Soil Properties

Different soil have different physical, chemical and biological properties. These properties result from different processes (such as weathering or decomposition or burrowing of worms) occurring in the soil at different rates and in conjunction with other processes.

Some important physical properties used for soil classification

Soil texture is the proportion os sand, silt and clay in a soil. In the triangle below, 12 different soil textures are recognized. A texture designation is often modified by how much gravel or coarse material is in a soil. The soil texture triangle below shows the proportion of sand, silt and clay in the 12 different texture types. Another soil property related to soil texture that is used in classification are clay films which indicate the movement and accumulation of clay in a horizon.

Soil structure is the arrangement of soil particles into groups or clusters called aggregates (also known as peds). Structure is the form that the soil naturally breaks into when you pull it out some out of the ground and let it fall apart naturally. Unlike a beach sand, it doesn't usually completely separate into individual grains, but stays together in larger clumps. These clumps can have characteristic shapes:

spherical (granular or crumb structures)

blocky (subangular blocky or blocky, trapezoidal)

platey

prism-like (prismatic or columnar)

massive (all particles are cemented together)

single-grained (no structure--separates like beach sand)

Soil color is another important physical property of soil used in classification. Color changes in response to quantity of organic matter (brown), iron oxides (orange-red), calcium carbonate (white), or natural color of sandy grains or minerals (usually grey because individual light and dark minerals are averaged to a greyish color by the human eye). We denote color by its hue (how yellow-red or grey it is), value (how light or dark it is), and chroma (how intense the color is). For example, a soil color indicated as 5YR6/8 is a bright orange hue. The YR indicates the hue, 6 indicates value and 8 indicates chroma.

Bulk Density is the mass of dry soil per volume of soil. It is usually expressed as g cm-3 or kg m-3. As bulk density includes the volume of all pores as well as the volume of solids in the soil, bulk densities of soil are much lower than that of particle density (which is approximately 2.65 g cm-3). Bulk density can give information about a variety of soil components or properties such as compacted layers, pans, rock content or presence of volcanic ash.

Mineralogy is used many ways in classification. Primary minerals(those inherited from the parent materials) can indicate age, degree of formation and the presence of lithologic discontinuities in a soil. Skeleton grains are sand and silt sized particles not readily translocated. Secondary minerals are those that form in soils. The presence or type of layer silicate clays, amorphous clays, or oxides alters physical and chemical properties of the soil, such as plasticity or CEC, and indicates the degree of weathering a soil has experienced.

Temperature Regime is used throughout soil classification from the level of soil orders to family. Temperature is used so widely in classification because it affects soil processes and affects plant growth. Soil temperatures vary by season and by depth, with the most seasonal variability in near surface horizons and least variability at depth. Mean annual soil temperature is determined at a depth of 50cm, but can be estimated by adding 1 degree centigrade to mean annual air temperature. See the Keys to Soil Taxonomy for a listing of soil temperature regimes.

Moisture Regime is important in soil classification because of the influence of water availability on plant growth. Not just the total quantity of rainfall is important. How the rain is distributed throughout the year and how well the soil retain this moisture without holding it so tightly that plants can access it determines moisture regime. Thus, texture plays an important role in moisture regime. For example, a very coarse texture soil does not retain water well and water availability may limit plant growth during a dry season. An example of a moisture regime with limited water availability during the growing season is the xeric moisture< regime shown below. See the Keys to Soil Taxonomy for a description of all the moisture regimes. Moisture regimes are frequently used at the suborder level of classification due to the importance of water availability./P>

Redox features indicate a soil that is always or frequently water saturated, and thus has a low oxygen content (anoxic). Examples of redox features are mottles of different colors (such as orange splotches in a grey matrix), blackish/purplish nodules, or an all grey soil (has a low chroma).

Some important soil chemical properties used in soil classification

pH indicates how acid or basic a soil is. A pH of 1-6 is acidic, 7 is neutral and 8-14 is basic. pH can provide information about a variety of other soil properties (and processes) such as whether a soil has high or low base saturation, or if it has been highly leached.

Cation exchange capacity tell you how many exchange site there are on the surfaces of the soil particles. In other words, it is a measure of how many positively charged ions can be temporarily held by the soil but not held so tightly that they can't be utilized by roots or eventually 'bumped-off' by some other ion. In some soil orders, CEC is used at the family level to say how 'active' a soil is.

Base saturation is a measure of how many non-acidic cations are on the exchange sites of a soil. The surfaces of soil particles (particularly the clay-sized particles and the humus) can develop a negative charge that is balanced by cations that are attracted to there surfaces. The percentage of 'basic' cations that are on the surfaces relative to the total number of charges on all the surfaces of the particles is the %base saturation. A high base saturation usually indicates higher fertility and low leaching of cations.

Accumulations of salts (Salinity), calcium carbonate, and calcium sulfate indicate different types of diagnostic soil horizons. These soil horizons typically develop in drier climates (the salts are leached out in wetter areas). Th presence of these horizons can indicate the soil order or may be used a other lower levels of classification.

Carbon/O.Mcontent of a soil is an important property because it affects so many other physical and chemical soil properties as well as the quantity and type of organisms in a soil. For example, a soi with a high o.m. content typically has a lower bulk density, and will have a higher CEC (especially if the pH is higher). Carbon content is used to distinguish different genetic and diagnostic horizons which characterize different soil orders and many other lower levels of soil classification. The degree of decomposition distinguishes different types of O horizons, with Oi

Oxalate extractable Fe and Al indicates the amount of Fe and Al bound to amorphous (non-crystalline or pseudocrystalline) soil minerals, such as oxides, allophane, or imogolite. One way that oxalate extracts are used in soil taxonomy is to distinguish spodic materials and andic soil materials.

P retention is high in more developed volcanic soils, as P can be tightly bound by the Al-humic complexes that form in these soils. Thus, P-retention is used as an indicator for andic soil materials. This property is not used extensively in classification.

There are many other soil chemical and physical properties that are used in classification. If you have suggestion for others that you would like to see included here, let me know.