How human activities affect soil fertility — AQA A-Level Environmental Science

Soil fertility is determined by a dynamic balance of physical structure, biological activity, and chemical nutrient availability. Human activities including intensive arable farming, livestock grazing, deforestation, irrigation, and urbanization significantly alter these natural parameters, often degrading the capacity of soils to sustain vegetative growth.

Within AQA A-Level Environmental Science, this topic bridges fundamental biogeochemical cycles and terrestrial ecosystem dynamics with contemporary challenges in global food security and sustainable land management. Mastering this topic requires understanding the mechanistic links between human interventions, soil property degradation, and restorative conservation practices.

• →Soil organic matter (SOM) and humus: Crucial for soil crumb structure, cation exchange capacity (CEC), moisture retention, and sustaining beneficial decomposers.
• →Physical degradation mechanisms: Compaction via machinery or overgrazing reduces infiltration and aeration, causing accelerated surface runoff and soil erosion by wind or water.
• →Chemical degradation mechanisms: Over-fertilisation causes eutrophication, acidification, and nutrient imbalance; over-irrigation in arid regions leads to salinisation via capillary action.
• →Biological degradation: Removal of crop residue and pesticide applications decrease biodiversity of soil biota (earthworms, mycorrhizae, nitrifying bacteria), slowing down organic decomposition and nutrient recycling.

• States that aeration increases air-filled pore space and oxygen availability in soil.
• Explains how ploughing loosens and opens compacted soil to improve gas exchange.
• Explains how drainage removes excess water so air re-enters pore spaces and aerobic conditions return.
• Links improved aeration to root respiration and aerobic microbial activity that supports fertility.

• 💡Check whether an option describes adding air or adding water before choosing it.
• 💡For drainage, trace the sequence: water leaves the pores, then air enters.
• 💡Consider both the benefit and the possible cost of each activity when options present trade-offs.
• 💡Use accurate terminology: write 'cation exchange capacity', 'crumb structure', 'macropores', and 'nitrifying bacteria' rather than vague terms like 'soil quality' or 'good nutrients'.
• 💡Structure anthropogenic impact answers into physical, chemical, and biological impacts to ensure comprehensive mark scheme coverage.
• 💡When explaining salinisation, always explain the direction of water movement via osmosis from the plant roots to the hypertonic soil solution.

• Confuses aeration with irrigation; recognise that aeration concerns air in pore spaces, not adding water.
• Assumes drainage always improves soil; note that over-drainage can dry soil and lower the water table.
• Describes ploughing as only adding nutrients; explain that its main aeration effect is opening pore space for gas exchange.
• Thinking synthetic NPK fertilizers improve overall soil health: They supply dissolved inorganic ions for plant growth but do not rebuild soil organic matter, humus, or soil crumb structure, and can suppress native mycorrhizal associations.
• Believing soil erosion and soil fertility loss are distinct, unrelated issues: Erosion preferentially strips the nutrient-dense topsoil layer, which contains the highest concentrations of organic matter and microbial life.
• Assuming irrigation always improves agricultural fertility: In semi-arid regions with poor drainage, irrigation causes salinisation, making the soil chemically infertile through osmotic desiccation of roots.

1. 1Day 1-2: Review natural soil properties (soil horizons, texture, crumb structure, and biotic components).
2. 2Day 3-4: Construct a detailed comparative mind map of agricultural practices (tillage, grazing, drainage, irrigation, fertilizer use) versus their impacts on physical, chemical, and biological fertility.
3. 3Day 5-6: Study soil conservation techniques (conservation tillage, contour ploughing, cover cropping, addition of organic matter) and evaluate their real-world efficacy.
4. 4Day 7: Complete timed 6-mark and 9-mark past paper questions focusing on data interpretation and evaluation command words.

• 📋Data analysis and calculations: Interpreting tables or graphs measuring soil erosion rates, bulk density changes, or leaching concentrations under different land uses.
• 📋Mechanistic short-answer questions (2-4 marks): Explaining step-by-step processes such as soil compaction leading to denitrification, or irrigation causing salinisation.
• 📋Extended evaluation questions (6-9 marks): Critically assessing the environmental advantages and limitations of intensive versus regenerative agricultural methods on maintaining soil fertility.