5. Land

Soil as a system

Dynamic open system with inputs and outputs and storages and flows; part of the larger ecosystem

Components of soil

Inorganic (sand; silt; clay) + organic (living organisms and dead matter or humus) + air + water

Soil inputs

Litter and roots; weathered rock; precipitation; gases (O2 and CO2); fertiliser and compost and manure

Soil outputs

Heat; nutrients lost by leaching; gases (respiration; N2O; CH4); eroded material; plant uptake; evaporation

Soil storages

Organic matter (humus); organisms (decomposers and roots); minerals; air; water

Soil flows and transfers

Infiltration; percolation; biological mixing; erosion; leaching; groundwater flow; aeration

Soil transformations

Decomposition; nutrient cycling; weathering; salinisation

Soil structure

Depends on particle size (sand > silt > clay) and on organic matter binding particles into aggregates

Sand

Particles 2-0.05 mm; high drainage and well aerated but low water and nutrient retention

Clay

Particles under 0.002 mm; holds water and nutrients well (high CEC) but drains slowly and compacts easily

Silt

Particles 0.05-0.002 mm; intermediate properties and helps bind particles

Loam

Balanced mix of sand and silt and clay; best for agriculture because it combines drainage and retention

Humus

Partially decomposed organic matter; improves fertility and water retention and structure

Role of soil in plant growth

Medium for roots; seed bank; stores nutrients (N and P and K) and water

Soil and biodiversity

Provides habitats and niches for microorganisms and fungi and animals

Soil and biogeochemical cycles

Key in carbon and nitrogen and phosphorus cycles; detritivores and saprotrophs decompose matter and recycle nutrients

Soil carbon

Soils store more carbon than the atmosphere and plants combined; boreal soils store a lot because decomposition is slow

Salinisation

Irrigation raises soil salt levels; irrigated cropland gains productivity but also salinity

Land constraints

About 70% of ice-free land is used for agriculture and forestry; not all suits crops; steep or poor soils are used for livestock

Inequality in land use

Marginalised groups (indigenous peoples and women and low-income communities) are excluded from land decisions and access to resources

Distribution paradox

The world produces enough food for 8+ billion people; hunger comes from poverty and waste and unequal distribution; not absolute scarcity

Food waste

One third of production is wasted (post-harvest and storage and distribution and retail and consumption); SDG 12 aims to halve per capita waste at retail and consumer level by 2030

Factors shaping agricultural systems

Soils; climate; culture; economics

Classification of farming systems

Outputs: arable; pastoral; mixed; monoculture; diverse; purpose: subsistence vs commercial and sedentary vs nomadic; inputs: intensive vs extensive and irrigated vs rain-fed and organic vs inorganic

Intensive vs extensive farming

Intensive: high inputs and high output per area; extensive: low inputs and low output per area and generally more sustainable

Nomadic pastoralism and slash-and-burn

Sustain low-density populations but are less sustainable under modernisation

Local soils and biome and crops

Soil and biome decide suitable crops (e.g. rice in the Ganges Valley and cereals in the prairies)

Green Revolution

1950s-60s: high-yielding varieties + irrigation + synthetic inputs increased yields

Green Revolution success

Yields doubled or tripled (e.g. IR8 rice in India and the Philippines); reduced hunger in Asia and Latin America

Green Revolution costs

Dependence on synthetic inputs; monocultures; soil degradation; water depletion; inequality as small farmers were excluded

High yield vs sustainability

High yields do not mean sustainable; ecosystem health was traded for short-term productivity

Sustainable alternatives to intensive farming

Fallowing; organic fertiliser; nitrogen-fixing cover crops; mycorrhizae; agroforestry; crop rotation; benefits are fewer inputs and healthier soil and more biodiversity and resilience

Erosion control

Water: terracing and contour ploughing and bunding and drainage and cover crops; wind: windbreaks and cover crops

Fertility conservation

Lime; compost; green manure; herbal leys; mycorrhizae; agroforestry

Sustainable cultivation practices

Avoid marginal land and overgrazing; strip cropping; mixed cropping; crop rotation; reduced tillage; reduced heavy machinery

Diets and trophic levels

Humans are omnivores; plant-based diets are lower on the food chain so use less land and cost less and are more sustainable

Strategies for sustainable food supply

Reduce demand and waste; cut greenhouse gas emissions; increase productivity without more land (e.g. plant-based meat substitutes and low-methane rice and GM crops and solar-powered fertiliser)

Food security

Physical and economic access to sufficient and balanced food

Why does food insecurity persist?

Mainly distribution and waste and poverty rather than too little food production

Amazon deforestation case study (scale and causes)

Peaked at 27000 km2 per year in 2004; fell to about 4000-7000 (2012-2018); surged again over 10000 (2019-2022); causes: cattle ranching (about 80%) and soy and logging driven by cheap land and subsidies

Amazon deforestation impacts

Environmental: about 1.5 Gt CO2 per year and biodiversity loss and less rainfall and soil degradation; social: indigenous displacement and violence (400+ groups affected)

Amazon tipping point

Savannisation is triggered at 20-25% deforestation (currently about 17%) with positive feedback

Amazon management

Soy Moratorium (2006) cut deforestation 75% in soy areas; satellite monitoring and fines work with strong political will; indigenous territories have the lowest deforestation; challenge: agribusiness is about 25% of GDP