Page 610 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

574 part IV Soils, Ecosystems, and Biomes
and have characteristics recognizably different from horizons directly above or below. The four “master” hori- zons in most agricultural soils are known as the O, A, B, and C horizons (Figure 18.2).
The boundary between horizons usually is distin- guishable when viewed in profile, owing to differences in one or more physical soil characteristics, such as colour, texture, structure, consistence (meaning soil consistency or cohesiveness), porosity, or moisture. These and other soil properties, which all affect soil function, are dis- cussed in the next section.
O Horizon At the top of the soil profile is the O hori- zon, named for its organic composition, derived from plant and animal litter that was deposited on the surface and transformed into humus, a mixture of decomposed and synthesized organic materials that is usually dark in colour. Microorganisms work busily on this organic debris, performing a portion of the humification (humus- making) process. The O horizon is 20%–30% or more organic matter, which is important because of its ability to retain water and nutrients and because of the way its behaviour complements that of clay minerals.
The A, Ae, B, and C horizons extend below the O horizon to the R horizon, which is composed of sedi- ment or bedrock. These middle layers are composed of sand, silt, clay, and other weathered by-products.
Soil scientists also employ lowercase letters to desig- nate subhorizons within each master horizon, indicating particular conditions. For example, the Ap horizon refers to an A horizon that has undergone ploughing; the Bh horizon refers to the presence of organics (humic material).
A Horizon In the A horizon, humus and clay particles are particularly important, as they provide essential chemical links between soil nutrients and plants. This horizon usually is richer in organic content, and hence darker, than lower horizons. Human disruption through ploughing, pasturing, and other uses takes place in the A horizon. This horizon is commonly called topsoil.
Ae Horizon The A horizon grades downward into the Ae horizon, which is made up mainly of coarse sand, silt, and leaching-resistant minerals. From the lighter- coloured E horizon, silicate clays and oxides of aluminum and iron are leached (removed by water) and carried to lower horizons with the water as it percolates through the soil. This process of removing fine particles and miner- als by water, leaving behind sand and silt, is eluviation— thus, the e designation for this horizon. As precipitation increases, so does the rate of eluviation.
B Horizon In contrast to the A and Ae horizons, B hori- zons accumulate clays, aluminum, and iron. B horizons are dominated by illuviation, in which materials leached by water from one layer enter and accumulate in another. Both eluviation and illuviation are types of translocation, in which material (such as nutrients, salts, clays) is moved
downward in the soil. In contrast to eluviation, which is erosional, illuviation is a depositional process. B horizons may exhibit reddish or yellowish hues because of the pres- ence of illuviated minerals (silicate clays, iron and alu- minum, carbonates, gypsum) and organic oxides. Some materials occurring in the B horizon may have formed in place from weathering processes rather than arriving there by translocation, especially in the humid tropics.
Together, the A, Ae, and B horizons are designated the solum, considered the true definable soil of the pro- file (and labeled in Figure 18.2). The horizons of the solum experience active soil processes.
C Horizon Below the solum is the C horizon, made up of weathered bedrock or weathered parent material. This zone is identified as regolith (although the term some- times is used to include the solum as well). The C horizon is minimally affected by soil operations in the solum and lies outside the biological influences experienced in the shallower horizons. Plant roots and soil microorganisms are rare in the C horizon. It lacks clay concentrations and generally is made up of carbonates, gypsum, or soluble salts of iron and silica, which can form cementing agents. In dry climates, calcium carbonate commonly forms the cementing material that causes hardening of this layer.
R Horizon At the bottom of the soil profile is the R (rock) horizon, consisting of either unconsolidated (loose) mate- rial or consolidated bedrock. When bedrock physically and chemically weathers into regolith, it may or may not contribute to overlying soil horizons.
Soil Characteristics
A number of physical and chemical characteristics dif- ferentiate soils and affect soil fertility and the resistance of soils to erosion. Soil fertility is the ability of soil to sus- tain plants. Billions of dollars are expended to create fer- tile soil conditions, yet the future of Earth’s most fertile soils is threatened because soil erosion is on the increase worldwide.
Here we discuss the most widely applicable proper- ties for describing and classifying soils; however, other properties exist and may be of value depending on the particular site. Manuals for describing, mapping, survey- ing, and analyzing soils can be found at the Agriculture and Agri-Food Canada website (sis.agr.gc.ca/cansis/taxa/ cssc3/index.html).
Physical Properties
The physical properties that distinguish soils and can be observed in soil profiles are colour, texture, structure, consistence, porosity, and moisture.
Soil Colour Colour is important, for it sometimes sug- gests composition and chemical makeup. If you look at exposed soil, colour may be the most obvious trait.