Acid soils are becoming an important issue in Kansas, even in the
western reaches of the state where most people think of high pH as a bigger
issue. The primary reason for our 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
Amount of ag lime (ECC) required to neutralize the acidity
created by nitrogen fertilizer.
of ECC needed as lime
neutralize the acidity
1 lb. of actual N
Ammonium nitrate 34%
Anhydrous ammonia 82%
UAN Solutions 28-32%
Ammonium Sulfate 21%N 7.2
Monoammonium Phosphate 11%
Diammonium Phosphate 18%
How nitrification increases soil acidity
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:
+ O2 → NO2- + 3H+ + 2e-
Nitrosomonas is the most frequently identified
genus associated with this step.
the second step of the process, nitrite-oxidizing bacteria oxidize nitrite to
nitrate according to the following equation:
+ H2O → NO3- + 2H+ +2e-
Nitrobacter is the most frequently identified genus
associated with this second step.
(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.
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
* Acidification potential: Neutral
nitrate (13% N)
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
ammonia (82% N)
nitrate (34% N)
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
phosphate (DAP) (18% N, 46% P2O5)
Diammonium phosphate has a moderate acidifying effect
* Acidification potential: High
sulfate (21% N, 24% S)
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+
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+
This reaction is immediately followed by another reaction
that takes H+ ions out of soil solution:
* (2) HCO3- + H+ → CO2
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:
4 ⇄ NH4+
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+
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
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.
-- Dave Mengel, Soil Fertility
-- Dorivar Ruiz Diaz, Nutrient