Exposed rock surfaces are the source of soil and soil nutrients.
Weathering results in the genesis of new soil and the introduction of vital minerals and elements into the environment and results from both chemical and physical processes. Chemical weathering involves exposed rock and mineral surfaces undergoing chemical reactions which in turn result in a loss of material. Physical weathering also results in loss of exposed materials, but the processes involved do not change the chemical composition of the minerals.
Soil Formation
Physical and chemical reactions that occur on the surfaces of rock formations and mineral deposits break these structures down into smaller and smaller pieces. Some of the minerals, such as those containing quartz, iron and aluminum undergo further alterations from chemical reactions with oxygen, carbon dioxide and acids in the atmosphere. These all result in the mineral portions of soil, which are sand, silt and clay. Soil is the substrate for plant growth, which is the driving factor for atmospheric oxygen thereby allowing life on earth. Additionally, the soil is home to populations of microorganisms that fuel the majority of biogeochemical processes of carbon, nitrogen, phosphorus and sulfur.
Soil Chemical Reactions
Elements and compounds release from weathering processes are not automatically available for assimilation by plants and soil microorganisms. Furthermore, elements differ in their overall availability for uptake based on how strongly they held within and on soil particles. For example, clay particles have an overall negative charge therefore positively charged elements, such as magnesium, potassium and iron, are held onto the clay surface. Because they are absorbed onto the surface of the clay particles, these elements are not readily lost from the soil profile due to water movement. Conversely, negatively charged ions such as nitrate are not readily held by soil particles are therefore easily lost from the system. The positively charged ions are also able to buffer the soil acid rain. Acid rain introduces positively charged hydrogen ions into the soil profile, which can be toxic to plants and animals. However, the hydrogen displaces other positively charged ions, like iron and potassium, on the surface of the clay particles and becomes unreactive. This process is known as soil buffering because the soil literally "buffers" changes in pH as the hydrogen becomes unavailable for reactions.
Changes in Weathering Rates
Many chemical weathering reactions are driven by water, and are largely a factor of what all is dissolved in water. Carbon dioxide strongly influences chemical weathering, and as the concentration of atmospheric carbon dioxide has increased there has been more dissolved carbon dioxide in rain water, thereby increasing the rate of chemical weather. Furthermore, weathering is a slow process and prior to human interference, was able to keep up with soil loss from erosion. However, activities such as agriculture and urban development have increased the amount of soil erosion and weathering cannot entirely keep up. Therefore, there have been decreases in soil fertility, increased sediment loads to aquatic systems and alterations in global biogeochemical cycles.
Tags: positively charged, charged ions, chemical reactions, clay particles, carbon dioxide, chemical weathering
