The process of nitrification is vital in ensuring the availability of the essential element nitrogen to many plants. This is so when fertilizers that contain ammonic nitrogen (NH4+, ammonium ion) are applied into the soil.
Because the soil particles are predominantly negatively charged, the ammonium ions are adsorbed following the rule that opposite electrical charges are attracted to each other. But the retention capacity of the soil is affected by its cation exchange capacity which varies with soil texture.
The more clay particles the soil contain, the more ammonium ion will be held. The maximum amount of ammonium ion that the soil can hold is further reduced by other positively charged elements. These cations are also held by some of the negatively charged clay particles.
While attached to the soil particles, the ammonium ions are available for plant absorption via roots that are able to reach them. However, because of this requisite for actual contact, the advantage of ammonic over nitrate form is only minimal.
According to Thorup (1984), about 98% of the nitrogen absorbed by most crops is in the nitrate (NO3-) form. In mature corn, only about 2% of the total volume of the soil in which it is rooted is occupied by its root system. However, some plants, such as rice and cranberries, favor ammonic nitrogen over nitrate.
The ammonium ion can also be converted to mobile nitrate nitrogen (NO3-). Like ammonium, nitrate is a form in which nitrogen can be absorbed by plants.
Under favorable conditions for the activity of microorganisms, ammonic nitrogen (NH4+) can be rapidly converted to nitrate nitrogen (NO3-) by nitrifying bacteria. The conversion process is called nitrification and involves two steps: the conversion of ammonium (NH4+) to nitrite (NO2-) through the action of Nitrosomonas and Nitrosococcus bacteria; and the conversion of nitrite to nitrate by Nitrobacter bacteria. As shown below, these two reactions generate energy which the bacteria use for their body functions
By Nitrosomonas and Nitrosococcus bacteria:
2NH4+ (ammonium) + 3O2 ----> 2NO2- (nitrite) + 4H+ + 2H2O + energy
By Nitrobacter bacteria:
2NO2- (nitrite) + O2 ----> 2NO3- (nitrate) + energy
The factors that affect the nitrification process include soil temperature, soil pH, soil texture, population of nitrifying bacteria, rate of nitrogen application, soil moisture, and oxygen supply. Ideal conditions that favor this process include warm temperature (70-96°F or about 21-36°C), moist soil, adequate soil aeration, and soil pH near 7.0. Corrective farm practices to enhance include drainage to improve aeration in otherwise waterlogged soils, cultivating compacted soils, and watering or providing irrigation water to dry soils.
Once converted to nitrate form, nitrogen becomes mobile. It diffuses into the soil solution and moves with water. It can be absorbed by plants or move downward or sideward with water and back again to the root zone through capillary action.
But nitrogen loss can occur when it is in the nitrate form. Depending on such factors as root depth, frequency and volume of water applied either by rainfall or irrigation, and the amount of fertilizer applied relative to the requirement of crop at certain stage of growth, nitrogen may leach downward below the rooting zone.
Further, nitrogen may be lost from the soil where nitrate nitrogen is converted to lesser oxidized forms and finally to N2. This occurs when the soil is depleted of oxygen such as in flooded soils or due to prolonged waterlogging. Without oxygen, some soil organisms utilize nitrate for their growth processes. This conversion process is called denitrification and the pathway is shown below:
NO3- ----> NO2- ----> NO ----> N2O ----> N2
Once converted to NO2, NO, N2O, and N2, which are gaseous forms of nitrogen, they can escape into the atmosphere. This will constitute nitrogen loss. In contrast, there is no nitrogen loss through denitrification from ammonic nitrogen even under the same anaerobic conditions. It must first be oxidized to nitrate under conditions that favor the process. This is the reason why ammonic nitrogen is preferred as source of nitrogen for rice grown under flooded soils.
THORUP RM (ed.). 1984. Ortho Agronomy Handbook: A Practical Guide to Soil Fertility and Fertilizer Use. San Francisco, CA: Chevron Chemical Company. 454 p.
(Ben G. Bareja Jan. 2013)