Soil+(HL)

Soil (HL) By Yap Yi Zhen  12.3 Discuss the effects of soil pH on cation-exchange capacity and availability of nutrients 12.4 Describe the chemical functions of soil organic matter (SOM)

Once again, SOM is defined as soil organic mater and is used to represent the organic constituents in the soil, including undecayed plant and animal tissues, their partial decomposition products and the soil biomass.

= Cation-exchange capacity of soil (CEC) =

Humus and smectites have the greatest CEC   -both negatively charged à able to attract positively charged cations. Can be either   Acidic: H+, Fe2+, Mn2+ and Al3+    Or    Basic: Ca2+, Mg2+, K+    They are not removed from the soil with water; instead other cations have to be added for an exchange to take place. ∴ The CEC of soil is defined as its capacity to exchange cations with the soil solution   - Used as a measure of potential soil fertility : higher CEC = more fertile soil    E.g. Soil that has low percentage of clay can have its CEC increased by the addition of manure    Quality of soil linked to its pH level. - pH of soil influences how efficiently a crop grows by affecting nutrient availability   - Optimal pH for most plants is 6.0-8.0 or slightly acidic or slightly basic   o Exceptions: Azaleas, rhododendrons, blueberries and conifers   o Thrive best in acidic soils with pH ranging between 5.0-5.5   o pH scale and plant growth (Government of Ontario, 2010) Low pH = high levels of acidic cations such as aluminum and manganese, which are toxic to plants Acidic soils can be produced by:  o Nutrient depletion   o Acid deposition Addition of lime to moist acidic soils can raise the pH of the soils to a more suitable pH range The acidity or basicity of these cations can be shown using hydrolysis equations: - Acidic cations form H+ ions when mixed with water - Basic cations form OH- ions when mixed with water Hydrolysis of aluminum ions Al3+(aq) + 3H2O (l) -> Al(OH)3(aq) + 3H+ (aq) Result: a weak base and hydrogen ions Hydrolysis of Calcium ions Ca2+ (aq) + 2H2O (l) -> Ca (OH) 2(aq) + 2H+ (aq) O (l) Results: a strong base and hydrogen ions We are able to determine the concentration of various chemical species when they form precipitates in water. In a reaction where precipitate is formed, the precipitate dissolves to some extent which allows for a dynamic equilibrium to be established between the precipitate and the dissolved ion. The equilibrium between solid silver chloride and its saturated solution is given AgCl(s) ↔ Ag+ (aq) + Cl-(aq) The equilibrium constant for this heterogeneous equilibrium is called the solubility product, Ksp and can be written for slightly or nearly insoluble ionic compounds Ksp = [Ag+][ Cl-] Generally, solubility product for a salt MmNn is shown as: Ksp = [M]m [ N]n =Bibliography= Works: Clark, J. (2002). Retrieved October 2, 2010, from Buffer solutions: http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.htm   Environmental Chemistry. In Chemistry: For use with the IB Diploma Programme (pp. 309-310) Neuss, G. (2007). //Chemistry Course Companion.// New York: Oxford University Press. Pictures: Government of Ontario. (2010, July 12). //Ministry of Agriculture Food and rural affairs.// Retrieved October 2, 2010, from Soil Management - Chemical Properties: http://www.omafra.gov.on.ca/english/environment/soil/chemical.htm    Picture of layers of soil from: http://www.enchantedlearning.com/geology/soil/
 * Due to technical difficulties, some equations do not have sub-script or super-script. Sorry for the inconvenience caused.