During a field study, morphological and physical characteristics of soil such as parent material, soil texture, topography, soil drainage classes, gravel content, slope, effective soil depth, and the rate of erosion are often used to determine the series, order, and age of a soil. Topography, soil drainage classes, soil texture, effective soil depth, and gravel content not only play important roles in determining soil series, but along with vegetation and climate are significant and fundamental factors for both understanding the formation of soils and analyzing the characteristics of farmland.
Determination of parental material is the first step in conducting a soil survey. A rock may be an assemblage of minerals bound together or it may be a mass of a single mineral. Thousands of rocks have been identified but they can be classified into three categories of the rock-forming processes: igneous rocks,
sedimentary rocks, and metamorphic rocks. Igneous rocks are sub-classified into acidic igneous rocks, neutral igneous rocks, and alkalic igneous rocks. Sedimentary rocks are usually formed from sedimentary deposits that are mechanically transported by water. Sedimentary rocks can be classified by their grain size into shale, sandstone, and conglomerate, and by their mineral content into limestone, zeolite, and diatomite, and by the degree of petrification into hard rocks and semi-petrified rocks. Soil scientists classify parent rocks into the following categories: acidic rocks, neutral rocks, alkalic rocks, sedimentary rocks, metamorphic rocks, Tertiary deposits, Quaternary deposits, and volcanic ash.
(Source: Korean Rural Development Administration, National Institute of Agricultural Science and Technology, 2007)
Soil drainage class refers to the frequency and duration of periods of saturation or partial saturation and affects runoff, infiltration, permeability, and internal soil drainage (percolation). In this manner, soil drainage classification is based on the moisture index of a soil profile which is determined by runoff, permeability, and proximity to the ground water table. However, artificial drainage and irrigation are not considered unless they significantly affect a soil's morphological characteristics. Soil drainage class descriptions include somewhat excessively drained, well drained, moderately well drained, somewhat poorly drained, and poorly drained. The following table shows morphological characteristics of soil profiles at for each soil drainage class.
(Source: Korean Rural Development Administration, National Institute of Agricultural Science and Technology, 2007)
| - Morphological characteristics of soil profile at each soil drainage class |
|
| Class |
Morphological characteristics of soil profile |
Land Use |
| Somewhat Excessively Drained |
Free of mottles
(high relief gravel or sand) |
Montane soil, Riverbed (sand) |
| Well Drained |
Brown or yellowish brown color but
free of grey mottles
(sandy loam to loam) |
Most farmlands |
| Moderately Well Drained |
A few grey mottles in deep soil
(primary soil color is brown or yellowish brown) |
Dry farmlands (some farmlands) |
| Somewhat Poorly Drained |
Primary soil color is grey
(a few brown, yellowish brown, and reddish brown mottles) |
Semi-dry farmlands |
| Poorly Drained |
Primary soil color is grey or dark grey
(yellowish or reddish mottles down to 50cm in the soil ) |
|
| Very Poorly Drained |
Dark grey color with
a very high ground water |
very wet farmlands |
|
Gravel in top soil is subject to damage by agricultural activities, whereas the content of gravel in deep soil is strongly associated with root growth. Higher gravel content (over 35% of gravel) impedes the growth of root crops and reduces the content of water and nutrients in the soil. Soil scientists classify gravel content in soil into the following three categories: below 10% (none), between 10 to 35% (some), over 35% (plenty).
(Source: Korean Rural Development Administration, National Institute of Agricultural Science and Technology, 2007)
Effective soil depth represents the depth of soil horizons. Areas with deep soil are often found either in the areas with well-weathered bedrock or in the area where sediments are deposited. While areas with shallow soil represent either poor soil formation or soil removal by erosion. Soil scientists classify soil by effective soil depth into 'very shallow (< 20 cm)', 'moderate(20-50cm)', and 'deep (> 100cm)'. It is recommended that effective soil depth should be at least 50cm to be used as rice paddies or farmlands.
(Source: Korean Rural Development Administration, National Institute of Agricultural Science and Technology, 2007)
Soil texture refers to the size and organization of non-organic particles in the soil which range from the finest clays, to silts, to coarser sands (below 2mm in diameter) and is a most important attribute that controls soil's physical and chemical characteristics. Soil texture is one of the sub-categories of Soil Taxonomy and it classifies soil into the following six categories: sand, sandy loam, silt, silt loam, clay loam, and clay. There are two methods to determine soil texture: textural analysis in the laboratory and feel method in the field. First, textural analysis uses a soil texture triangle to determine soil texture from the proportions of sand (coarse sand and fine sand), silt and clay. In the field, the textural class of a soil can be determined by rubbing a mass of wet soil between your fingers, but this method requires extensive experience.
(Source: Korean Rural Development Administration, National Institute of Agricultural Science and Technology, 2007)
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