Soil Textures
Soil texture is a parameter used in both the field and laboratory as an instrument of classification. The physical texture of the soil is used to determine such classification. This texture can be determined using qualitative methods like texture by feel, and also by using multiple quantitative methods such as the hydrometer method, the pipette method, the POM (particulate organic matter) method, or the rapid method. The hydrometer method is the most widely used of the quantitative methods. Soil texture focuses primarily on particle sizes that are less than 2 mm in diameter. Those that fit this criterion include sand, silt, and clay. Classification systems are typically based on the observed percentages of sand, silt, and clay. Class systems most used are the USDA soil taxonomy and WRB soil classification systems which both use 12 classes of texture, and also the UK-ADAS system which uses 11 classes [1].
Texture Classifications
The US has 12 soil texture classifications that are defined by the USDA [1]. These classifications include sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay [2]. The classifications are all determined by the fractions of sand, silt, and clay present for a particular soil sample. They are typically named for the dominating soil particle size (clay, silt, sand) or a combination of the most abundant ones (sandy clay, silty clay). Loam is more of a mixture of particle sizes composed mostly of sand, silt, and a smaller amount of clay. By weight, its mineral composition is about 40–40–20% concentration of sand-silt-clay, respectively. These proportions can vary, resulting in 5 of the previously mentioned classifications other than loam itself (clay loam, sandy loam, etc..). When one wants to determine soil texture, assistance from a soil texture triangle is usually needed [2]. The image to the right is an example of a typical texture triangle [9]. Each side of the triangle represents the percentages of sand, silt, and clay within a given soil sample. The closer you get to one side of the triangle, the percentage of that soil particle size will be greater. The triangle provides aid only if you know what percentages of sand, silt, and clay that you have in your given soil sample (see Hydrometer method). Once those percentages are found the triangle can be used in a very grid like fashion to determine which of the 12 classifications your sample falls into. To use the triangle, start with one of your particle size percentages, let’s say silt, and locate where that is on its side of the triangle. Then follow the slanted line down to the left until you arrive at your percentage of clay. That point’s location will tell you what soil class you have. Essentially after you choose your starting particle size side, move parallel to your second particle size side to locate your point. For example, if you find that your soil sample is 60 percent silt and 35 percent clay then your soil is classified as silty clay loam. This method can be used in a similar manner starting from any side of the texture triangle. The triangle can also be used inversely to see what percentages of each particle size is generally present for each soil class if one is using the texture by feel method. The texture is also related to chemical and physical properties of the soil. The distribution and particle sizes have an effect on the soil’s capacity for holding nutrients and water. Soils with finer textures will exhibit a higher water retention capacity, but this will decrease as particle size increases with a coarser soil texture [3].
Particle Sizes
For soil, there are specific particle size ranges that determine what its components are classified as. Be it clay, silt, or sand. Clay particles are amongst the smallest having diameters less than 0.002 mm. Clay is structured in a plate like manner which also allows for its hydroplastic properties induced by the increased surface area due to its small particle size [4]. Following clay are silt particles having diameters between 0.002 mm and 0.05 mm with the USDA classification. Sand has the largest of the particle sizes with diameters ranging from 0.05 mm to 2.00 mm. Sand is divided further due its large diameter range into the categories of very fine sand, fine sand, medium sand, coarse sand, and very coarse sand. The USDA and WRB classifications differ only slightly in the ranges of particle diameter, but both classify clay particles with the same value. The chart to the right provides a more detailed description of the soil particles and their classifications [8].
