I have sent my soil sample for analysis. One of the results obtained shows hydrolytic acidity is not detected. May I know what is the meaning of hydrolysis acidity?
In my 30+ years of teaching soil chemistry, this is the first time see the term "hydrolytic acidity". Nevertheless, my guess is that the term refer to the following reaction:
Al(3+) + H2O Al(OH) (2+) + H(+)
If your soil sample has no hydrolytic acidity, then it has no soluble aluminum.
Yen Teng Tai:
Total soil acidity is sum of two types of acidity: exchange acidity (more common) and hydrolytic acidity.
Exchange acidity is defined by the amount of acidity contributed by free hydrogen ion (very little in typical soil) and acidic cations (e.g., Al3+) that are neutralized by base. This is called exchange acidity because neutral cations from added salt (Say K+ from KCl) exchange or displace these cations such as Al3+ from soil, which contributes to the acidity of soil.
Exchange acidity, however, does not account for acidic hydroxyl group or carboxyl group (e.g., R-COOH; R is organic group comprised of C and H). Soil acidity contributed by these groups is known as hydrolytic acidity (because these weak acids, from soil surface, undergoes hydrolysis to contribute to soil acidity).
Example, R-OH + H2O = R-O- + H3O+ (acidity)
So one can assume that hydrolytic acidity is contributed by humus (or organic matter) in soil.
(refer to any soil science encyclopedia).
Encyclopedia of Soil Science edited by Ward Chesworth page 53. Do search in google book to find this page.
or here: http://books.google.com/books?id=EOYYM0-DAGQC&lpg=PA53&dq=hydrolytic%20acidity%20soil&pg=PA52#v=onepage&q=hydrolytic%20acidity&f=false
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