What you’ll learn21 learning objectivesChoose one objective for a focused lesson, or study the complete topic.5.1.1Soil as dynamic system• Own inputs, outputs, storages, flows within larger ecosystemSyllabus objective5.1.2Soil composition• Inorganic and organic components, water, airSyllabus objective5.1.3Soil profiles and horizons• Soils develop a stable, layered structure known as a profile made up of several horizons, produced by interactions within the system over long periods of time• Soil profiles have distinctive horizons that show a transition from more organic components in the upper surface to inorganic below• 2.5 Application of skills• Sample two soils from the subsoil (B horizon): one from a local garden or field, and one from a natural ecosystemSyllabus objective5.1.4Soil system inputs• Soil system inputs include those from dead organic matter and inorganic minerals• Dead organic matter inputs may include, but not be limited to, plant litter, dead animal biomass, manure• Organic mineral inputs may include weathering, deposition or decomposition, precipitation (water with dissolved minerals), gases, air, humidity and solar energy• In managed soil systems, many inputs are anthropogenic: compost, fertilizer, agrochemicals, irrigation, salinizationSyllabus objective5.1.5Soil system outputs• Soil system outputs include losses of dead organic matter due to decomposition, losses of mineral components and loss of energy due to heat loss• Mineral component outputs include wind or water erosion, water and mineral absorption by plant roots, leaching of dissolved plant nutrients and water, diffusion of gases• These outputs can cause the loss or modification of soil components and are different from total loss of soil by erosion; howeverSyllabus objective5.1.6Soil transfers• Transfers occur across soil horizons, into and out of soils• Include: infiltration, percolation, groundwater flow, biological mixing, aeration, erosion and leachingSyllabus objective5.1.7Soil transformations• Transformations within soils can change the components or the whole soil system• Include: decomposition, weathering, nutrient cycling and salinizationSyllabus objective5.1.8Soil flow diagrams• Systems flow diagrams show flows into, out of and within the soil ecosystem• Soil systems are essential for the water, carbon and nitrogen cyclesSyllabus objective5.1.9Soil foundation role• Medium for plant growth (seed bank, water store, nutrients)• Stores nitrogen, phosphorus, potassium• Carbon obtained from atmosphereSyllabus objective5.1.10Soil and biodiversity• Provides habitat and niche for many species• Microorganisms, animals, fungiSyllabus objective5.1.11Soil in element recycling• Role in biogeochemical cycles• Detritivores (earthworms) and saprotrophs (fungi, bacteria) decompose matterSyllabus objective5.1.12Soil texture• Physical make-up of mineral soil• Proportions of sand, silt, clay, humusSyllabus objective5.1.13Texture affects productivity• Influences nutrient retention, water retention, drainage, aerationSyllabus objective5.1.14Soil carbon role• Can act as sinks, stores, or sources• Depends on input rate vs. decomposition rateSyllabus objective5.1.15(HL)—Soil classification• By appearance of whole soil profile• Profile diagrams show transfer/transformation processesSyllabus objective5.1.16(HL)—Soil horizons• O: organic layer• A: mixed layer (topsoil)• B: mineral soil• C: parent rockSyllabus objective5.1.17(HL)—A horizon (topsoil)• Rich in organic matter• Most valuable for plant growth• Most vulnerable to erosion and degradationSyllabus objective5.1.18(HL)—Soil formation factors• Climate, organisms, geomorphology (landscape)• Geology (parent material), timeSyllabus objective5.1.19(HL)—Sand, silt, clay differences• Particle size and chemical properties• Sand/silt: low cation-exchange capacity (CEC)• Clay: high CEC (increases mineral availability)Syllabus objective5.1.20(HL)—Soil property analysis• Sand/silt/clay percentages, organic matter, water• Infiltration, bulk density, colour, pHSyllabus objective5.1.21(HL)—Carbon release from soils• Released as methane or CO₂• Causes: global warming, agriculture, wetland drainage• May lead to tipping pointSyllabus objective
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