Морфология почв и строение почвенного профиля

Description of soil horizons

Soil morphology is the branch of soil science dedicated to the technical description of soil, particularly physical properties including texture, color, structure, and consistence. Morphological evaluations of soil are typically performed in the field on a soil profile containing multiple horizons. Along with soil formation and soil classification, soil morphology is considered part of pedology, one of the central disciplines of soil science.

Since the origin of agriculture, humans have understood that soils contain different properties which affect their ability to grow crops. However, soil science did not become its own scientific discipline until the 19th century, and even then early soil scientists were broadly grouped as either "agro chemists" or "agro geologists" due to the enduring strong ties of soil to agriculture. These agro geologists examined soils in natural settings and were the first to scientifically study soil morphology. A team of Russian early soil scientists led by V. V. Dokuchaev observed soil profiles with similar horizons in areas with similar climate and vegetation, despite being hundreds of kilometers apart. Dokuchaev's work, along with later contributions from K. D. Glinka, C. F. Marbut, and Hans Jenny, established soils as independent, natural bodies with unique properties caused by their equally unique combinations of climate, biological activity, relief, parent material, and time. Soil properties had previously been inferred from geological or environmental conditions alone, but with this new understanding, soil morphological properties were now used to evaluate the integrated influence of these factors. Soil morphology became the basis for understanding observations, experiments, behavior, and practical uses of different soils. A soil profile is one face of a pedon, or an imaginary three dimensional unit of soil that would display the full range of properties characteristic of a particular soil. Pedons generally occupy between 1 and 10 m2 of surface land area and are the fundamental unit of field based soil study. Many soil scientists in the United States document soil morphological descriptions using the standard Pedon Description field sheet published by the USDA NRCS. In addition to location, landscape, vegetation, topographic, and other site information, soil morphology descriptions generally include the following properties:

To estimate by hand in the field, soil scientists take a handful of sifted soil and moisten it with water until it holds together. The soil is then rolled into a ball nearing 1 2 inches in diameter and squeezed between the thumb and side of the index finger. Ribbons should be made as long as possible until it naturally breaks under its own weight. Longer ribbons indicate a higher clay percentage. The relative smoothness or grittiness indicates the sand percentage, and with practice, this technique can provide accurate textural class determinations. From here, the soil can be classified using a soil texture triangle, which labels the type of soil based on the percentages of each particle in the sample.

Soil particles naturally aggregate together into larger units or shapes referred to as "peds". Peds have planes of weakness between them are generally identified by probing exposed soil profiles with a knife to pry out and gently break apart volumes of soil. Soil porosity is complex. Traditional models regard porosity as continuous. This fails to account for anomalous features and produces only approximate results. Furthermore, it cannot help model the influence of environmental factors which affect pore geometry. A number of more complex models have been proposed, including fractals, bubble theory, cracking theory, Boolean grain process, packed sphere, and numerous other models.

Soil micromorphology refers to the description, measurement, and interpretation of soil features that are too small to be observed by the unassisted eye.

Soils are formed from their respective parent material, which may or may not match the composition of the bedrock that they lie on top of. Through biological and chemical processes as well as natural processes such as wind and water erosion, parent material can be broken down. The chemical and physical properties of this parent material is reflected in the qualities of the resulting soil. Climate, topography, and biological organisms all have an impact on the formation of soils in various geographic locations.

A steep landform is going to see an increased amount of runoff when compared to a flat landform. Increased runoff can inhibit soil formation as the upper layers continue to get stripped off because they are not developed enough to support root growth. Root growth can help prevent erosion as the roots act to keep the soil in place. This phenomenon leads to soils on slopes being thinner and less developed than soils found on plains or plateaus.

Varying levels of precipitation and wind have impacts on the formation of soils. Increased precipitation can lead to increased levels of runoff as previously described, but regular amounts of precipitation can encourage plant root growth which works to stop runoff. The growth of vegetation in a certain area can also work to increase the depth and nutrient quality of a topsoil, as decomposition of organic matter works to strengthen organic soil horizons.

Varying levels of microbial activity can have a range of impacts on soil formation. Most often, biological processes work to disrupt existing soil formation which leads to chemical translocation. the movement of these chemicals can make nutrients available, which can increase plant root growth.