An understanding of physical properties of soil leads to an understanding of how other processes may take place. The majority of these properties also change very slowly, therefore, a small difference can mean a lot. Soil texture is determined based on the proportions of the soil that contains sand, silt, and clay. This effects specific surface area, water relations, tillage, erosion, agrichemicals relations, and environmental considerations related to soil. In particular specific surface area plays a key role in determining the interface or contact zone between particles and their surrounding environments. Organic content is important for soil fertility, water absorption, and nutrients for the soil. Water content is important as plants need water to grow, and it is also needed to help replace water in the water table. Color helps to determine what the specific context of the soil may be. For example, a reddish soil indicates exposure to oxygen and possibly iron. Lastly, pH affects the surrounding environment of the soil, and its ability to hold minerals such as calcium.
There are basically three schools of thought when it comes to classifying soils; engineering, soil science, and OSHA. I am going to go into the most detail with the soil science approach because it is what I am most familiar with. Unified Soil Classification System (USCS) is the most common engineering classification system used in North America. It breaks soils down into three groups; coarse-grained soils, fine-grained soils, and high organic soils. Coarse-grained soils are sand and gravels, while fine-grained soils are things like clay and silt. High organic soils are peat soils, which I discussed in my wetlands overview post. These three groups are then further broken down. The coarse-grained soil classification is broken down into sand and gravel based on the particle size. Typically this is done using a stack of sieves that sort soil by particle size. The top sieve has the largest mesh, and progressively the mesh gets smaller and smaller. The stack can be shaken by hand or placed in a shaker.