Soil pH
Soil pH is measurement of the hydrogen activity in the soil and not the concentration of hydrogen in the soil. The ionized H+ is only part of the total hydrogen that is in the soil. In most soils, the ionized H+ is only a small percentage of the total H+. While pH is a measurement of the intensity of the hydrogen, the exchangeable hydrogen is a measurement of the capacity of the soil to resist changes in pHs which is called the soil's buffering capacity.
pH is defined by the reaction H2O<--> H+ + OH- which has a solubility constant of 10 -14. When H+ exceeds OH- in solution, it is considered acidic and is called a sour soil and will have a pH below 7. When they are equal, the solution is considered neutral and has a pH of 7. When there are more OH- then H+ in solution, the pH is considered alkaline and is called a sweet soil. The change in pH is represented by the following equation pH = -log(H+). Thus every 1 drop in the pH represents a 10 fold increase in the H+ concentration in the soil solution. While a increase of one on the pH scale represents a 10 fold increase in OH- activity.
In very acidic soils such as found on acid mine sites, the pH is controlled often by the oxidations of sulfides. At a pH of below 4, Fe+3 removes OH ions from solution causing an increase H+ ions thus the low pH. At pHs between 4.5 and 6.5, Al+3, Al(OH)+2, and Al(OH)2+ control the pH of the soil by adsorbing OH- ions and thus releasing more H+ ions. At a pH of 6.5, the Al(OH)3 will precipitate out if the solubility constant of Al(OH)3 is exceeded and the soil is essentially based saturated. At pHs above 7.5, the soil pH shows the presence of CaCO3 which is hydrolyzed removing H+ ions and releases OH- into the soil solution.
In terms of the reserve acidity or H+ ions in soils, it is primary adsorbed to clay minerals and organic matter. The more clay and organic matter in a soil the more difficult it is to change the pH of the soil. This is called the buffering capacity of the soil.
Soil pH is the most important measurement that you should take of your soil. The reasons are it affects what nutrients are available, solubility of toxic metals, microbial activity, rate of decomposition of organic matter, and what specific plants you can grow on a site. It also affects soil structure because of the large hydration radius of H+, soils with a low pH will often be more dispersed.
The pH of the soil orginates with what type of parent materials form the soil. Tropical and high rainfall areas have lower pHs because of the rainfall leaching most of the more alkaline metals such as calcium and magnesium leaving the more acidic metals of aluminum and iron. Additionally fertilizers can affect the pH of the soil. ammonium based fertilizers go through the process of nitrification. The decomposition of plant materials and root respiration releasing CO2 into the solution results in a decrease in soil pH
Many of the micronutrients are much more available at lower pHs to point that they can reach excessive levels. Iron, Zinc, Copper, manganese, boron, The exception is molybdenum which are more available at higher pHs. Macronutrients vary more in terms of their availability with soil pH. Phosphorous is available less in soils with high Fe and Al contents at pHs below 6.0 and also at pHs above 7.5 because it ppt out with Ca. Ammonium based fertilizers can lose a sizeable portion of their nitrogen as (NH3) by the process of ammonia volatization at pHs above 7.2. At very low pHs, due to a restriction in the rate of nitrification, what nitrogen is available is as a ammonium ion.
The pH along with solution concentrations of soil affects the anion and cation exchange capacities. At higher pHs because of the removal of H+ from the exchange sites on variable charged organic matter and soil minerals, they are better at retaining cations in the soils.
Is their such thing as a perfect pH? For a long time 6.5 was thought to be the best pH but it has been found the liming a soil to above 5.5 eliminates most of soil pH related problems. The major one being reduced Al in solution thus elimination of Al toxicity problems. Most plants do have a range of pHs where they grow and produce better at. The main reason is what nutrients they require for optimum growth. Blueberries grow best at a pH range of 5.0 to 5.5 due to high Fe requirement and their ability to not be affected by Al toxicity. Azaleas can grow well at pHs below 5.0. Corn, wheat and bluegrass grows best in a range of 5.5 to 6.5 while sugar beets prefer a pH above 6.5 Legumes such as peanuts and peas need higher pHs because Rhizobium species activity decreases below at pHs below 6.0.
The first aspect of measuring your soil's pH is soil sampling which involves taking cores from 10 locations where you are growing plants. Additionally if you plan to grow different crops in different areas, you should take separate soil samples for each area. The reason is that soil pH can vary in a short distance so this gives a better estimate of what you will need to do to admend the soil. Soil pH is best measured by the use of a pH meter. The pH of the soil is measured either in a water or a 0.01M CaCl2 solution. The ratio of soil to water varies depending on what that laboratory procedure is. You can also use dyes, pH probes and other similiar pH measuring systems. But, they only give a relative measurement of the soil pH.
To admend the pH of the soils, agriculture lime and quicklime are used to raise the pH os the soils. Agriculture lime is slow release due to lower solubility and takes longer to raise the pH while quicklime is quick release but can burn crops because of the high pH it produces. To lower the pH of soils, sulfur based compounds are used.
Walter's Gardening Guide
Soil Information Links
Sources Used
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<http://www.esf.edu/pubprog/brochure/soilph/soilph.htm>
Hazelton, P. and Murphy, B. 2007. "Soil Chemical Properties" Chapter 5 in "Interpreting Soil Test Results". CSIRO Publishing, 150 Oxford Street, P. O. Box 1139, Collingwood, VIC 3066, Australia.
Katou, H " A pH-dependence implicit formulation of cation- and anion-exchange capacities of variable-charge soils"
JUL-AUG 2002. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL Volume: 66 Issue: 4 Pages: 1218-1224 19 Feb. 2010. <http://soil.scijournals.org/cgi/content/full/66/4/1218?maxtoshow=&hits=10&RESULTFORMAT=&fulltext=
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Mc Lean, E. O. 1982 "Soil pH and Lime Requirement" Chapter 12 in "Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties" Agronomy Monograph no. 9 (2nd Edition) ASA-SSSA 677 S. Segoe Rd., WI 53711.
Tisdale, S. L. et al. 1993. "Soil Fertility and Fertilizers" (5th Edition) MacMillan Publishing Company, 886 Third Avenue, New York, NY 10022.
Wikipedia contributers "Soil pH" 9 February 2010 Wikipedia, The Free Encyclopedia. 20 February 2010
<http://en.wikipedia.org/w/index.php?title=Soil_pH&oldid=342923603>