Module 47. World Soils & Biomes

The Earth’s surface is a mosaic of diverse ecological regions and soil types that support life in various forms. Soils and biomes are interconnected elements of the biosphere — soils provide nutrients and anchorage for vegetation, while biomes represent large ecological units defined by climate, flora, and fauna. Together, they influence biodiversity, agricultural productivity, and global environmental stability. Understanding their distribution and characteristics is crucial to managing natural resources sustainably.

Formation and Classification of Soils

Soil is formed through the process of weathering — the physical and chemical breakdown of rocks and minerals combined with organic matter accumulation. The formation process, known as pedogenesis, depends on several key factors:

• Parent material: The mineral composition of the rock from which soil develops.
• Climate: Temperature and precipitation determine the rate of weathering and organic decomposition.
• Topography: Influences drainage, erosion, and depth of soil horizons.
• Biological activity: Plants, animals, and microorganisms contribute organic matter and nutrients.
• Time: Soils mature and differentiate into layers, or horizons, over centuries.

Globally, soils are classified into distinct types based on their composition, fertility, and climatic conditions. The FAO (Food and Agriculture Organization) and USDA (United States Department of Agriculture) have developed systems to standardise classification. The principal soil types include:

• Tundra (Gelisols): Found in polar regions, characterised by permafrost and limited biological activity.
• Podzols: Acidic soils in coniferous forests of cool, moist climates, with distinct leaching and ash-coloured horizons.
• Chernozems: Rich, dark, humus-filled soils found in temperate grasslands such as the Eurasian Steppe and North American Prairies, ideal for agriculture.
• Laterites: Common in tropical regions, heavily leached, and rich in iron and aluminium oxides, often reddish in colour.
• Aridisols: Sandy, saline soils of desert environments with minimal organic matter.
• Alfisols: Moderately fertile soils under deciduous forests and savannas.
• Andisols: Volcanic ash-derived soils rich in minerals, found near active volcanoes in Japan, the Andes, and the Pacific Ring of Fire.

Distribution and Characteristics of Major Biomes

A biome is a large ecological zone defined by its climate, soil, vegetation, and wildlife. The main terrestrial biomes of the world include:

• Tundra Biome: Found in polar latitudes (Arctic and Alpine regions), characterised by permafrost, low temperatures, and moss or lichen vegetation. Soils are shallow and nutrient-poor (Gelisols).
• Taiga or Boreal Forest Biome: Extends across Canada, Scandinavia, and Russia. Dominated by coniferous trees, it has acidic podzolic soils and long, cold winters.
• Temperate Forest Biome: Present in Europe, eastern North America, and East Asia, featuring broad-leaved deciduous trees. The region’s soils (Alfisols) are fertile, supporting dense vegetation and agriculture.
• Grassland Biome: Comprises temperate prairies, pampas, and steppes. These regions experience moderate rainfall and have nutrient-rich chernozem soils, making them global grain-producing zones.
• Desert Biome: Located in subtropical high-pressure belts such as the Sahara, Arabian, and Kalahari deserts. Vegetation is sparse, and soils (Aridisols) are sandy, alkaline, and low in organic matter.
• Tropical Rainforest Biome: Found near the Equator, including the Amazon Basin, Congo Basin, and Southeast Asia. Soils (Laterites) are deeply weathered and nutrient-poor due to heavy rainfall, though the dense canopy maintains nutrient recycling.
• Savanna Biome: Transitional zones between rainforests and deserts in Africa, Brazil, and India. The soils are ferruginous and moderately fertile, supporting mixed grasses and scattered trees.
• Mediterranean Biome: Occurs in regions with hot, dry summers and mild, wet winters — such as California, South Africa, and the Mediterranean Basin. The soils are thin, stony, and prone to erosion.
• Mountain Biomes: Exhibit vertical zonation, with vegetation and soils varying by altitude. Lower slopes may have fertile alluvial soils, while higher elevations have shallow, stony profiles.

Relationship Between Soils and Biomes

The interaction between climate, vegetation, and soil composition forms a dynamic ecological system. For example:

• In tropical rainforests, rapid decomposition maintains nutrient cycling despite poor soil fertility.
• Grassland soils accumulate humus due to slow decay of roots, enhancing agricultural potential.
• In arid deserts, limited rainfall restricts organic matter formation, leading to sparse vegetation.

This interdependence means that any disturbance to soil — through deforestation, overgrazing, or pollution — can lead to biome degradation, loss of biodiversity, and desertification.

Human Impact and Environmental Concerns

Human activity has dramatically altered the natural balance of soils and biomes. Deforestation, industrial agriculture, and urbanisation have led to soil erosion, nutrient depletion, and contamination. The excessive use of fertilisers and pesticides disrupts soil microbiota, while climate change exacerbates droughts, floods, and permafrost thawing.
Desertification, especially across the Sahel and parts of Asia, threatens livelihoods and food security. The conversion of grasslands into croplands and the destruction of tropical forests for timber and farming reduce carbon sequestration, accelerating global warming.
Efforts to counter these effects include:

• Soil conservation practices such as contour ploughing, crop rotation, and afforestation.
• Sustainable land management programmes promoted by the United Nations Convention to Combat Desertification (UNCCD).
• Rewilding and restoration ecology initiatives aimed at rehabilitating degraded ecosystems.

Global Significance and Conservation

The world’s soils and biomes play a vital role in maintaining the Earth’s ecological balance. Soils act as carbon sinks, regulate hydrological cycles, and support plant growth. Biomes, in turn, provide essential ecosystem services — oxygen production, climate regulation, and genetic diversity.
Recognising the ecological and economic importance of these natural systems, international frameworks such as the FAO Global Soil Partnership and the Convention on Biological Diversity (CBD) advocate for conservation and sustainable use. Modern research increasingly focuses on soil microbiomes, carbon sequestration, and biome resilience to climate change.