As the parent rock weathers, it forms particles of differing sizes. Based on their diameter, these particles are classified into groups called soil separates. In decreasing order of size, the separates are:
1. Gravel (coarse and fine)—2.0 mm or more in diameter.
2. Sand (very coarse—[2.00-1.00 mm], medium [.50—.25 mm], fine [.25-.10 mm] and very fine [.10—.05 mm])
3. Silt—(.05—.002 mm in diameter)
4. Clay—less than 0.002 mm in diameter
The relative proportions of these separates of different sizes in any one soil create the soil texture. The proportions can only be determined precisely in a soil laboratory. There, a series of sieves are used to separate out the sand (and gravel, if present), while a suspension and settling technique is used to separate and measure the percentages of silt and clay (Figure 3—2).
Most soils in nature contain sand, silt, and clay in some proportions. The textural names given to soils are ways of describing these proportions. For example, if a soil contains about 40 percent sand, 20 percent
figure 3-2. To use soil sieves, a premeasured amount of dried soil is passed through the stack. Each sieve’s mesh is finer than the one before. Sands are separated by particle size. An additional suspension and setting technique is required to measure the percentage of silt and clay. (Delmar/Cengage Learning. Photo by Jack Ingels)
of the finer clay, and 40 percent silt separates, it is termed a loam. Loams are generally favored for horticultural field production because they have enough sand to provide good drainage and aeration, yet enough of the finer particles to retain moisture and provide necessary plant nutrients (as will be discussed later). When one or two separates dominate the mixture, the textural name given the soil reflects that domination, as in sandy loam, silt loam, silty clay loam, sandy clay, silty clay, and so on.
In order to appreciate soil textural names, the physical and chemical properties of the separates should be considered.
Sand particles have assorted shapes and sizes depending on how they were weathered. They range from smooth and round to sharp and angular. The spaces between sand particles are large compared to the spaces between silt and clay particles. Water passes through sand quickly because of this large pore space, and air is present in greatest quantity in sand. Sand is low in mineral nutrients and is generally inactive chemically.
Silt particles are irregularly shaped and much smaller than most sand particles. Given identical volumes of sand and silt, there would be many more silt than sand particles; hence silt has a greater surface area than sand. Since water clings to particle surfaces, the result is that silt holds water in the soil far better than sand. It does not provide as much space for air, however. Like sand, silt has a low nutrient level and is not very active chemically.
Clay has very small, plate-like particles. It possesses the greatest surface area of all the separates. Water is held tightly to the clay particles and passes very slowly through the soil. Predictably, air is often in short supply in a heavy clay soil, especially when it is wet. Clay has an adhesive quality when moistened and squeezed. This is what gives cohesiveness to soil, sometimes too much, creating sticky, hard-to-plow fields. Clay is active chemically. Many of the particles have surface charges that attract water and ions. The term for such a chemical state is colloidal. It is the colloidal quality of clay that makes it important for chemical activity and nutrient exchange in the soil.
The names of soil textures should now assume descriptive meaning. For example, a sandy loam would possess the following characteristics:
• sand, silt, and clay separates present, with sand dominant
• drainage good and perhaps slightly excessive
• nutrient content good since clay is present
• aeration good due to the sand, assuming that it is not too fine
• water-holding capability fair to good, depending on the amount of organic material and clay
The United States Department of Agriculture (USDA) soil texture triangle illustrates how soils are named, based on a laboratory determination of their composition (Figure 3-3).
HOW TO USE THE TRIANGLE:
I. Needed information: the mechanical analysis data for the soils to be named.
Examples: Soil #1 Soil #2 Soil #3
60 percent sand 22 percent sand 28 percent sand
30 percent silt 60 percent silt 36 percent silt
10 percent clay 18 percent clay 36 percent clay
II. Using any two of the percentages for each soil, project them into the triangle following the direction indicated by the arrows and the grid lines. Within the compartment where the two lines intersect, the textural name of the soil is read.
Soil #1 Soil #2 Soil #3
Sandy loam Silt loam Clay loam
figure 3-3. The soil texture triangle (Courtesy USDA)