IB Environmental systems and societies 5.1B Contribution of Soil Notes

Soil Underpins Terrestrial Life and Ecosystems

1. Soil gives plants anchorage, water, oxygen, and essential nutrients.
2. It also acts as a seed bank, a water reservoir, a nutrient store, and a habitat.
3. Soil supplies almost every mineral nutrient plants need, working through the carbon, nitrogen, phosphorus, and water cycles.

Soil as a Medium for Plant Growth

1. Anchorage
   1. Mineral and organic particles give roots the resistance they need to hold the plant upright.
2. Water
   1. Water held in pore spaces supports photosynthesis, nutrient transport, and turgor.
3. Oxygen
   1. Roots respire aerobically, so compacted or waterlogged soils that block oxygen slow their growth.
4. Nutrients
   1. Plants take up nitrogen, phosphorus, potassium, and other minerals from the soil solution.

Soil as a Nutrient Store

1. Nutrient availability depends on pH, organic matter, and microbial activity.

1. Soils also supply calcium for cell walls, magnesium for chlorophyll, and sulfur for amino acids.

Soil as a Seed Bank

1. Buried seeds can stay viable for years or even centuries until conditions allow germination.
2. Germination needs water, oxygen, and a suitable temperature.
3. Seed depth matters: shallow seeds risk drying out, while deep seeds may exhaust their reserves before reaching the surface.

Soil as a Biodiverse Habitat

1. Soil is one of Earth's most biodiverse habitats, holding bacteria, fungi, protozoa, nematodes, arthropods, worms, and small mammals.
2. Variations in moisture, temperature, particle size, and organic content create many microhabitats.
3. This biodiversity drives nutrient cycling, decomposition, and structure formation.

Why Soil Biodiversity Matters

1. Organisms link into food webs of producers, decomposers, detritivores, predators, and mutualists.
2. Mycorrhizal networks let plants share nutrients and signals, the so-called "Wood Wide Web".
3. Greater soil biodiversity raises ecosystem stability and resilience.

Soil Drives the Biogeochemical Cycles

1. Soil is where dead organic matter is broken down and its nutrients are returned to living things.
2. Detritivores such as earthworms shred litter into fragments, increasing surface area for microbes.
3. Saprotrophs (fungi and bacteria) then digest these fragments into simple molecules that re-enter the cycles.

Soil carbon storage is covered in detail in 5.1D, and the full nitrogen cycle in Topic 2.3.

Soil in the Nitrogen Cycle

1. Ammonification: bacteria convert organic nitrogen into ammonium.
2. Nitrification: bacteria convert ammonium into nitrites and then nitrates.
3. Denitrification: anaerobic bacteria return nitrates to the air as nitrogen gas.
4. Nitrogen fixation: bacteria such as Rhizobium convert atmospheric nitrogen into ammonium plants can use.

Soil in the Phosphorus Cycle

1. Weathering of rock and decomposition of organic matter release phosphate ions into the soil.
2. Plants take up phosphate, and unlike nitrogen it has no significant gaseous phase.
3. Phosphorus returns to the soil through decomposition, but binding and leaching control how much is available.

Summary of Soil's Contributions

1. Soil supports plants, stores nutrients and seeds, and houses a vast community of organisms.
2. Through decomposition it keeps the carbon, nitrogen, and phosphorus cycles turning.