Soil development

Rock and soilClick to enlarge
Soil develops from rock – an extremely slow process indeed!
Source: S. Marahrens / Umweltbundesamt

What is needed above all for soil to develop is time, and of course rock, which is the material from which soil develops. Likewise necessary are organic substances, water, air and microorganisms.

A lengthy process

Soil is formed and develops over extremely lengthy periods and results from the interplay between myriad factors. The most important natural factors are rock, climate, plants, animals, terrain form and slope, and water availability. Of great importance in this process are the development period and the scope of anthropogenic ground use, which in recent centuries has induced major changes in the soil.

Soil develops through a process involving weathering, fracturing and comminution of rock into mineral soil particles. These processes evolve at varying speeds, depending on the intensity of the factors that are brought to bear. But soil is far more than a mere mixture of exclusively mineral particles of varying sizes; for it is above all a mixture of decomposed organic substances, humus and mineral elements that are subjected to water and air, as well as myriad flora and fauna organisms. It takes an extremely long time for this mixture to achieve the familiar quality and necessary depth of the entity we term “soil.” It can take anywhere from 100 to 300 years for one centimeter of a thick, humus soil layer to develop; and this can all be washed away by a single severe rainstorm.

Rock fracturing and comminution

Soil is formed from surface rock that is fractured and comminuted by a vast array of weathering processes. For example, when water penetrates rock cracks and crevices, causing the rock to expand by around nine per cent via the force exerted when the water freezes, the resulting pressure causes the rock to burst, fracture and comminute. For dark rock in particular, the alternation of heating and cooling also exerts mechanical stress on the rock, causing it to fracture.

Chemical reactions alter rock composition, whereby particularly for rock containing a large amount of silica, hydrolysis (chemical decomposition) destabilizes the rock structure owing to ion loss from the rock’s crystal lattice. In a virtually identical process known as hydratation, water molecules are uptaken in the crystal lattice of a mineral, and the consequent swelling causes the rock to lose its rigidity.
The pressure exerted by growing roots can also cause rock to fracture and comminute – an explosive force that can be observed in the roots that surface on forest walking paths.

Humification, browning and loaming

Rock fracturing and mineral particle comminution are accompanied by a host of other soil formation processes, such as humification, browning, and loaming.

Humification involves the formation of highly stable substances, known as humic substances, consequent upon plant residue decomposition that is mainly attributable to soil organisms. This breakdown process can be slowed or brought to a virtual halt by water and/or oxygen deprivation, low temperatures or other conditions that are suboptimal for the organisms.

Browning is a process in which soil minerals such as olivine or biotite react with oxygen. The resulting oxides and hydroxides lend the soil its characteristic reddish brown coloration.

Loaming, which almost always occurs in conjunction with browning, results in clay minerals that are engendered by the comminution of the silicate minerals feldspar and mica. Inasmuch as the particle size of clay minerals is the same as that of clay, originally sandy soil becomes more loamy and fertile through comminution.

Lessivage, podzolisation and gleyification

All ions released into the soil during the comminution process, as well as the resulting clay minerals and humic substances, are ultimately transported downward with percolating soil water. The outcome of this process is determined by transport intensity, soil permeability, and process duration. Depending on climate, the process can also entail the dissolution of highly soluble salts (which are then refixed) and of poorly soluble carbonates, as the result of acidifying rainfall. Lessivage involves the displacement of clay particles, a process that is triggered when soil pH falls below 6.5. The clay particles are fixed in a lower stratum, resulting in clay deposition there.

Podzolisation likewise occurs in the presence of a severe drop in topsoil pH values. This acidification is attributable to acid and eco-unfriendly litter layers composed of elements such as pine needles, in conjunction with anthropogenic atmospheric acid deposition. A light yellowish stratum is observed in cases where upper soil strata buffer capacity is saturated, a rapid phenomenon that occurs mainly in low-mineral sandy soils. This light coloration results from (a) the dark colored humic substances being leached out of the soil; and (b) the reddish iron oxide being re-fixed at a lower stratum.

Gleyification involves permanent oxygen deprivation in a zone that is permanently saturated with water and that is mainly observed in lower soil strata. The chemical reduction of manganese and rust-colored iron results in formation of a greenish blue stratum with light colored striations and intermittent bleaching.

All of these processes affect soil composition and properties, and lend the soil its characteristic appearance comprising a vertical structure with its layers and strata.

Scheme Development of soils
The slow development of soil is driven by many factors.
Source: S. Marahrens / Umweltbundesamt

Good composition

The soil formation processes described above determine the grain size composition of mineral soil particles. This property is referred to as soil type. This parameter indicates particle size composition, which can range from extremely fine particles that are invisible to the naked eye, to two millimeter in diameter grains of sand. Hence soil type is determined by the relative proportions of clay, silt and sand in the soil. For example, loamy sand is a mixture of all three elements, sand being the main one.

Particle size composition, which is one of the main properties of soil, affects its key functions and processes. Soil type is the determining factor in particular for water deposition, soil fertility, and susceptibility to flooding. The extent to which soil is prone to erosion, or to compaction resulting from the use of heavy machinery, is largely determined by particle size distribution.

Clay minerals are the smallest of the mineral soil particles, thus making clay such a valuable substance, as its silicate structure enables it to bind ions in the crystal-lattice, – thus making the clay nutrients phytoavailable.

Even more invaluable are the humic substances or humus in soil. Apart from its carbon sequestration capacity, thanks to its extensive surface area humus can also store and release nutrients and absorb massive amounts of water. Clay minerals, along with humus, are key pollutant filters. Humus is also a key source of energy for all soil organisms, which can only contribute to soil quality in the presence of optimal conditions. Hence it is essential that soil management techniques be used that do not reduce humus content.