Soil Geomorphology and Identification
By Adam Krumbein
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.
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 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:
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:
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.