By Donald G. McGahan, Ph.D.
Published: February 3, 2013
Refreshed:March 15, 2016
Soil Surface and Subsurface Texture Class
Historically terms such as heavy soil and light soil have been used to describe soils texture. These term are currently mostly unsatisfactory. This is because they simply do not provide enough detail. We still have simplifications, but heavy and light are no longer used.
Today twelve textural classes are recognized in the United States Department of Agriculture (USDA) system of classification. The idea behind textural classes is actually quite logical.
First, texture is the size separates of sand (2 mm to 0.05 mm), silt (0.05 mm to 0.002 mm), and clay size (< 0.002 mm). The “gold standard” for determination of these size separates is based on a weight of the sum of each size separate (sand, silt, and clay). The mass of the size separates of sand, silt, and clay are expressed as a percent. The sum of the sand, silt, and clay equal 100%.
It is important to recognize that coarse fragments (this is land/soil judging speak for rocks) and organic matter are not included or considered part of the soil textural classes.
The twelve textural classes are typically conveyed in a “ternary diagram” that is more commonly called a textural triangle (Figure: Textural Classes). The size separate(s) weight, as a percent, intersect to give one of the textural classes.
Individuals involved in careers that revolve around soil, use a technique called “texture-by-feel” to determine the amounts of sand, silt, and clay in a sample. Scientifically, and to practitioners, this is termed “apparent texture,” but do not let that “apparent” word fool you because there are specific, and well defined instances, that the apparent texture trumps the gold standard of weighing the size separates to determine their relative percentage (gravity sedimentation by pipet method). A flow chart was devised by Thein (1979) to help the decision process for texture-by-feel (Texture-by-Feel Analysis Figure).
Additionally, individuals involved in careers that revolve around soil can become very, very, good at correctly determining the correct textural class. It is a routine procedure to determining the ‘clay separate’ percentage by this “texture-by-feel” method. Also, when the texture is “sand” or “loamy sand” the size of the ‘sand separate’ is further separated into size classes: very coarse sand, coarse sand, medium sand, fine sand, and very fine sand.
It is important for the student judger to understand that clay, silt, and sand refer to a ‘size separate,’ or a ‘textural class’. What is being discussed, sometimes, can only be determined by the context in which the term is used. This is knowledge that the student soil judger gains by being involved, and training for, the event.
When the student judger is determining the textural class for the Future Farmers of America (FFA) Career Development Event (CDE) the size separates are not determine individually. In fact, the FFA CDE student judger does not even have to determine one of the 12 textural classes.
Different members of these 12 classes are sometimes combined to give smaller numbers of textural groupings. The twelve classes are combined to give five classes: moderately coarse, medium, and moderately fine sandy, loamy, and clayey. These five groupings are further grouped into three groupings.
The moderately coarse, medium, & moderately fine are further grouped to give a loamy category. The fine is called clayey, and the coarse is called sandy in the more general grouping of three classes.
The five classes are what the student judgers will be asked to identify in the FFA CDE Land Evaluation and the Groupings of Soil Textural Classes diagrams their relationship (Figure: Groupings of Soil Textural Classes). The use of the five (5) member grouping of textural classes is not a compromise specifically for the FFA CDE Land Evaluation event, but is professionally defined and the Land Evaluation Event helps develop the students skills, abilities, and knowledge.
Suggestions regarding identification of textural classes follow.
There is no substitute for experience in evaluating soil texture, but there are some generalizations that can serve as useful guidelines. The usual approach is to moisten a soil sample and ribbon it out between the thumb and forefinger. Estimates of the textural class may then be made based on the way the soil feels, and on the way it ribbons out. Generally, sand feels gritty, silt feels powdery and floury or smooth when wet, and clay feels sticky. Soils that are high in clay tend to form longer, stable ribbons. As an approximation, the following guidelines, and the “texture-by-feel” analysis flow diagram should be useful in evaluating texture.
One word of caution is that organic matter is not considered when determining soil textural classes, but organic matter can sometimes be felt as greasy and it can increase the ability of the soil to ribbon. As such, the person ribboning soil must be on guard not to allow the “greasy” feeling organic matter to influence the textural determination. This is often most troublesome when differentiation between coarse and moderately coarse textures where a coarse textural class is fine enough to almost be a moderately course, but is not moderately course. Also, consider that the coarseness of the sand content also influences our senses. With adequate organic matter and when the sand is fine or very fine a weak ribbon can be produced, but without the organic matter the texture might clearly be a coarse textural class.
Staining of the fingers is an indication of aggregating agents such as iron oxides (yellow to orange in color) and organic matter (dark browns and blacks) that are often associated with the clay size separate. The ‘gold standard’ laboratory method for most soils is to remove the organic matter, iron oxides, and any carbonates prior to determining the mass of the sands, silts, and clays in a sample. As stated previously organic matter is to be ignored. While these colors infer much about other process happening in the soil to a trained soil scientist we do our best to ignore, or adjust for, the staining of the fingers for textural analysis.
- Coarse – A soil that feels gritty and is loose, very friable, and the individual grains can be readily seen or felt. When squeezed between the thumb and forefinger, it feels gritty and will not ribbon longer than 1/2 inch in length. If squeezed when dry it will fall apart when pressure is released. When moist, a mold may be formed which is unstable and crumbles as the soil is handled.
- Moderately Coarse – Textured soil feels gritty but contains enough silt and clay to make moist soil hold together. The individual sand grains can readily be seen and felt. A very weak and loosely structured ribbon less then 1 inch long may be formed between the thumb and forefinger when wet. If squeezed when wet it will form a mold that can be carefully handled without breaking.
- Medium – Textured soils have a slightly gritty (less than Moderately Coarse though!), smooth, or velvety feel when moist. When moistened, a weak to moderate ribbon generally not greater than 1 inch can be formed. The smeared surface of the ribbon is usually dull. The mold formed by squeezing when moist can be handled freely without breaking.
- Moderately Fine – Textured soils feel smooth to slightly gritty when wet. When the soil is moist, it will form a 1 to 2 inch ribbon generally with a shiny surface. Moderately fine soils usually break into clods or lumps when dry.
- Fine – Textured soil is often very sticky and very plastic when wet. It will form a long ribbon exceeding 2 inches in length. When dry, the soil forms hard, massive clods or lumps.
When gaining the skill of determining Texture-by-Feel calibration against known textures is very helpful. Some individuals consistently produce longer ribbons and some individuals consistently produce shorter ribbons. Reproducibility is the desired skill.
While determining the textural class following the flow diagram is helpful the professional skill also consists of determining the percent of clay mass in the sample. The Texture-by-Feel flow diagram including the eleven Textural Classes above includes tests for silty and sandy loam, clay loam and clay Textural Classes. While this can be helpful also for FFA judgers it is rarely applied.
The size of sand can have a profound impact on the beginner. The courser the sand the greater initial impact it makes on our perceptions. Fine and very fine sand do not seem to impact our senses as strongly. Nevertheless, sand mass is sand mass regardless of the sand size and experiencing a range of sand sizes in various mixtures and contents can be helpful.
Following is a video created at Univeristy of California Davis demonstrating some dexterity at texture-by-feel.
That is all.