The skin of the Earth: All about soil

Soil is the foundation of life on Earth. It provides essential functions such as supporting plant growth, storing and filtering water, cycling nutrients, buffering and degrading pollutants, participating in the Earth’s carbon cycle, providing a habitat for a vast array of organisms, supplying raw materials, and serving as a foundation for infrastructure. This makes it one of the planet’s most vital and precious natural resources. Yet, despite its importance, soil is often undervalued and is increasingly threatened by unsustainable land use practices, urbanisation, climate change, and pollution.

Soil is a finite natural resource. Rapid human population growth has put unprecedented pressure on the use of soils. About a third of all soils are moderately to highly degraded (Smith et al, 2024), resulting in reduced fertility, increased erosion, and diminished water retention. The decline in soil health harms ecosystems, accelerates climate change, and threatens human livelihoods and food security.

Protecting and restoring soil health is essential for achieving the United Nations Sustainable Development Goals. ISRIC is committed to advancing soil knowledge and delivering soil information that is fundamental for managing soils sustainably and building a food-secure, climate-resilient future.

Often referred to as the “skin of the Earth,” soil is the thin outer layer of the Earth’s crust and the interface between the atmosphere, lithosphere, biosphere and anthrosphere. It is where most of the material and energy fluxes at the Earth’s surface take place. It forms slowly through a process called pedogenesis, in which rock or sediment is transformed under the influence of climate (rainfall and temperature), topography, and living organisms. This process leads to the development of distinct soil layers, known as horizons, which differ in particle-size distribution (gravel, sand, silt and clay concentration), colour, chemical compositions and physical properties. Depending on environmental conditions and human influence, soils can be shallow or extend several meters deep.

Soil is much more than dirt beneath our feet. It is a life-support system that underpins many aspects of our environment, economy and well-being. Its impact reaches far beyond agriculture, playing a critical role in food security, water management, climate regulation, biodiversity, raw material supply and cultural heritage. Here we highlight some important soil functions.

Soils vary from place to place because of differences in parent material, climate, organisms (vegetation and soil fauna), topography, and age of the landscape as it evolves over the long time of soil formation. These factors influence soil characteristics such as colour, particle-size distribution, structure, density, restrictive layers and chemical composition. Many tropical soils are typically deep, highly weathered, and nutrient-poor largely due to high rainfall and stable landscapes. Desert soils tend to be low in organic matter and may contain high concentrations of salts. Soils in the temperate regions are generally less weathered and thinner than tropical soils and tend to contain more organic matter and have higher natural fertility. Mountain soils tend to be thin and rocky. In Arctic regions, cold and wet conditions favour the accumulation of organic matter. Soil development is heavily influenced by the freezing-thawing cycle of the permafrost.

Soil threats are processes that degrade soil quality and reduce its capacity to perform essential functions and provide ecosystem services. Major threats include soil erosion by wind and water, soil sealing (the covering soil with impervious surfaces like buildings and roads), contamination from pollution and waste, and loss of soil organic matter, which reduces fertility and water retention. Other threats are soil compaction, destruction of soil structure, salinisation (the accumulation of salts), acidification, waterlogging, and the decline of soil biodiversity.

Sustainable soil management (SSM) offers a way to counter these threats and even restore degraded soils. The Food and Agriculture Organization of the United Nations (FAO) defines SSM as practices that maintain or enhance the soil’s supporting provisioning, regulating and cultural services without significantly impairing either the soil functions that enable those services or biodiversity. Key SSM practices include minimising tillage, maintaining continuous soil cover, practicing crop rotation and diversification, applying soil and water conservation techniques, using organic amendments, and managing irrigation and grazing effectively.

For more information, refer to the FAO’s Voluntary Guidelines for Sustainable Soil Management Practices.