What are soil horizons and why are they important?
Soil horizons are layers of soil that are underground, which develop from the combined actions of living organisms and percolating water. They have different physical, chemical and biological characteristics that affect the growth and health of plants. Soil horizons can be identified by their color, texture, structure and composition.
There are seven main soil horizons: Oi, Oa, A, E, B, C and R. The O horizon is the organic surface layer, which consists of plant and animal litter that is partially or fully decomposed. The Oi horizon is the upper part of the O horizon, where the organic matter is less decomposed. The Oa horizon is the lower part of the O horizon, where the organic matter is more decomposed and humified.
The A horizon is the topsoil layer, which contains a mixture of organic matter and mineral particles. It is usually dark in color and rich in nutrients. The A horizon is where most plant roots grow and where most biological activity occurs.
The E horizon is the eluviation layer, which is a zone of leaching or removal of minerals and organic matter by water. It is usually light in color and low in nutrients. The E horizon is common in forest soils, where acidic water dissolves minerals and carries them downward.
The B horizon is the subsoil layer, which is a zone of accumulation or deposition of minerals and organic matter from above. It is usually reddish or brownish in color and high in clay content. The B horizon may have distinct features such as nodules, coatings or bands of minerals that reflect the soil-forming processes.
The C horizon is the parent material layer, which is a zone of little or no alteration by soil-forming processes. It consists of loose or weathered rocks and minerals that have not been transformed into soil. The C horizon may have different origins such as glacial deposits, volcanic ash or bedrock fragments.
The R horizon is the bedrock layer, which is not considered part of the soil. It consists of solid rock that underlies the soil and may be exposed at the surface or buried deep underground. The R horizon may have different types such as granite, limestone or sandstone.
Soil horizons are important for gardeners and farmers because they provide information about the soil quality, fertility and suitability for different crops. By examining the soil profile (a vertical section of soil showing all the horizons), one can determine the depth, thickness, texture, structure and composition of each horizon and how they affect plant growth. Soil horizons also indicate the history and evolution of the soil and how it has been influenced by climate, vegetation, topography and human activities.
How to identify and classify different types of soil
Soil types are categories of soils that share similar characteristics and properties. Soil types are based on the proportions and types of soil particles, such as sand, silt and clay, as well as the organic matter and mineral content. Soil types also reflect the soil-forming factors, such as climate, vegetation, parent material, topography and time.
Soil types can be classified into different systems according to different criteria and purposes. For example, the USDA soil taxonomy classifies soils into 12 orders, such as alfisols, aridisols, entisols, histosols, etc., based on their diagnostic horizons and features. The World Reference Base for Soil Resources (WRB) classifies soils into 32 reference groups, such as acrisols, cambisols, fluvisols, podzols, etc., based on their diagnostic criteria and qualifiers. The FAO-UNESCO soil map of the world classifies soils into 30 major groups, such as chernozems, ferralsols, luvisols, solonetz, etc., based on their morphological characteristics and genetic processes.
To identify and classify different types of soil, one needs to examine the soil profile, which is a vertical section of the soil showing all the horizons or layers. The soil profile can be observed in a soil pit or a soil core sample. The soil profile can reveal the texture, structure, color, depth, thickness and sequence of the horizons, as well as the presence of roots, pores, cracks and other features.
The texture of the soil refers to the relative proportions of sand, silt and clay particles in each horizon. The texture determines the water holding capacity, drainage, aeration and nutrient availability of the soil. The texture can be estimated by feeling the soil or measured by laboratory methods. The texture can be expressed by using a textural triangle that shows the percentage of sand, silt and clay in each horizon. The texture can also be described by using terms such as sandy loam, silty clay loam, clayey sand, etc., based on the dominant particle size.
The structure of the soil refers to the arrangement of soil particles into aggregates or peds that have distinct shapes and sizes. The structure influences the porosity, permeability and stability of the soil. The structure can be observed by breaking apart a clod of soil or by examining a thin section under a microscope. The structure can be described by using terms such as granular, prismatic, blocky, platy, etc., based on the shape of the peds; and terms such as weakly developed, moderately developed or strongly developed based on the degree of aggregation.
The color of the soil refers to the hue (redness or yellowness), value (lightness or darkness) and chroma (intensity or purity) of the soil in each horizon. The color indicates the organic matter content, mineral composition and oxidation state of the soil. The color can be observed by using a moistened sample of soil or by comparing it with a standard color chart such as Munsell Soil Color Chart. The color can be expressed by using three numbers that represent hue (e.g., 10R), value (e.g., 4) and chroma (e.g., 6), followed by a modifier (e.g., moist).
The depth and thickness of each horizon refer to the vertical distance from the surface to the bottom of each horizon and from the top to the bottom of each horizon respectively. The depth and thickness indicate the degree of weathering and development of the soil. The depth and thickness can be measured by using a ruler or a tape measure in a soil pit or a core sample. The depth and thickness can be expressed by using units such as centimeters (cm) or meters (m).
The sequence of horizons refers to the order in which different horizons appear from top to bottom in a soil profile. The sequence reflects the history and evolution of the soil and its response to various soil-forming factors. The sequence can be observed by using symbols that represent each horizon such as O for organic horizon; A for topsoil horizon; E for eluviation horizon