Soils Provide the Foundation of Terrestrial Ecosystems as a Medium for Plant Growth
Definition
Seed bank
A seed bank is a natural store of viable seeds present in the soil, capable of germinating when favorable conditions arise.
- Soils provide anchorage, water, oxygen, and essential nutrients for plants.
- Soils act as a seed bank, a water reservoir, and a nutrient store for terrestrial ecosystems.
- Soils supply almost all plant nutrients except carbon, which plants obtain from atmospheric CO₂.
- Key nutrients stored in soils include nitrogen (N), phosphorus (P), and potassium (K).
Soils as a Medium for Plant Growth
1. Anchorage
- Strong soil structure stabilizes roots.
- Roots require resistance from mineral and organic particles to anchor the plant.
2. Supply of Water
- Water held in pore spaces supports photosynthesis, nutrient transport, and turgor pressure.
- Soil texture determines water-holding capacity (e.g., clay holds more water than sand).
3. Supply of Oxygen (O₂)
- Roots respire aerobically and require continuous oxygen from soil air spaces.
- Compacted or waterlogged soils restrict O₂ diffusion, slowing root respiration.
4. Nutrient Source
- Plants absorb N, P, K, magnesium, calcium, sulfur, and micronutrients from soil solution.
- Nutrient availability depends on pH, organic matter, and microbial activity.
When asked why soils are essential for terrestrial ecosystems, emphasize anchorage, water, oxygen, and nutrients, and the crucial point that carbon is the only essential element not obtained from soil.
Soil Depth and Root Growth
- Deeper soils allow roots to access more water and nutrients.
- Most nutrients occur in the topsoil (A horizon)
- Shallow soils limit nutrient uptake.
- Restrictions affecting root growth include:
- Mechanical impedance (compacted layers)
- Lack of cracks or pore spaces
- Waterlogging causing oxygen shortage
- Extreme temperatures damaging root tissue
- High aluminium toxicity in acidic soils
- Low nutrient supply
- Phytotoxic chemicals in contaminated soils
Essential Plant Nutrients Stored in Soil
| Nutrient | Function in Plants | Soil Source | Example of deficiency |
|---|---|---|---|
| Nitrogen (N) | Promotes leaf and stem growth (proteins, chlorophyll) | Decomposed organic matter, fertilizers, nitrogen-fixing bacteria | Yellowing leaves, stunted growth |
| Phosphorus (P) | Important for root development, flowering, and fruiting | Weathered rocks, organic matter, phosphate fertilizers | Poor root growth, weak stems |
| Potassium (K) | Regulates water balance, enzyme activation | Clay minerals, fertilizers | Weak stems, scorched leaf edges |
Additional Soil Nutrients
- Calcium (Ca): Strengthens cell walls, prevents diseases (from limestone or gypsum).
- Magnesium (Mg): Essential for chlorophyll production (from dolomite, fertilizers).
- Sulfur (S): Important for amino acids and enzymes (from organic matter, volcanic soil).
Serotinous pine cones only release seeds after wildfires.
Soils as Natural Seed Banks
- Seeds stored in soil remain viable for years to centuries.
- Germination requires water, oxygen, and optimal temperature.
- Seed depth determines germination success:
- Shallow seeds risk drying out before germination.
- Deep seeds may not reach the surface before reserves are exhausted.
- Small seeds must be close to surface due to smaller energy stores.
Soils as Nutrient Reservoirs
- Soils store key nutrients needed for plant function:
- Nitrogen for proteins and nucleic acids
- Phosphorus for DNA, ATP, membranes
- Potassium for water regulation and enzyme activation
- Nutrient supply depends on:
- Weathering of parent rock
- Decomposition of organic matter
- Microbial transformations (e.g., nitrification, mineralization)
- Soil pH influencing solubility and cation exchange
- Humus increasing nutrient-holding capacity
