how to test soil ph

Physical, chemical, and biological weathering act to weaken the particle bonds found in rock materials. As a result, weathered materials are normally more susceptible than unaltered rock to the forces of detachment. The agents of erosion can also exert their own forces of detachment upon the surface rocks and soil through the following mechanisms: Once a particle is entrained, it tends to move as long as the velocity of the medium is high enough to transport the particle horizontally. Within the medium, transport can occur in four different ways: Figure 1 is an image that was created from DEMs (Digital Elevation Model) for the following 1:24,000 scale topographic quadrangles: Telescope Peak, Hanaupah Canyon, and Badwater, California. To the left is the Panamint Mountain Range. To the right is Death Valley. Elevation spans from 3,368 to -83 meters and generally decreases from left to right. The blue line represents an elevation of 0 meters. Large alluvial fans extending from a number of mountain valleys to the floor of Death Valley can be seen in the right side of the image. The sediments that make up these depositional features came from the weathering and erosion of bedrock in the mountains located on the left side of the image. (This image was created with MacDEM software). Erosion begins with the detachment of a particle from surrounding material. Sometimes detachment requires the breaking of bonds which hold particles together. Many different types of bonds exist each with different levels of particle cohesion. Some of the strongest bonds exist between the particles found within igneous rocks. In these materials, bonds are derived from the growth of mineral crystals during cooling. In sedimentary rocks, bonds are weaker and are mainly caused by the cementing effect of compounds such as iron oxides, silica, or calcium. The particles found in soils are held together by even weaker bonds which result from the cohesion effects of water and the electro-chemical bonds found in clay and particles of organic matter. Entrainment also has to overcome the resistance that occurs because of particle cohesive bonds. These bonds are weakened by weathering or forces created by the erosion agent (abrasion, plucking, raindrop impact, and cavitation). Figure 2 shows a graph that describes the relationship between stream flow velocity and particle erosion, transport, and deposition. The curved line labeled "erosion velocity" describes the velocity required to entrain particles from the stream's bed and banks. The erosion velocity curve is drawn as a thick line because the erosion particles tends to be influenced by a variety of factors that changes from stream to stream. Also, note that the entrainment of silt and clay needs greater velocities then larger sand particles. This situation occurs because silt and clay have the ability to form cohesive bounds between particles. Because of the bonding, greater flow velocities are required to break the bonds and move these particles. The graph also indicates that the transport of particles requires lower flow velocities then erosion. This is especially true of silt and clay particles. Finally, the line labeled "settling velocity" shows at what velocity certain sized particles fall out of transport and are deposited.