Soil Geomorphology and Identification

By Adam Krumbein
Stevens Water Monitoring Systems, Inc.

When working with or studying the soil, it’s important to know what type of soil is being examined. Each type of soil has different characteristics, and will have different effects on water infiltration rates, water holding capacity, evapotranspiration rate, and other soil characteristics.

Soil sensors (also known as soil probes) are one popular way of measuring soil moisture, temperature level, conductivity, and other characteristics that are important to researchers, farmers, and others who rely on soil data for their work.

Measurements taken by the Stevens Hydra Probe II are not influenced by soil texture and bulk density, as the soil moisture calibration is very accurate. With this level of accuracy the user can make soil moisture comparisons between different types of soils.

The following guide is an overview of soil properties (soil geomorphology), with follow-up links to online resources for more information.

The Stevens Hydra Probe II Soil Sensor

Soil Texture

An easy way to help determine what type of soil you have is to simply feel it to determine texture and thus what the primary makeup of the soil is. Grab a baseball size portion of the soil in your hands and wet the soil with water, working the moist soil with your hands. The stickier it is, the more clay there is. The soapier the soil feels the higher the silt content. Grittiness is indicative of sand. The soil texture triangle to the right shows the 12 major soil texture classes and what percent of each type soil makes them up.

Soil textural classes based on the percentage of sand, silt, and clay.

Soil Horizons and Sensor Depth

Like selecting a topographical location, selecting the sensor depth depends on the interest of the user. Farmers will be interested in root zone depth while soil scientists may be interested in the soil horizons. Soil horizons are distinct layers of soil that form naturally in undisturbed soil over time. The formation of soil horizons is called soil geomorphology and the types of horizons are indicative of the soil order. Like other natural processes, the age of the horizon increases with depth.

The reason why it is so useful to have a soil sensor in each horizon is because different horizons have different hydrological properties. Some horizons will have high hydraulic conductivities and thus have greater and more rapid fluctuations in soil moisture. Some horizons will have greater bulk densities with lower effective porosities and thus have lower saturation values. Some horizons will have clay films that will retain water at field capacity longer than other soil horizons.

Knowledge of the soil horizons in combination with an accurate soil sensor will allow the user to construct a more complete picture of the movement of water in the soil. The horizons that exist near the surface can be 6 to 40 cm in thickness. In general, with increasing depth, the clay content increases, the organic matter decreases and the base saturation increases. Soil horizons can be identified by color, texture, structure, pH and the visible appearance of clay films.

Soil Horizon Names and Descriptions:
O: Decaying plants on or near surface
A: Top Soil, Organic Rich
B: Subsoil, Most Diverse Horizon and the Horizon with the most sub classifications
C: Weathered/aged parent material
E: Horizon leached of organic or mineral content (light in color)

soil horizons (image courtesy USDA).

Soil Orders and Taxonomy

Soil, just like plants and animals, has been broken down by scientists into a hierarchical classification system, which is as follows: orders, suborders, great group, subgroup, family, and series. While there are thousands of types of soil around the world, they can all be classified under 12 major orders. Taken from the USDA NRCS Soil Taxonomy Website, these are the 12 major orders:

	Alfisols: Alfisols are found in in semiarid to moist areas. They formed under forest or mixed vegetative cover and are productive for most crops.
	Andisols: Andisols tend to be highly productive soils. They are common in cool areas with moderate to high precipitation, especially those areas associated with volcanic materials.
	Aridisols: Aridisols are soils that are too dry for the growth of mesophytic plants. They often accumulate gypsum, salt, calcium carbonate, and other materials that are easily leached from soil in more humid environments. Aridisols are common in the world’s deserts.
	Entisols: Entisols occur in areas of recently deposited parent materials or in areas where erosion or deposition rates are faster than the rate of soil development; such as dunes, steep slopes and flood plains.
	Gelisols: Gelisols are soils that have permafrost near the soil surface, have evidence of frost churning, or ice segregation. These are common in the higher latitudes or high elevations.
	Histosols: Histosols have a high content of organic matter and no permafrost. Most are saturated year round, but a few are freely drained. They are commonly called bogs, moors, pears or mucks.
	Inceptisols: Inceptisols are soils of semiarid to humid environments that generally exhibit only moderate degrees of soil weathering and development. These occur in a wide variety of climates.
	Mollisols: Mollisols are soils that have a dark colored surface horizon relatively high in content of organic matter. The soils are base rich throughout and therefore are quite fertile.
	Oxisols: Oxisols are highly weathered soils of tropical and subtropical regions. They characteristically occur on land surfaces that have been stable for a long time. They have low natural fertility as well as a low capacity to retain additions of lime and fertilizer.
	Spodosols: Spodosols formed from weathering processes that strip organic matter combined with aluminum from the surface layer and deposit them in the subsoil. These tend to be acid and infertile.
	Ultisols: Ultisols are soils in humid areas. They are typically acid soils in which most nutrients are concentrated in the upper few inches. They have a moderately low capacity to retain additions of lime and fertilizer.
	Vertisols: Vertisols have a high content of expanding clay minerals. They undergo pronounced changes in volume with changes in moisture. Because they swell when wet, vertisols transmit water very slowly and have undergone little leeching. They tend to be fairly high in natural fertility.

In order to more accurately determine what type of soil you are dealing with, the USDA NRCS has put thousands of their soil surveys online in an easy-to-use program called the Web Soil Survey. Use their online tools to find out what types of soils reside in your area.