
- Many soil animals (e.g., earthworms, beetle larvae) lose habitat in deeper layers and concentrate in thin upper horizons
Organisms show adaptations to these conditions—for example, plants with shallow but widespread roots in compacted soils, or animals that specialize in living in cracks and surface litter.
Soil Water Content and Water Availability
Soil water is not simply “wet” or “dry.” Two aspects are important:
- Total water content
- Water availability to organisms
After a rain:
- Water first occupies large pores and drains quickly due to gravity
- Some water remains in small pores, held by surface tension; this is the main water supply for plants between rainfall events
Plants and soil organisms respond to:
- Drought-prone soils (e.g., sandy):
- Strong fluctuations between wet and dry periods
- Favor drought-tolerant species (deep-rooters, plants that can close stomata quickly, seeds that wait through dry periods)
- Many soil animals enter resting stages (cysts, cocoons) or move deeper where moisture persists
- Waterlogged or saturated soils:
- Large pores are filled with water; air is displaced
- Oxygen becomes limiting; anaerobic conditions can develop
- Specialized plants (e.g., marsh plants) form:
- Aerenchyma (air-filled tissues) to transport oxygen to roots
- Shallow or adventitious roots near the surface
- Soil animal communities shift: earthworms and many insects decline, while some aquatic or semi-aquatic larvae, nematodes, and anaerobic microbes dominate
- Field capacity:
- The water content after excess water has drained away
- Represents a relatively stable supply for plants
- Soils with high field capacity buffer drought better and support more continuous biological activity
Soil Temperature
Soil temperature often differs from air temperature. It is influenced by:
- Soil color and surface cover (dark soils and vegetation absorb more heat)
- Water content (wet soils warm and cool more slowly)
- Texture (sandy soils fluctuate more than clayey or humus-rich soils)
- Depth (temperature becomes more stable with depth)
Biological consequences:
- Seed germination: Many seeds require a minimum soil temperature and sometimes specific temperature cues for germination
- Root activity: Nutrient uptake and growth slow in cold soils; warm soils (within species-specific limits) promote faster root growth
- Microbial processes: Decomposition and mineralization rates increase with temperature (up to an optimum), feeding back on nutrient availability
- Soil fauna:
- Many invertebrates avoid extreme soil temperatures by moving deeper or entering dormant stages
- Overwintering stages (eggs, pupae, resting cysts) often occur at specific depths where temperature fluctuation is reduced
In mountain or polar regions, permafrost soils (permanently frozen subsurface layers) severely restrict root depth and soil biota to a thin active layer that thaws only in summer.
Chemical Soil Factors
Soil pH
Soil pH describes how acidic or alkaline the soil solution is. It has far-reaching effects:
- Nutrient availability:
- Certain nutrients (e.g., phosphate, iron, manganese) become less available at very high or very low pH
- In acidic soils, some metals (e.g., aluminum, manganese) can reach toxic levels
- Nitrogen transformations by microbes (e.g., nitrification) are sensitive to pH
- Soil organisms:
- Bacteria generally prefer neutral to slightly alkaline conditions
- Many fungi tolerate lower pH values and dominate in acidic forest soils
- Earthworm diversity and abundance often decrease in strongly acidic soils
- Plants and pH specialization:
- Acidophilic (acid-loving) plants: adapted to low pH and often low nutrient availability, e.g., some heathland species
- Calciphilic (lime-loving) plants: prefer soils with higher pH and calcium carbonate, common on limestone bedrock
- Many crops grow best in a moderate pH range and decline outside it
Soil pH also interacts with pollution: acidic conditions can mobilize harmful metals and affect organism health.
Nutrient Content and Fertility
Soils contain essential plant nutrients, including:
- Macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S)
- Micronutrients (trace elements): iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), etc.
The availability (not just total quantity) of these nutrients determines soil fertility.
- Nutrient-poor soils:
- Lead to slow growth and favor stress-tolerant plants with efficient nutrient use (e.g., evergreen leaves, slow turnover)
- Foster specialized mutualisms, such as mycorrhizal fungi that help plants access limited nutrients
- Support soil food webs adapted to low resource input (slower decomposition, long-lived organisms)
- Nutrient-rich soils:
- Support fast-growing, competitive plant species
- Stimulate microbial activity and rapid decomposition of organic matter
- Can lead to dense plant stands that shade out less competitive species, reducing plant diversity but perhaps increasing animal biomass
- Imbalances or extreme nutrient conditions:
- Excess nitrogen deposition (e.g., from air pollution, fertilization) can change species composition, favoring nitrophilous (N-loving) plants
- Deficiencies of specific micronutrients can limit growth or cause visible disorders in plants (chlorosis, necrosis), indirectly affecting herbivores and other trophic levels
Salinity
Salts in soil (often measured as electrical conductivity of the soil solution) affect organisms by:
- Lowering the water potential, making it harder for plants to absorb water
- Introducing specific ion toxicities (e.g., Na⁺, Cl⁻ at high concentrations)
Salty soils occur naturally (e.g., in coastal areas, arid regions) or are caused by human activities (irrigation without adequate drainage).
Effects on organisms:
- Most crop plants and many wild species are salt-sensitive:
- Show reduced growth, leaf burn, or die in high salinity
- Halophytes (salt-tolerant plants) are specially adapted to salty soils:
- Strategies include salt-excreting glands, storage of salts in vacuoles, succulent tissues to dilute salts
- Soil microbial and animal communities change:
- Sensitive species disappear, halotolerant organisms become dominant
In irrigated agriculture, salinization of soils is a major environmental problem that alters entire plant communities and soil biota.
Toxic Substances and Contaminants
Soils may contain naturally occurring toxic elements or pollutants from human activities, for example:
- Heavy metals (lead, cadmium, mercury, arsenic)
- Excessive copper or zinc near industrial or mining sites
- Organic pollutants (pesticides, hydrocarbons, persistent industrial chemicals)
Biological consequences:
- Direct toxicity to plants (growth inhibition, chlorosis, root damage) and soil organisms (reduced reproduction, mortality, altered behavior)
- Bioaccumulation of certain substances along food chains, affecting higher trophic levels
- Selection for tolerant genotypes:
- Some plants evolve resistance to high metal concentrations and can colonize contaminated soils
- Microorganisms may gain metabolic pathways to break down or immobilize pollutants
These changes can drastically simplify or alter soil communities and aboveground ecosystems.
Organic Matter and Humus
Organic matter (dead plant and animal material) is slowly decomposed, transformed, and stabilized as humus.
Key functions for organisms:
- Nutrient reservoir:
- Humus stores nutrients and releases them gradually through mineralization
- Water-holding capacity:
- Humus-rich soils retain more water than mineral soils with low organic matter
- Soil structure:
- Humus helps form stable aggregates, improving porosity and root penetration
- Buffering capacity:
- Humus can bind toxins and buffer pH fluctuations, moderating extreme chemical conditions
Ecological implications:
- Humus-poor soils:
- Low biological activity, low fertility
- Support sparse vegetation and limited soil fauna
- Humus-rich soils:
- Support dense root systems, diverse microbial communities, and rich soil fauna (earthworms, mites, springtails, insects)
- Provide a more buffered environment against drought and nutrient pulses
The balance between input (litter, root exudates) and decomposition (driven by climate and organisms) determines humus levels and therefore many soil factors simultaneously.
Vertical Differentiation: Soil Horizons and Microhabitats
Soils are typically stratified into horizons:
- Surface litter layer (O horizon)
- Humus-rich topsoil (A horizon)
- Mineral subsoil (B horizon)
- Weathered parent material (C horizon)
- Bedrock (R horizon)
Each horizon offers different abiotic conditions:
- Topsoil: higher organic matter, better aeration, more temperature fluctuations
- Subsoil: lower organic content, often more stable moisture and temperature, but reduced oxygen and sometimes higher compaction or salinity
- Deeper layers: can be anoxic, cold, or chemically harsh
Organisms distribute themselves according to their needs:
- Plant roots often concentrate in the topsoil where nutrients are most abundant, with some species sending deeper roots mainly for water
- Many soil animals and microbes are most active in the litter and upper horizons
- Specialized organisms inhabit deeper or more extreme layers (e.g., anaerobic microbes, some deep-burrowing animals)
Thus, even within a single soil profile, there are multiple microhabitats shaped by abiotic soil factors.
Soil Factors as Ecological Filters
Taken together, soil texture, structure, water and air balance, temperature, pH, nutrients, salinity, toxic substances, and organic matter act as ecological filters:
- Only organisms with suitable adaptations can establish and persist
- Different combinations of soil factors create characteristic soil types and habitat conditions
- Changes in soil properties (natural or human-induced) can rapidly alter community composition and ecosystem functioning
For plants and soil-dwelling animals, these abiotic soil factors are often as decisive as climate or other abiotic influences in determining their ecological niches and geographic distributions.