Acid soils are becoming an important issue. The primary reason for soils becoming more acid over time is the use of nitrogen (N) fertilizers containing ammonium-N, including mono and diammonium phosphates, 11-52-0 and 18-46-0. As the ammonium-N in fertilizers nitrifies, acidity is released.
One common way to express the relative acidifying effects of N fertilizers is the pounds of Effective Calcium Carbonate, ECC, required to neutralize the acidity from 1 pound of actual N. That value varies from 3.6 to 7.2 for the fertilizers we commonly use. The table below shows the actual pounds of ECC needed to neutralize the acidity produced by the N from common fertilizer materials.
Here's the amount of ag lime (ECC) required to neutralize the acidity created by nitrogen fertilizer.
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Fertilizer N Concentration Pounds of ECC needed as lime
to neutralize the acidity
from 1 lb. of actual N
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Ammonium nitrate 34% N 3.6
Anhydrous ammonia 82% N 3.6
Urea 46% N 3.6
UAN Solutions 28-32% N 3.6
Ammonium Sulfate 21%N 7.2
Monoammonium Phosphate 11% N 7.2
Diammonium Phosphate 18% N 5.4
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How nitrification increases soil acidity
To understand the acidification potential of various nitrogen fertilizers, it is helpful to understand how the process of nitrification increases soil acidity. In the first step of nitrification, ammonia-oxidizing bacteria oxidize ammonia to nitrite according to the following equation:
* NH3 + O2 → NO2- + 3H+ + 2e-
Nitrosomonas is the most frequently identified genus associated with this step.
In the second step of the process, nitrite-oxidizing bacteria oxidize nitrite to nitrate according to the following equation:
* 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.
B. Why is urea less acidifying than MAP and DAP, if they are all applied at the same N rate?
In soil solution, urea first reacts with water and free H+ ions to form ammonium-N and bicarbonate.
* (1) CO(NH2) 2 + 2H2O + H+ → 2NH4+ + HCO3-
This reaction is immediately followed by another reaction that takes H+ ions out of soil solution:
* (2) HCO3- + H+ → CO2 + H2O
Both these reactions “soak up” free H+ ions in soil solution, which reduces acidity. This reduction in acidity is more than balanced out by the acidifying reaction of the nitrification of ammonium-N. As with anhydrous ammonia, the overall net effect is acidifying.
C. Why is MAP slightly more acidifying than DAP when applied at the same N rate?
Ammonium phosphates, such as MAP and DAP fertilizers, are extremely soluble in soil solution, and dissolve easily. Knowing what happens to each product after it dissolves helps explain this effect.
MAP -- The pH of MAP in saturated solution is 3.5. MAP contains one ammonium-N and one H2PO4- ion. The reaction in soil solution is:
* NH4H2PO 4 ⇄ NH4+ + H2PO4-
This reaction does not use up any H+ ions in soil solution, so the full acidifying effect of nitrification impacts the soil pH level.
DAP -- The pH of DAP in saturated solution is 8.0. DAP contains two ammonium-N ions and one HPO4-2 ion. In soil solution, DAP initially undergoes the following reaction:
* (1) (NH4)2HPO 4 ⇄ 2NH4+ + HPO42-
If the soil solution pH is greater than 7.0, the orthophosphate ion will be stable and not react any further. If the soil solution pH is less than 7.0, the orthophosphate ion will react with the free hydrogen ions and reduce the acidity somewhat.
* (2) HPO42- + H+ ⇄ H2PO4-
To the extent that it occurs, the second reaction balances some of the acidifying effect of the nitrification of the ammonium-N ions. That’s why DAP has a slightly less dramatic acidifying effect on the soil than MAP.
Soil acidity and aluminum toxicity
We should also note that strictly speaking, soil acidity is a measure of the concentration of H+ ions in the soil solution. But the greatest injury to crop growth from low pH soils comes not from the H+ ions, but from the release of aluminum into the soil solution at low pH levels.
As the pH decreases below 5.5, the availability of aluminum and manganese increase and may reach a point of toxicity to the plant. Excess Al in the soil solution interferes with root growth and function, as well as restricting plant uptake of certain nutrients, namely, Ca and Mg. Liming acid soils reduces the activity of Al and Mn.
