The role of nitrogen fertilizer in soil pH levels
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 fertilizer materials.
Amount of ag lime (ECC) required to neutralize the acidity created by nitrogen fertilizer.
Fertilizer N Concentration Pounds of ECC needed as lime
to neutralize the acidity
from 1 lb. of actual N
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
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: