Nitrogen transformations in the soil
Nitrogen undergoes transformations in the soil, depending on the chemical composition of the nitrogen applied.
Nitrate is taken up directly by plants, while ammonium and urea need to be first transformed into nitrate.
Nitrogen from nitrate
Nitrate (NO3 -) is easily absorbed by plants at high rates. Unlike urea or ammonium, it is immediately available as a nutrient. Nitrate is highly mobile in the soil and reaches the plant roots quickly. The negative charge of nitrate carries along positively charged nutrients such as magnesium, calcium and potassium. It is important to note that essentially all the nitrogen in the soil, whether it was applied as urea, ammonium or nitrate, ends up as nitrate before plants take it up. If nitrate is applied directly, losses from the transformation of urea to ammonium and from ammonium to nitrate are avoided.
Nitrogen from ammonium
Ammonium (NH4+) is directly absorbed by plants at low rates. The positively charged ion fixes to soil minerals and is less mobile than nitrate (NO3 -). Plant roots therefore need to grow towards the ammonium. Most of the ammonium is transformed into nitrate by soil microbes. This nitrification process depends on temperature and takes between one and several weeks. Another part of the ammonium is immobilized by soil microbes and released only over longer periods of time, thus building up soil organic matter.
Nitrogen from urea
Plant roots do not directly absorb the ureic form of nitrogen in significant quantities. Urea needs to be first hydrolyzed to ammonium by soil enzymes, which takes between a day and a week, depending on the temperature. Moisture is required for hydrolysis.
The ammonium generated by hydrolysis does not, however, behave exactly as the ammonium from ammonium nitrate. Hydrolysis of urea results in a short-term alkalization in the immediate vicinity of the urea grain applied. It shifts the natural balance between NH4+ and NH3 to the latter form, resulting in volatilization losses.
These losses are the main reason for the lower N-efficiency observed with urea. This is also the reason why urea, whenever possible, should be incorporated into the soil immediately upon application.
