Physical properties of soil
The physical properties of soil, in order of decreasing importance for ecosystem services such as crop production, are texture, structure, bulk density, porosity, consistency, temperature, colour and resistivity.
Properties that are influenced by soil texture include porosity, permeability, infiltration, shrink-swell rate, water-holding capacity, and susceptibility to erosion.
In the illustrated USDA textural classification triangle, the only soil in which neither sand, silt nor clay predominates is called loam.
While even pure sand, silt or clay may be considered a soil, from the perspective of conventional agriculture a loam soil with a small amount of organic material is considered "ideal", inasmuch as fertilizers or manure are currently used to mitigate nutrient losses due to crop yields in the long term.
[15] Sand is the most stable of the mineral components of soil; it consists of rock fragments, primarily quartz particles, ranging in size from 2.0 to 0.05 mm (0.0787 to 0.0020 in) in diameter.
There is no clear relationship between the size of soil mineral components and their mineralogical nature: sand and silt particles can be calcareous as well as siliceous,[18] while textural clay (0.002 mm (7.9×10−5 in)) can be made of very fine quartz particles as well as of multi-layered secondary minerals.
Soil structure affects aeration, water movement, conduction of heat, plant root growth and resistance to erosion.
[26] Water, in turn, has a strong effect on soil structure, directly via the dissolution and precipitation of minerals, the mechanical destruction of aggregates (slaking)[27] and indirectly by promoting plant, animal and microbial growth.
[29] Clayey soil, due to its differential drying rate with respect to the surface, will induce horizontal cracks, reducing columns to blocky peds.
[33] At the same time, root hairs and fungal hyphae create microscopic tunnels (micropores) that break up peds.
[36] The addition of the raw organic matter that bacteria and fungi feed upon encourages the formation of this desirable soil structure.
[38] At the same time, the edges of the clay plates have a slight positive charge, due to the sorption of aluminium from the soil solution to exposed hydroxyl groups, thereby allowing the edges to adhere to the negative charges on the faces of other clay particles or to flocculate (form clumps).
[49] However, it has been shown that, depending on species and the size of their aggregates (faeces), earthworms may either increase or decrease soil bulk density.
[50] A lower bulk density by itself does not indicate suitability for plant growth due to the confounding influence of soil texture and structure.
[52] Hence the positive correlation between the fractal dimension of soil, considered as a porous medium, and its bulk density,[53] that explains the poor hydraulic conductivity of silty clay loam in the absence of a faunal structure.
[59] Tillage has the short-term benefit of temporarily increasing the number of pores of largest size, but these can be rapidly degraded by the destruction of soil aggregation.
A soil's resistance to fragmentation and crumbling is assessed in the dry state by rubbing the sample.
[87] The color of the ground cover and its insulating properties have a strong influence on soil temperature.
[93] All physical, chemical, and biological processes in soil and roots are affected in particular because of the increased viscosities of water and protoplasm at low temperatures.
In some northwestern parts of the range, white spruce occurs on permafrost sites[95] and although young unlignified roots of conifers may have little resistance to freezing,[96] the root system of containerized white spruce was not affected by exposure to a temperature of 5 to 20 °C.
[99][101][102][103][104] Such insensitivity to soil low temperature may be common among a number of western and boreal conifers.
[105] Soil temperatures are increasing worldwide under the influence of present-day global climate warming, with opposing views about expected effects on carbon capture and storage and feedback loops to climate change[106] Most threats are about permafrost thawing and attended effects on carbon destocking[107] and ecosystem collapse.
The Red River of the South carries sediment eroded from extensive reddish soils like Port Silt Loam in Oklahoma.
In general, color is determined by the organic matter content, drainage conditions, and degree of oxidation.
Munsell color dimensions (hue, value and chroma) can be averaged among samples and treated as quantitative parameters, displaying significant correlations with various soil[115] and vegetation properties.
[114] The development and distribution of colour in a soil profile result from chemical and biological weathering, especially redox reactions.
[112] As the primary minerals in soil parent material weather, the elements combine into new and colourful compounds.
