The role of nitrogen fertilizer in soil pH levels
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.