Nitrogen Management in Soil

Nitrogen, a macro and primary nutrient element is limiting nutrient in most of the soil. Its proper management requires the knowledge of several factors which reduces its availability in soil. In Nepal Urea is the main source of fertilizer used to supply N to the crop. Urea contains 46 percent of N, and it is present in amide form. Chemically it is in organic form because of its carbon content (NH₂ CO NH₂), but it is considered inorganic or chemical fertilizer because it is not derived from organs.

Urea when surface applied and not thoroughly incorporated is highly prone to loss via ammonia volatilization. Volatilization is the loss of n in the form of ammonia gas, and this type of loss is found to be increase rapidly if the soil is in alkaline condition and the temperature is high. Through incorporation helps to reduce loss because the ammonia thus released after hydrolysis absorbs water and from NH₄OH which release NH₄⁺ and OH^– ions. Also Urease, the enzyme which hydrolyzes Urea, activity increases with increase in temperature. Applying the fertilizer with some depth helps to decrease the Urease activity thus release of ammonium takes place in gradual way.

Even though net effect of Urea is to increase the acidity of soil, during its hydrolysis it consumes some hydrogen ions and makes soil alkaline first, so it has high volatilization as compared to ammonium sulfate. Ammonium sulfate has more acidic effect in soil than Urea and donot absorb hydrogen ions upon hydrolysis.

Loss of N in the gaseous form of N and its oxides (N₂, N₂O, NO) is called denitrification. The process of denitrification is also called dissimilatory reduction of nitrate. Nitrate when moves or present in reduced zone (i.e. where there is lack of oxygen) goes to the process of denitrification. There are a large groups of organisms which helps in denitrification. In rice field denitrification loss can be reduced by applying the ammonium form of fertilizers in reduced zone. In such zone ammonium will not be converted into nitrate thus preventing the loss through denitrification.

Alternate wetting and drying cycle are found to be buttressing for high N loss, because in aerated condition most of the N is present in nitrate form. When those aerated condition soils are wetted there will be lack of oxygen and reduction reactions dominate. The nitrate form of N is used by microorganisms as an electron acceptor i.e. alternative of oxygen, so dessimilatory reduction of nitrate takes place causing loss in the form of N and its oxide gases. So, continuous submergence will be beneficial to minimize the N loss in rice field.

Immobilization is process of assimilatory reduction of nitrate by the microorganism to build up their cell proteins. If the organic matter in the soil has wide C:N ratio, the N present in OM would not be sufficient for the N required for their protein synthesis, they utilize the mineralized nitrate and make it unavailable for some period. Application of organic matter having wide C:N ratio like saw dust increase the immobilization.

Leaching is the process of washing out of nutrient with percolating water. Nitrate form of N is not tightly held by soil particles is prone to leaching. Use of heavy single irrigation should be replaced by multiple light irrigations.

Ammonium fixation is the process of fixation of ammonium ion in the inter-layer spacing of clay colloids. Application of K fertilizer will help to release the fixed ammonium, because of nearly equal sizes of NH4+ and K+ ions.

Note: use of Urease inhibitors (e.g. hydroquinone, PPD), nitrification inhibitors (e.g. nitrapyrin, DCD),  slow release fertilizers (e.g. Urea-form), Urea in super granules, Urea in mud balls, coated Urea (sulfur, wax, lac, resin, neem etc) etc all help to minimize the N loss by reduced solubilization  of Urea and slow release of nitrate.