The alteration in the mechanical composition of soils is characterized by the accumulation of smaller particles over time, as rock materials and minerals undergo degradation through physical and chemical processes. These processes, collectively known as weathering, encompass the destructive effects of flowing water (erosion), as well as the freezing and expansion of water within rock fissures; wind (deflation) and glaciers (exaration), which contribute to the disintegration of rocks. As a result of weathering, rock fragments are transformed into sand, silt, or clay, which represent the three primary classes of soil granulometric composition, listed in descending order of particle size. Although this process is destructive to rocks, it generates a porous substrate that is accessible to water and plant roots. This substrate can be thin-layered even prior to the commencement of soil formation, that is, before the introduction of plant roots and other organisms. Such conditions are characteristic of many deposits that have been transported by water, wind, or ice, as well as volcanic ash.
The initial phase of soil formation, which spans several hundred years, is characterized by the accumulation of organic substances. During this period, the input of organic matter from plant remains surpasses the decomposition capacity of microorganisms. When soil develops on solid rock surfaces, it is initially colonized by lower plants such as lichens and mosses, followed by grasses or trees. Over time, a balance is established in developing soils between the input and decomposition of organic substances, which can be maintained if the soil is not subjected to degradation. Organic matter, primarily located in the upper part of the soil profile known as horizon A, serves as an indicator of soil fertility. Humus, formed from the partially decomposed remains of plants and animals by microorganisms, constitutes a significant component of the upper horizons of certain soils.
Plants derive nitrogen and other nutrients from humus. In the context of soil cultivation, the rates of organic matter accumulation are altered, as crops extract nutrients from the soil. Following harvest, the natural cycle resumes, leading to a decrease in humus content. Consequently, farmers, aiming to utilize the same plot of land over multiple years, are compelled to supplement the soil with fertilizers. Leaching, or the transition of substances into solutions, also plays a role in the process of soil formation. Rainwater dissolves some solids that move with the water. The direction of movement can be lateral (surface runoff) or vertical if water is absorbed into the soil. Water entering the soil can either pass through it or penetrate only to a certain depth within the root-inhabited layer.
There it is temporarily absorbed and then returned to the atmosphere either through extraction by plants of moisture through roots or through evaporation from the soil surface. The suction of water by roots leads to a decrease in some substances in the upper soil layer, which accumulate within the depth reached by the water. This is how soil horizons are formed. Water filtered through the soil and replenishing groundwater reserves also contributes to leaching, but this does not lead to horizon differentiation. Horizon A is the zone of maximum leaching.
Weathering is most pronounced in humid climatic conditions; however, even in arid climates, certain easily soluble salts are transported down the soil profile. During intense rainfall events, these salts can be redistributed and accumulate on the surface in topographical depressions, as water evaporation occurs in these areas. This pattern of salt movement contributes to the formation of saline soils in the deserts of western America, Africa, Central Asia, and other global regions. In the most humid climates, easily soluble salts accumulate in deeper soil horizons. Eluviation (the washing out) and illuviation (the washing in) are soil-forming processes whereby substances in an aqueous medium transition into a dissolved state and subsequently precipitate.
Particles migrate with water but are not chemically dissolved, with clay particles being the most involved in this process. This results in a reduction of their quantity in horizon A (eluviation) and their accumulation in horizon B (illuviation). Dispersion is hindered in the presence of easily soluble salts. Consequently, eluvial and illuvial processes are characteristic of regions with a humid climate, where horizon A is typically situated above clay-rich horizon B.
The formation of soils is largely determined by five principal soil-forming factors: climate, the activity of animals and microorganisms, soil-forming (parent) rocks, relief, and time.