5.1B Contribution of soil Notes

Soils Provide the Foundation of Terrestrial Ecosystems as a Medium for Plant Growth

1. Soils provide anchorage, water, oxygen, and essential nutrients for plants.
2. Soils act as a seed bank, a water reservoir, and a nutrient store for terrestrial ecosystems.
3. Soils supply almost all plant nutrients except carbon, which plants obtain from atmospheric CO₂.
4. Key nutrients stored in soils include nitrogen (N), phosphorus (P), and potassium (K).

Soils as a Medium for Plant Growth

1. Anchorage

1. Strong soil structure stabilizes roots.
2. Roots require resistance from mineral and organic particles to anchor the plant.

2. Supply of Water

1. Water held in pore spaces supports photosynthesis, nutrient transport, and turgor pressure.
2. Soil texture determines water-holding capacity (e.g., clay holds more water than sand).

3. Supply of Oxygen (O₂)

1. Roots respire aerobically and require continuous oxygen from soil air spaces.
2. Compacted or waterlogged soils restrict O₂ diffusion, slowing root respiration.

4. Nutrient Source

1. Plants absorb N, P, K, magnesium, calcium, sulfur, and micronutrients from soil solution.
2. Nutrient availability depends on pH, organic matter, and microbial activity.

Soil Depth and Root Growth

1. Deeper soils allow roots to access more water and nutrients.
2. Most nutrients occur in the topsoil (A horizon)
3. Shallow soils limit nutrient uptake.
4. Restrictions affecting root growth include:
   1. Mechanical impedance (compacted layers)
   2. Lack of cracks or pore spaces
   3. Waterlogging causing oxygen shortage
   4. Extreme temperatures damaging root tissue
   5. High aluminium toxicity in acidic soils
   6. Low nutrient supply
   7. Phytotoxic chemicals in contaminated soils

Essential Plant Nutrients Stored in Soil

Additional Soil Nutrients

1. Calcium (Ca): Strengthens cell walls, prevents diseases (from limestone or gypsum).
2. Magnesium (Mg): Essential for chlorophyll production (from dolomite, fertilizers).
3. Sulfur (S): Important for amino acids and enzymes (from organic matter, volcanic soil).

Soils as Natural Seed Banks

1. Seeds stored in soil remain viable for years to centuries.
2. Germination requires water, oxygen, and optimal temperature.
3. Seed depth determines germination success:
   1. Shallow seeds risk drying out before germination.
   2. Deep seeds may not reach the surface before reserves are exhausted.
   3. Small seeds must be close to surface due to smaller energy stores.

Soils as Nutrient Reservoirs

1. Soils store key nutrients needed for plant function:
   1. Nitrogen for proteins and nucleic acids
   2. Phosphorus for DNA, ATP, membranes
   3. Potassium for water regulation and enzyme activation
2. Nutrient supply depends on:
   1. Weathering of parent rock
   2. Decomposition of organic matter
   3. Microbial transformations (e.g., nitrification, mineralization)
   4. Soil pH influencing solubility and cation exchange
   5. Humus increasing nutrient-holding capacity

Soils as Biodiverse Habitats

1. Soils host one of the most biodiverse ecosystems on Earth, containing bacteria, fungi, algae, protozoa, insects, nematodes, arthropods, annelids, and small mammals.
2. Soil conditions, moisture, temperature, particle size, organic content, create diverse microhabitats.
3. Many soil organisms are unknown or unclassified, especially microorganisms.
4. Soil biodiversity drives essential processes such as nutrient cycling, decomposition, and soil structure formation.

Organisms in Soil

Soil as a Habitat & Niche

1. Shelter & Protection: Maintains stable temperature and moisture levels, shielding organisms from extreme conditions.
2. Food Source: Organic matter from decaying plants and animals serves as nutrients for microorganisms and larger species.
3. Nutrient Recycling: Decomposers break down waste and return essential nutrients to the soil.
4. Symbiotic Relationships: Fungi (e.g., mycorrhizae) and bacteria (e.g., Rhizobium) assist plants in absorbing nutrients.

Soils as Structural Ecosystems

1. Soil organisms interact to form complex food webs, linking producers, decomposers, detritivores, predators, parasites, and mutualists.
2. Mycorrhizae networks facilitate nutrient sharing and communication between plants (“Wood Wide Web”).
3. Soil organisms create microclimates that regulate moisture and nutrients.
4. Soil biodiversity enhances ecosystem stability and resilience.

Soils and Their Role in Biogeochemical Cycles

1. Soils are essential in the carbon, nitrogen, phosphorus, and hydrological cycles.
2. Major input to soils comes from dead organic matter, including leaf litter, roots, and dead organisms.
3. Detritivores (e.g., earthworms) break organic matter into smaller fragments, increasing surface area for microbial action.
4. Saprotrophs (fungi, bacteria) digest organic compounds into simpler molecules that can re-enter nutrient cycles.
5. Cycling prevents nutrient loss, regulates plant productivity, and maintains ecosystem functioning.

Key Processes in Soil Recycling

Soil’s Role in Major Biogeochemical Cycles

Soils in the Carbon Cycle

1. Soils store more carbon than the atmosphere and vegetation combined.
2. Decomposition releases carbon dioxide through respiration of soil organisms.
3. Formation of humus stores carbon long-term.
4. When plant matter accumulates faster than decomposition occurs, carbon sequestration increases.

Soils in the Nitrogen Cycle

1. Ammonification: Bacteria convert organic nitrogen into ammonium.
2. Nitrification: Nitrifying bacteria convert ammonium into nitrites and nitrates.
3. Denitrification: Anaerobic bacteria convert nitrates into nitrogen gas, returning it to the atmosphere.
4. Nitrogen fixation: Symbiotic bacteria (e.g., Rhizobium) convert atmospheric nitrogen into usable ammonia.

Soils in the Phosphorus Cycle

1. Decomposition releases phosphate ions from organic matter.
2. Weathering of rocks supplies phosphates to soil.
3. Phosphates are taken up by plants but are not returned to the atmosphere.
4. Soil binding and leaching regulate phosphorus availability.

Role of Detritivores & Saprotrophs in Soil Nutrient Cycling

1. Earthworms: Increase soil aeration and mix nutrients.
2. Fungi: Break down complex organic compounds like lignin and cellulose.

Threats to Soil Nutrient Recycling

1. Deforestation → Reduces organic matter input, decreasing soil fertility.
2. Soil Erosion → Washes away topsoil containing nutrients and decomposers.
3. Climate Change → Alters decomposition rates due to temperature and moisture shifts.
4. Excess Fertilizer Use → Can disrupt natural cycles and lead to pollution.