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Volume 1: Organic Components

Soil formation is the result of a combination of five factors: parent material, climate, topography, biological factors, and time. Soil formation is the consequence of a combination of biological, physical, and chemical processes.

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Soil should ideally contain 50 percent solid material and 50 percent pore space. About one-half of the pore space should contain water, while the other half should contain air. The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. Most soil microorganisms, bacteria, algae, or fungi, are dormant in dry soil, but become active once moisture is available.

Soil distribution is not homogenous because its formation results in the production of layers; the vertical section of the layers of soil is called the soil profile. Within the soil profile, soil scientists define zones called horizons: a soil layer with distinct physical and chemical properties that differ from those of other layers. Five factors account for soil formation: parent material, climate, topography, biological factors, and time. The organic and inorganic material in which soils form is the parent material. Mineral soils form directly from the weathering of bedrock, the solid rock that lies beneath the soil; therefore, they have a similar composition to the original rock.

Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-moving river could have sediments of rocks and sand, whereas a slow-moving river could have fine-textured material, such as clay. Soft sediment deformation : Soil distribution is not the same at all depths.

Soil Components

The vertical section of soil layers is called the soil profile. The soil profile contains defined zones called horizons whicht have distinct physical and chemical properties that differ from those of other layers. An example is shown here in the soft sediment deformation in the Navajo Sandstone. Temperature, moisture, and wind cause different patterns of weathering, which affect soil characteristics.

The presence of moisture and nutrients from weathering will also promote biological activity: a key component of a quality soil. Topography affects water runoff, which strips away parent material and affects plant growth. Steep soils are more prone to erosion and may be thinner than soils that are relatively flat or level.

What is soil organic carbon? | Agriculture and Food

The presence of living organisms greatly affects soil formation and structure. Animals and microorganisms can produce pores and crevices. Plant roots can penetrate into crevices to produce more fragmentation. Plant secretions promote the development of microorganisms around the root in an area known as the rhizosphere.

Additionally, leaves and other material that fall from plants decompose and contribute to soil composition. Time is an important factor in soil formation because soils develop over long periods. Soil formation is a dynamic process. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil. Soils are made up of combinations of four distinct layer types or horizons: O horizon, A horizon, B horizon, and C horizon.

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Soils are named and classified based on their horizons. The soil profile has four distinct layers:.

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Soil profile : This soil profile shows the different soil layers O horizon, A horizon, B horizon, and C horizon found in typical soils. Some soils may have additional layers, or lack one of these layers. The thickness of the layers is also variable, depending on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons, whereas mature soils may display all of these, plus additional layers. Privacy Policy. Skip to main content. When the soil moisture content is optimal for plant growth, the water in the large and intermediate size pores can move about in the soil and be easily used by plants.

Sandy soil will retain very little water, while clay will hold the maximum amount. Water moves through soil due to the force of gravity , osmosis and capillarity. At zero to 33 kPa suction field capacity , water is pushed through soil from the point of its application under the force of gravity and the pressure gradient created by the pressure of the water; this is called saturated flow. At higher suction, water movement is pulled by capillarity from wetter toward drier soil.

This is caused by water's adhesion to soil solids, and is called unsaturated flow. Water infiltration rates range from 0. Tree roots, whether living or dead, create preferential channels for rainwater flow through soil, [] magnifying infiltration rates of water up to 27 times. Flooding temporarily increases soil permeability in river beds , helping to recharge aquifers.

Water applied to a soil is pushed by pressure gradients from the point of its application where it is saturated locally, to less saturated areas, such as the vadose zone. In the United States percolation water due to rainfall ranges from almost zero centimeters just east of the Rocky Mountains to fifty or more centimeters per day in the Appalachian Mountains and the north coast of the Gulf of Mexico.

Water is pulled by capillary action due to the adhesion force of water to the soil solids, producing a suction gradient from wet towards drier soil [] and from macropores to micropores. Preferential flow occurs along interconnected macropores, crevices, root and worm channels, which drain water under gravity. Of equal importance to the storage and movement of water in soil is the means by which plants acquire it and their nutrients.

Most soil water is taken up by plants as passive absorption caused by the pulling force of water evaporating transpiring from the long column of water xylem sap flow that leads from the plant's roots to its leaves, according to the cohesion-tension theory. It is these process that cause guttation and wilting , respectively. Root extension is vital for plant survival. A study of a single winter rye plant grown for four months in one cubic foot 0. The total surface area of the loam soil was estimated to be 52, square meters.

However, root extension should be viewed as a dynamic process, allowing new roots to explore a new volume of soil each day, increasing dramatically the total volume of soil explored over a given growth period, and thus the volume of water taken up by the root system over this period. Roots must seek out water as the unsaturated flow of water in soil can move only at a rate of up to 2.


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Only a small fraction 0. The majority is ultimately lost via transpiration , while evaporation from the soil surface is also substantial, the transpiration:evaporation ratio varying according to vegetation type and climate, peaking in tropical rainforests and dipping in steppes and deserts. Evapotranspiration plus water held in the plant totals to consumptive use, which is nearly identical to evapotranspiration. The total water used in an agricultural field includes surface runoff , drainage and consumptive use.

The use of loose mulches will reduce evaporative losses for a period after a field is irrigated, but in the end the total evaporative loss plant plus soil will approach that of an uncovered soil, while more water is immediately available for plant growth.

Transpiration ratios for crops range from to The atmosphere of soil, or soil gas , is very different from the atmosphere above. The consumption of oxygen by microbes and plant roots, and their release of carbon dioxide , decrease oxygen and increase carbon dioxide concentration. Atmospheric CO 2 concentration is 0. Movement of gases is by diffusion from high concentrations to lower, the diffusion coefficient decreasing with soil compaction.

It is the total pore space porosity of soil, not the pore size, and the degree of pore interconnection or conversely pore sealing , together with water content, air turbulence and temperature, that determine the rate of diffusion of gases into and out of soil. Soil atmosphere is also the seat of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms, e.

We humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated, [] a bulk property attributed in a reductionist manner to particular biochemical compounds such as petrichor or geosmin. Soil particles can be classified by their chemical composition mineralogy as well as their size. The particle size distribution of a soil, its texture , determines many of the properties of that soil, in particular hydraulic conductivity and water potential , [] but the mineralogy of those particles can strongly modify those properties.

The mineralogy of the finest soil particles, clay, is especially important. Gravel , sand and silt are the larger soil particles , and their mineralogy is often inherited from the parent material of the soil, but may include products of weathering such as concretions of calcium carbonate or iron oxide , or residues of plant and animal life such as silica phytoliths. Due to its high specific surface area and its unbalanced negative electric charges , clay is the most active mineral component of soil.

Many soil minerals, such as gypsum, carbonates, or quartz, are small enough to be classified as clay based on their physical size, but chemically they do not afford the same utility as do mineralogically-defined clay minerals. Before the advent of X-ray diffraction clay was thought to be very small particles of quartz , feldspar , mica , hornblende or augite , but it is now known to be with the exception of mica-based clays a precipitate with a mineralogical composition that is dependent on but different from its parent materials and is classed as a secondary mineral. Typically there are four main groups of clay minerals: kaolinite , montmorillonite - smectite , illite , and chlorite.

The spatial arrangement of the oxygen atoms determines clay's structure.