How "Humus" contribute to soil acidity in acidic soil?

Question

I have come asross following sentence. I am always under the impression 'humus' reduces soil acidity. This is cotaraducting to common belief. Please explain in depth.

{The continued leaching Dissolving out by the action of a percolating liquid of soils results in the replacement of calcium, magnesium, potassium and sodium ions by hydrogen ions and the formation of acid soils with low pH. In acid soils, the dissolution of aluminosilicate minerals occur and the aluminium ions thus released increase the acidity owing to hydrolysis. Similarly, humus and hydrous oxides contribute to soil acidity at low pH.}

Answer 1

Increased amount of humus is associated with a lowered soil pH.   Remember the definition of pH is the negative log of the hydrogen ion activity (or concentration, it means an increase the amount of hydrogen).   With the increase in humus concentration in the soil comes an increase in carboxyl groups and an increase in hydrogen ion concentration.  These same groups are also partially responsible for the associated increase in cation exhange capacity (cation holding capacity).

Answer 2

Ok, maybe it is true that organic matter will acidify the soil somewhat.  However, one must look at the broader picture instead of focusing on one very minor attribute.  Unless it's already an organic soil like peat or something you'd find in the Everglades area of Florida, I imagine the nutrient contribution and increased water holding properties of the added organic matter is going to increase crop yields a lot more than the minor reduction in pH is going to lower them.

So, if the acidifying statement is something your professor said, agree that it lowers the pH but challenge him on the overall benefits of adding OM.  Make him work for his money!

Answer 3

I think this series of articles will explain everything to you. What you need for everything to function like you need and expect it to you need to have a healthy soil. Starting with a very acid soil, it is "sick" and things just don't work as you would normally expect them to. If you do not correct the low pH of the soil, things just continue to work to make things worse and worse.

http://www.attra.org/attra-pub/soilmgmt....

Answer 4

Humus may be said to chelate, making  ions such as nitrate and phosphate less available for leaching. This is their major contribution to slightly increasing pH in soils close to neutral or above.

That effect is totally overwhelmed in a soil that is acidic. It still happens, but is insignificant.

Beakdown of humus does however produce CO2 and thus carbonic acid. But this process is inhibited by acidity, if free oxygen levels are depressed. We see acidic peat bogs with very low rates of humus decomposition, so the peat, while sustaining an acidity do not decompose enough to drive it very high.

Answer 5

I would suggest you look up "humus" in wikapeodia,  There you will find that humus acts a buffer with both acidic and alkaline soils.   Since it is a buffer, if may be difficult to raise the pH above the buffered point, by the same token it makes it difficult to lower the pH.

Answer 6

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