Soil morphology is the field observable attributes of the soil within the various soil horizons and the description of the kind and arrangement of the horizons. C.F. Marbut championed reliance on soil morphology instead of on theories of pedogenesis for soil classification because theories of soil genesis are both ephemeral and dynamic.
The observable attributes ordinarily 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.
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.
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 a 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 a 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.
Soil composition by laboratory methods
An experienced soil scientist can determine soil texture in the field with decent accuracy, but not all soils lend themselves to accurate field determinations of soil texture. The mineral texture can be obfuscated by high soil organic matter, iron oxides, amorphous or short-range-order aluminosilicates, and carbonates. Soil texture is the relative relations of the components sand, silt, and clay. Texture is most often reported as percentages on a mass basis. Laboratory methods employ chemical pretreatments to mediate the effects of organic matter, iron oxides, amorphous or short-range-order aluminosilicates, and carbonates.
- Buol, Stanley W.; Southard, Randal J.; Graham, Robert C.; McDaniel, Paul A. (2003). Soil Genesis and Classification, 5th Edition. Ames, Iowa: Iowa State Press, A Blackwell Pub. Co. p. 494. ISBN 0-8138-2873-2.
- Soil Survey Staff (1993). Soil Survey Manual. Washington D.C.: U. S. Government Printing Office. Soil Conservation Service, United States Department of Agriculture Handbook 18. Retrieved 2006-11-03.
- Horgan, Graham W. (1996). "A review of soil pore models" (PDF). Retrieved 2006-11-03.