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5.3 Acidification 131

prolonged leaching of bases, at higher elevations due to erosion, and under intensive
weathering over a long period of time due to predominance of iron and aluminum
oxides. But agricultural practices may accelerate or create soil acidity within a short
time. Acidification in agricultural soils may be due to application of nitrogenous
fertilizers, leaching of nitrates, removal of produce, and buildup of soil organic matter
(Upjohn et al. 2005).

5.3.1.1 Fertilizers
The amount of acidification that results from using nitrogenous fertilizers
depends on the fertilizer type (Table 5.4). Fertilizers that contain nitrogen as
ammonium, for example, ammonium sulfate, acidify the soil within weeks after
application. The most important acid-forming reaction for fertilizers is microbial
oxidation of ammonical fertilizers, which may themselves be strong bases, by
the following reactions:
−
+
1. NH 3 + 2O 2 = H + NO 3 + H 2 O (nitrification of ammonia)
−
+
2. NH 4 NO 3 + 2O 2 = 2H + 2NO 3 + H 2 O (nitrification of ammonium nitrate)
–
+
3. CO(NH 2 ) 2 + 4O 2 = 2H + 2NO 3 + H 2 O + CO 2 (hydrolysis of urea and nitrification
of products)
The materials above are the most common synthetic N inputs to agroecosys-
tems, either as single or mixed NPK fertilizers, and will oxidize to the equivalent
of nitric acid under the well-drained, aerobic conditions of most dryland agri-
cultural soils. Calcium nitrate and sodium nitrate have a neutralizing effect on
soil acidity, unless all the nitrate is leached, but they are expensive and use is
restricted to horticulture. Using superphosphate fertilizer on crops and pastures
does not directly acidify the soil. Applying pure sulfur or “flowers of sulfur”
will acidify the soil.
A long-term experiment was laid out in 1962 by Dr. Lloyd A. Peterson at the
Arlington Agricultural Research Station, located in South Central Wisconsin, as a
4 × 4 × 4 NPK fertility trial. After 30 years of fertilizer additions, soil samples were
analyzed and reported by Barak et al. (1997). Application of ammonium nitrate and
urea N fertilizers at the long-term fertility trial at Arlington, Wisconsin, had caused
soil acidification. Mean treatment values of pH ranged from 5.6 to 4.8 when
measured in water and from 5.5 to 4.1 when measured in 1 M KCl for 0–150 lb N/
acre annually treatments, respectively. A list of fertilizers with varying acidifying
potential is given below.
Slightly acidifying fertilizers: Urea, ammonium nitrate, and urea ammonium nitrate
solutions
These products are slightly acidifying because they contain ammonium or produce
ammonium when applied to the soil. For every kilogram actual nitrogen applied,
1.8 kg of pure calcium carbonate is required to neutralize the acidity.
Moderately acidifying fertilizers: Diammonium phosphate (DAP)