The role of nitrogen fertilizer in soil pH levels
* NO2- + H2O → NO3- + 2H+ +2e-
Nitrobacter is the most frequently identified genus associated with this second step.
Hydrogen (H+) is released in the process of nitrification, and free hydrogen ions increase acidity. The higher the percentage of ammonium (or urea) in the fertilizer, the greater the acidification potential.
Another reason that NH4+ increases acidity has to do with plant uptake. As plant roots absorb NH4+ they secrete H+ ions into the soil solution to maintain a chemical charge balance.
Acidification potential of nitrogen fertilizers
* Acidification potential: Neutral
Potassium nitrate (13% N)
Calcium nitrate (15.5% N)
Because all of the nitrogen in these fertilizers is in the nitrate form, these fertilizers are not acidifying so there is no need to apply lime to neutralize acidity.
* Acidification potential: Moderate
Anhydrous ammonia (82% N)
Urea (46% N)
Ammonium nitrate (34% N)
Urea ammonium nitrate solutions (32% and 28% N)
These products are acidifying because they contain ammonium, or produce ammonium when applied to the soil. But they are less acidifying than DAP, MAP, or ammonium sulfate. Unlike DAP and MAP, anhydrous ammonia and urea do not leave any phosphoric acid residue remaining after they dissolve in soil solution. Ammonium sulfate leaves sulfuric acid residue as it dissolves. With ammonium nitrate and UAN solutions, only part of the total N is in the ammonium form, so these products result in less nitrification than fertilizers in which all the N is in the ammonia or ammonium form.
* Acidification potential: Moderately high
Diammonium phosphate (DAP) (18% N, 46% P2O5)
Diammonium phosphate has a moderate acidifying effect when applied.
* Acidification potential: High
Ammonium sulfate (21% N, 24% S)
Mono-ammonium phosphate (MAP) (11% N, 52% P2O5)
These fertilizers are very acidifying. Ammonium sulfate not only results in acidification through the process of nitrification, but one of the dissolution byproducts in sulfuric acid.
This may raise some other questions, though, such as:
A. Why is anhydrous ammonia less acidifying than MAP and DAP if they are all applied at the same N rate?
When anhydrous ammonia (NH3) is applied to the soil, it reacts with water to form ammonium-N and the hydroxide ion, which is basic.
* NH3 + H2O ⇄ NH4+ + OH-
This reaction initially raises the pH of the soil. It is only after the NH4+ undergoes nitrification that it begins to acidify the soil (through the release of H+). These two reactions (the basic effect of ammonia reacting with water vs. the acidifying effect of nitrification) don’t entirely balance each other out. The end result is an acidifying effect.