Soil morphology encompasses the field-observable attributes of the soil within various soil horizons and the description of the kind and arrangement of these horizons. C.F.
Marbut (2003) championed reliance on soil morphology instead of theories of pedogenesis for soil classification because theories of soil genesis are both ephemeral and dynamic.
Observable Attributes of Soil Morphology in Agriculture
The observable attributes typically described in the field include the composition, form, soil structure, and organization of the soil, color of the base soil, and features such as mottling, distribution of roots and pores, evidence of translocated materials such as carbonates, iron, manganese, carbon, and clay, and the consistence of the soil.
The observations are typically performed on a soil profile. A profile is a vertical cut, two-dimensional, in the soil and bounds one side of a pedon. The pedon is the smallest three-dimensional unit, but not less than 1 meter square on top, that captures the lateral range of variability.
Soil Micromorphology in Agricultural Analysis
While soil micromorphology begins in the field with the routine and careful use of a 10x hand lens, much more can be described by careful description of thin sections made of the soil with the aid of a petrographic polarizing light microscope.
The soil can be impregnated with an epoxy resin, but more commonly with a polyester resin (crystic 17449) and sliced and ground to 0.03 millimeter thickness and examined by passing light through the thin soil plasma.
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Soil Porosity and Its Impact on Crop Growth
Porosity of topsoil typically decreases as grain size increases. This is due to soil aggregate formation in finer textured surface soils when subject to soil biological processes. Aggregation involves particulate adhesion and higher resistance to compaction.
Typical bulk density of sandy soil is between 1.5 and 1.7 g/cm³. This calculates to porosity between 0.43 and 0.36. Typical bulk density of clay soil is between 1.1 and 1.3 g/cm³. This calculates to porosity between 0.58 and 0.51. This seems counterintuitive because clay soils are termed heavy, implying lower porosity.
Heavy apparently refers to a gravitational moisture content effect in combination with terminology that harkens back to the relative force required to pull a tillage implement through the clayey soil at field moisture content as compared to sand.
Porosity of subsurface soil is lower than in surface soil due to compaction by gravity. Porosity of 0.20 is considered normal for unsorted gravel size material at depths below the biomantle. Porosity in finer material below the aggregating influence of pedogenesis can be expected to approximate this value.
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.
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Soil Compaction and Its Effects on Agricultural Productivity
Cultivation disturbs the soil, causing fragmentation, compaction, and displacement (Roger-Estrade et al., 2000). Consequently, two types of earth may appear within the soil profile: fine and compacted zones (most often defined as clods). The presence of a large portion of high penetration-resistant clods is one of the most serious factors limiting soil exploration by plant roots (Hoad et al., 1992).
Since water content fluctuates during the growing season and clod penetration resistance is strongly linked to moisture, impedance of clods to root growth changes considerably.
An additional factor closely linked to these interactions is the intensity of soil compaction, which can be readily interpreted as an increase of bulk density of a specific soil type (Goldsmith et al., 2001).
If soil becomes compacted to the level that plant growth is impaired, the compaction must be alleviated through several measures intended to restore satisfactory growth conditions. Especially, loosening and subsoiling aim at eliminating soil compaction and preventing reduced soil-rooting depth (Carter, 1988).
However, it is safe to assume that even optimal techniques and excellent timings of agronomic operations cannot fully eliminate soil compaction. Thus, soil impedance remains one of the most important factors influencing crop yield (Atwell, 1988, Stenitzer and Murer, 2003).
Some previous studies showed that soil compaction decreased root development and delayed root colonization of deeper soil layers (e.g., Ehlersb et al., 1982). Frequently, the relation between the relative root elongation rate and soil penetration resistance was used to demonstrate the importance of soil compaction for root growth (Bennie, 1991).
However, it appears that there is a lack of studies on alterations of other morphological properties of roots due to soil compaction, and hence on plant physiological processes and crop production (Bengough, 2003).
The relationship between soil compaction and root growth was usually studied using a homogeneous substrate (see for instance Unger and Kaspar, 1994), structured soil conditions were considered very rarely (Amato and Ritchie, 2002).
Soil morphology is composed of composition, form, soil structure, and organization of the soil, color of the base soil, and features such as mottling, distribution of roots and pores, evidence of translocated materials such as carbonates, iron, manganese, carbon, and clay, and the consistence of the soil.
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