Soil is a complex, dynamic mixture of mineral matter, organic matter, air, and water covering much of the Earth’s land surface. Scientists seek to understand what drives the diversity of soils across the globe. The characteristics of any given soil result from several interacting forces, but the central question revolves around two primary influences: the underlying rock type and the prevailing climate.
Understanding the Core Factors of Soil Formation
Soil scientists summarize the variables determining soil development using the CLORPT framework. This acronym stands for the five major factors: Climate, Organisms, Relief (topography), Parent Material, and Time. All five factors work together and are necessary for soil to form and evolve over geological timescales.
The most direct comparison is usually made between Parent Material and Climate. Parent Material refers to the initial geological material—the rock type—from which the soil originates. Climate encompasses the temperature and moisture regimes of a location, governing the energy and water available for transformation.
The Initial Influence of Parent Material (Rock Type)
The parent material sets the initial conditions for soil formation, providing the raw ingredients upon which all other processes act. This rock type directly influences the starting chemical composition and mineral content of the nascent soil. For instance, soils derived from limestone bedrock will be alkaline and high in calcium. Conversely, soils starting from quartz-rich granite may be more acidic and coarser, initially containing higher levels of potassium.
The texture of the soil, the proportion of sand, silt, and clay particles, is also heavily influenced by the parent material’s original grain size. Sandstone tends to yield sandy, coarse-textured soils, while materials like basalt or shale weather into finer, clay-rich soils. The inherent resistance of the parent rock dictates the initial rate of physical weathering and how quickly the soil begins to form. Softer rocks break down faster than hard, resistant rocks, controlling the soil’s initial composition and development trajectory.
Climate as the Engine of Soil Transformation
Climate acts as the dominant engine driving the rate and direction of chemical and biological transformation in the soil system. Temperature and precipitation control the speed of soil-forming processes. Warm temperatures accelerate chemical weathering reactions, such as mineral dissolution, and speed up the decomposition of organic matter by soil microbes.
Precipitation dictates the movement of water through the soil profile, a process known as leaching. In high-rainfall environments, excessive leaching washes soluble nutrients and minerals downward, often leading to the formation of distinct, highly weathered, and acidic soil layers. Conversely, in arid climates, low precipitation and high evaporation cause water to move upward, accumulating salts and carbonates near the surface in a process called salinization.
The climate regime also strongly determines the nature of the ecosystem, connecting it to the organisms factor. A warm, humid climate supports dense vegetation, contributing vast amounts of organic matter to the soil, while cold or arid climates limit biological activity. The type of vegetation determines the quality and quantity of organic material incorporated, further shaping the soil’s final properties regardless of the starting rock type.
Synthesis: Why Climate Dominates Over Time
While the parent material provides the initial chemical and physical foundation, climate dictates the long-term changes that occur, ultimately overshadowing the original rock type. Over extended periods, the powerful forces of climate—driven by temperature and moisture—strip the soil of its original chemical signature through weathering and leaching. This process causes soils derived from vastly different rock types to converge on similar properties if they are subjected to the same climate regime for a sufficient amount of time.
This long-term dominance is evidenced by “zonal soils,” which are mature soils whose characteristics primarily reflect the climate and vegetation of the region. For example, a tropical rainforest soil developed on granite will eventually resemble one developed on basalt. This occurs because intense heat and heavy rainfall leach away most of the original minerals from both.
In contrast, “intrazonal” or “azonal” soils are those where a local factor, such as the parent material, still exerts a strong influence. This typically happens because the soil is geologically young or exists under extreme, localized conditions. However, for most mature soils across the planet, the overriding effects of climate dictate the final profile, composition, and characteristics, proving it to be the more influential factor.