Changes of Soil and Nitrogen

This article throws light upon the five pathways through which the changes of soil as well as fertilizer nitrogen takes place. The pathways are: 1. Mineralisation 2. Immobilization 3. Fixation of Nitrogen in Soils 4. Losses of Nitrogen from Soils 5. De-Nitrification.

Pathway # 1. Mineralisation:

Mineralisation of nitrogen is defined as the process by which nitrogen in organic compounds becomes converted into the inorganic ammonium (NH₄⁺) and nitrate (NO₃^–) ions carried out by different soil micro-organisms.

This process operates through three step-by-step reactions namely, aminisation, ammonitication and nitrification. The first two steps are carried out by heterotrophic micro­organisms and the last one chiefly by auto trophic soil bacteria.

(i) Aminization:

This process will operate in different types of soils having differential soil reaction like, neutral, alkaline and acidic. This process is believed to be carried out by heterotrophic soil micro-organisms like various groups of bacteria and fungi, each of which is responsible for one or more steps in various reactions in the process of organic matter decomposition.

It is evident that bacteria pre-dominantly carried out the breakdown of proteins (combined with organic N compounds) in neutral and alkaline soil reactions, whereas under acidic soil reactions fungi become active.

Aminization is defined as the process by which the hydrolytic decomposition of proteins from combined nitrogenous compounds as well as release of amines and amino acids takes place by heterotrophic soil micro-organisms.

This process is also known as proteolysis (organic N compounds → Amino-N). However, this process can take place both under aerobic and anaerobic soil condition but end products are varied. The major end products under aerobic condition of proteolysis are CO₂, (NH₄)₂SO₄ and H₂O, whereas under anaerobic proteolysis, the end product are NH₃, NH₂, CO₂ organic acids, H₂S, mercaptans etc.

(ii) Ammonification:

The reduction of amines so released from the above reactions to ammoniacal compounds by some other groups of heterotrophic micro-organisms is known as ammonification.

After production of NH₄⁺ -N from organic materials, it undergoes several transformations in soils which are as follows:

(i) By the process of nitrification (carried out by soil micro-organisms), this NH₄⁺— N may be converted into NO₂^–—N and NO₃^–—N forms.

(ii) This so formed NH4—N may be absorbed by the plants.

(iii) It may be used by heterotrophic micro-organisms for further decomposition of organic carbon residues if any (Immobilisation).

(iv) It may be fixed by soil colloids especially by clay colloids.

(v) It may be released very slowly to the atmosphere through volatilization process.

The conversion of organic nitrogen to ammonium nitrogen (Org. N →NH₄⁺—N) does not require a specific kind of organism but is carried out by various groups of soil micro-organisms and hence it is said to be effected by non-specialized organisms. This conversion process is believed to slow.

However, the ammonification process operates in both aerobic and anaerobic soils. But in case of aerobic soils, this process further proceeds and converts NH₄⁺—N to NO₃^–—N via NO₂^–—N forms through the process of nitrification carried out by some specific organisms. In case of anaerobic soils (e.g. submerged soils), the transformation of nitrogen (Org. N → NH₄⁺—N) stops at the stage of NH₄⁺—N production due to lack of oxygen in soils required for carrying out the microbial activity.

(iii) Nitrification:

Nitrification is defined as the process by which the microbial oxidation of ammoniacal nitrogen to nitrate form of nitrogen takes place.

This process is a two-step process consisting of:

(i) Conversion of ammonium to nitrite nitrogen (NO₂^–—N), and

(ii) Conversion of nitrite to nitrate (NO₃^–—N).

However, the above two conversion pro­cesses are carried out by two separate kinds of micro-organisms that are schematically shown below:

(i)

It has been also found that various heterotrophic organisms viz. bacteria, actinomycetes and fungi are capable of converting reduced nitrogenous compounds to nitrite (NO₂). Nitrite is produced from the materials containing ammonium; amines, amides, hydroxylamines, oximes, other various reduced nitrogenous compounds etc.

However, the micro-organism, Nitrosomonas can effectively bring about the conversion of NH4 —N → NO₂ —N.

Various genera like Nitrosomonas, Nitrosolobus, Nitrosospira have been isolated from different types of soils, but soils receiving farm yard manure and other animal excreta respective micro-organisms like Nitrosomonas and Nitrosolobus are found. Nitrosolobus plays a much more significant part in carrying out the nitrification process in soils.

(ii) Nitrite so formed by the ammonium oxidize autotrophs is quickly converted to Nitrate form of nitrogen (NO₂^– —N →NO₃^– —N) through oxidation chiefly by Nitrobacter species as follows:

However, the net reactions of two step processes of nitrification

Nitrosomonas and Nitrobacter are collectively known as the Nitrobacteria. Both ammonium and NO₂^– oxidisers are obligately aerobic. Therefore, in waterlogged soils the oxidation of NH₄⁺ is thus restricted. In addition, the nitrifying bacteria act favourably in soils having neutral or slightly acidic reactions.

Under strong acid condition of the soils, the activity of those micro-organisms has been found to be depressed with the simultaneous reduction of NH oxidation.

Pathway # 2. Immobilization:

Nitrogen immobilization means the change of mineral nitrogen to organic forms by soil micro-organisms. Assimilation is sometimes used in the same sense as immobilization. Whether immobilization will take place or not depends on the initial nitrogen content in the organic materials applied to the soil.

In general, the amount of nitrogen content in soil is more or less when applied organic materials containing about 1.7% N.

When the N content of added organic materials is low (< 1.2%), the amount of inorganic nitrogen content in the soil immediately decreases as the soil micro-organisms take up that soil nitrogen for their body metabolism and subsequent multiplication. Under this condition, a shortage of nitrogen for growing crops may be met up by supplying adequate amount of nitrogenous fertilizers to the soil.

On an average, the magnitude of immobilization of N is usually less due to bacteria, intermediate due to fungi and higher due to actinomycetes. However, the C: N ratio of the added organic materials to the soil mostly determines mineralization and immobilization processes. Therefore, it will be worthwhile to discuss the C: N ratios of the organic materials affecting both processes in the soil.

Carbon: Nitrogen (C: N) ratios:

The C: N ratio organic materials applied to soils shows marked effect on positive and negative nitrogen release. A C: N ratio of approximately 20: 1 has been found to be as the dividing line (critical level) between immobilization and mineralization (release of inorganic nitrogen). However, the following C: N ratio determines the different processes like mineralization, immobilization etc.

The nitrogen deficit in the soil, due to the application of organic materials having wider C: N ratio is expressed as the N-factor. Nitrogen factor is defined as the number of units of inorganic nitrogen immobilized for each 100 units of material undergoing decomposition.

On the other hand, the nitrogen factor may be defined as the amount of nitrogen required to prevent a net immobilization. On an average, the values for the nitrogen factor vary from 0.1 or less to 1.3, depending on the nature and the stage of decomposition of the organic materials to be added in the soil.

Gains of N by Soils:

The gain of nitrogen by the soil takes place through various ways which are as follows:

(i) Biological nitrogen fixation.

(a) Symbiotic—bacteria which live in symbiotic relationship with the host plant (legume) and fix nitrogen in the nodules.

(b) Non-symbiotic—nitrogen fixed in the soil by certain free living and associative organisms deriving energy from the decomposing organic matter.

(ii) Addition through precipitation.

(iii) Addition through manures, fertilizers, composts including vermicomposts, green manures etc.

(i) Biological nitrogen fixation:

This is the most important process by which nitrogen from inorganic molecular form in the atmosphere is fixed and converted to an organic form by a number of various kinds of soil micro-organisms is known as biological nitrogen fixation.

It is evident that biological N₂ fixation contributes substantially to the supply of N to crops. The amount of N₂ fixed, however, varies from one place to another and it is very much dependent on soil factors namely soil pH, available P, K the presence of heavy metals and the soil moisture regimes.

The conversion of the inert N₂ molecule into combined nitrogen (NH₃, NO₃^– etc.) which can be utilized as a mineral nutrient is brought about either by reduction to ammonia (NH₃) or oxidation to nitrate (NO₃^–). This conversion, also referred to as fixation, is highly energy consuming. In both industrial and biological conversion the reaction N₂→ NH₃ dominates.

In industrial fixation, N₂ is catalytically reduced to NH₃ by reaction with hydrogen (produced from natural gas) in the Haber-Bosch process (N₂ + 3H₂→ 2NH₃) under conditions of high temperature and pressure. The increase in both the costs of fossil energy and the worldwide demand for nitrogen fertilizer used for food production are main reasons for renewed interest in biological N₂ fixation as an alternative or at least a supplement to the use of high analysis chemical nitrogen fertilizer.

The capability of biological fixation of atmospheric nitrogen (N₂) is restricted to organisms with prokaryotic cell structure viz. bacteria and cyanobacteria. Eleven out of 47 bacterial families and eight cyanobacteria families are diazotrophs, i.e. capable of N₂ fixation. Agriculturally significant N₂ fixation is carried out by Eubacteria, many of which are heterotrophic, depending on supply of reduced carbon (e.g. Azospirillum).

Others are autotrophic and able to reduce CO₂ (e.g. Anabaena). A number of species of the genera Frankia (Thallobacteria), Nostoc and Anabaena (Cyanobacteria) and the rhizobia (Proto-bacteria) are of particular importance because of their symbiotic capabilities.

Based on their growth rates, two distinct groups of rhizobia exist, the fast growing genus Rhizobium and the slow growing genus Brady rhizobium.

Symbiotic, associative and free-living nitrogen fixing organisms have different energy sources and fixation capabilities which are shown in Table 21.2.

(a) Symbiotic Nitrogen Fixation:

There are two types of symbiotic systems found in relation to the carbon requirement for N₂fixation:

1. Nodulated legumes and nodulated non-legumes.

2. Symbiosis with cyanobacteria (Blue-green algae)

In system 1, the N₂ fixing micro-organisms, are either bacteria belonging to the genera rhizobium and Brady rhizobium (in legumes), or actinomycetes, of the genus Frankia (non-legume, actinorhizal symbiosis e.g. Casuarina). Casuarinas play an important role in agriculture associated with shifting cultivation and in afforestation of eroded soils.

In system 2, the fixing bacteria mainly derive their carbon (energy) requirement from the photosynthesis. Examples of this group of organisms are cyanobacteria of the genus Nostoc living symbiotically with fungi (e.g. lichens) free floating fresh water ferns of the genus Azolla with the heterocyst-forming cyanobacteria of the genus Anobaena or woody species of the genera Cycas and Macrozamia with cyanobacteria living in the coralloid roots. In both the legume and non-legume systems, symbiosis is characterized by more or less distinct host preference (e.g. Rhizobium and Brady rhizobium) or even host specificity.

Infection and Host Specificity:

Therefore, if legumes are introduced into soils in which the same or a symbiotically related legume has not previously been grown, seed inoculants with the appropriate species is required if effective N₂ fixation is to be achieved.

However, within a given combination (i.e. bacteria or host plant) there are great differences between strains of bacteria and host genotype in relation to both infection, nodulation and effectivity in N₂ fixation which may offer the potential to increase N₂ fixation.

Depending upon the different plant types infection by the micro-symbiont may occur on developing root hairs (e.g. clover), at the junction of lateral roots through structurally modified cell walls of the root cortex (e.g. many actinorhizal systems) or at the base of the stem. In stem-nodulating plant species like sesbanis rostrata, the infection oftenly takes place at sites where lateral root primordia may pierce the epidermis.

Stem nodules are formed basically by the same bacteria as in the case of root nodules, and as a rule, root and stem nodules occur on the same plant. The first step of host plant infection by the micro-symbiont requires recognition of the host.

Distinct phenolic compounds (flavonoids) such as lutein released by the roots act at low concentrations as a signal for chemo-taxis of rhizobia in the rhizosphere, and stimulate expression of nodulation (nod) genes in the rhizobia at higher concentrations, which are to be expected at the root surface.

Induction of nod genes is required for the production of lectins and attachment of the bacteria on the development root hairs. Root hair curling and cortical cell division by bacterial “signals” probably cytokines are the main features of the subsequent step followed by development of the nodule meristem and invasion by rhizobia, mediated by the host plant derived infection thread.

Inside the dividing nodule cells rhizobia are released into the plant cell cytoplasm and packaged inside a plant derived membrane (peribacteroid membrane). During this period, bacteria are transformed into bacteroids which are several times larger than the original bacteria and devoid of a cell wall. Infected root cells may contain up to 20,000 bacteriods.

The transformation to bacteriods is closely related to the synthesis of hemoglobin, nitrogenase and other enzymes required for nitrogen fixation and this is the stage of nodule development.

However, the principles of host plant recognition and infection are shown in the following figure 21.1:

Associative Nitrogen Fixation:

It is evident that a considerable amount of the carbon fixed during photosynthesis in higher plants released into the rhizosphere in the form of root exudeates or decaying root cell. The activity as well as population of soil micro-organisms including diazotrophic bacteria in the rhizosphere is thus several times more as compared to bulk, soils.

For cases where the diazotrophic bacteria live preferentially at the rhizoplane (root surface) and also within intercellular spaces of the cortex cells, the term rhizosphere associations have been introduced. The collaboration between the different organisms (host, microbe) is mutually beneficial in case of sysbiosis whereas in the case of associations the partnership is more casual and the nitrogen transfer move indirect.

Diazotrophic bacteria found in the stele of roots and in the stems of graminaceous infect epidermal and cortical root cells in which their nitrogenase activity is not only retained but also depressed by external nitrate supply than in the free-living bacteria.

The most common genera in associated systems are: Azospirillum, Azotobacter, Klebsiella, Enterobacter and Pseudomonas, of which first two genera are dominant especially in tropics.

Diazotrophic associative bacteria may accelerate host plant growth either by No fixation and enhancing nitrogen nutrition or by production of phytohormones and there by modifying the morphology and physiology of the root as well as growth and developmental processes or by both.

However, a tentative scheme regarding associations between diazotrophic bacteria and plant roots on hormonal effects and N₂ fixation is shown in figure 21.2.

Nitrogen Fixation by Free-living Micro-organisms:

Diazotrophic bacteria are ubiquitous in soils. They are anaerobes (e.g. Clostridium pasteurianum), facultative anaerobes (e.g. Klebsiella) and aerobes (e.g. Azotobacter, Azospirillum). The contribution of these micro-organisms for the nitrogen fixation is, however, very small because of carbon limitation.

Free living diazotrophs are also found on leaf surfaces and therefore, there is some prospect of utilizing N₂ fixation in the phyllosphere in tropical agricultural systems. Spraying on the leaves of wet land rice several times with N₂ fixing bacteria has been found to be substantially increased nitrogenase activity of the leaves, the grain yield and the nitrogen content of the leaves.

Pathway # 3. Fixation of Nitrogen in Soils:

Fixation of native as well as applied nitrogen takes place mainly as ammoniacal (NH₄⁺) form by clay minerals and organic matter present in soils. Ammonium (NH₄⁺) is strongly adsorbed to negatively charged clay minerals particularly to 2: 1 lattice clay minerals (e.g., illites, vermiculites and montmorillonite).

The process through which NH₄⁺—N is fixed by clay minerals is known as ammonium fixation and is similar to K⁺ fixation. Ammonium and potassium thus compete for the same binding sites on the clay colloids and therefore, the fixation of K⁺ has been found to be lowered with the application of ammoniacal fertilizers.

However, the so-called fixed NH₄⁺cannot be exchanged by K⁺ and is mostly unavailable to plant roots. The recent fixed NH₄⁺ may be very slowly available to the plants under certain favourable soil conditions. Fixation of NH4 by high organic matter is also found and the presence of hydroxyl groups in the organic matter may be the possible reaction sites resulting NH4 fixation.

The mechanism of NH₄⁺ fixation does not always consider a harmful process, but it has some practical implications in certain soils where there is a considerable amount of leaching of nitrogen particularly in sandy and sandy loam soils. In such situations, ammonium fixation may help to supply nitrogen for the plant growth.

Pathway # 4. Losses of Nitrogen from Soils:

The losses of nitrogen from soils occur through various mechanisms like leaching, run-off, gaseous losses like NH₃ volatilisaiton and de-nitrification etc.

Nitrogen applied as commercial nitrate fertilisers or produced by nitrification of ammonium is very often subject to leaching losses from the soil. Losses of nitrogen through leaching mainly depend on nature and properties of soils, rainfall, sources of applied nitrogen and other climatic factors. Leaching losses usually occurs in coarse textured soils where the rate of percolation is high.

In soils where the primary source of negative charges is organic rather than inorganic, the ammonium ion (NH4) is also subject to leaching with the percolating water.

However, various factors may be responsible for the losses of ammonium due to leaching in flooded as compared to upland, well-drained soils which are as follows:

(i) Ammonium ion (NH₄⁺) is not so likely to accumulate in a well-drained as in flooded, anaerobic soils.

(ii) Reduction reactions in a flooded soil produce sufficient amount of ferrous (Fe²⁺) and manganous (Mn²⁺) ions, which displace ammonium from the exchange sites to the soil solution where it is susceptible to leaching.

(iii) The constant head of standing water in a flooded soil results in greater downward percolation of the soil solution than that which occurs in a non-flooded soil.

It is evident that the broadcasted urea is susceptible to leaching and run-off losses from soils with low cation exchange capacity (CEC) because urea is weakly adsorbed by the soil colloids.

Pathway # 5. De-Nitrification:

De-nitrification process is defined as the microbial reduction of nitrate and nitrite with the release and loss of molecular nitrogen, in some instances, of NO₂. The process is strictly a respiratory mechanism (nitrate respiration), in which nitrate replaces molecular oxygen. Nitrogen losses in submerged soils occur mainly from de-nitrification process.

It is evident that 10-95% of the total volume of gas evolved from submerged soils in India is nitrogen, of which some amounts come from de-nitrification. Under anaerobic conditions (sub-merged soils) some anaerobic micro-organisms are able to derive their oxygen from nitrates and nitrites with the release of nitrogen and nitrous oxide.

The possible bio-chemical pathway leading to such losses of N is indicated below:

A few types of facultative aerobic bacteria are responsible for de-nitrification and the active species belong to the genera Pseudomonas, Bacillus and Paracoccus. A few species of Chromo- bacterium, Hyphomicrobium, and Serratia are implicated in denitrificaiton. Besides these some autotrophs like, Thiobacilus denitrificans and T. thioparus are also capable of bringing about the de-nitrification process.

In an aerobic soil, large populations of such denitrifying micro-organisms are found particularly in the rhizosphere zone. Carbonaceous root exudates are believed to provide support for the growth of denitrifying bacteria in the root zone.

The potential for de-nitrification is immense in most field soils but conditions must arise which cause such micro-organisms to shift from aerobic respiration to a denitrifying type of metabolism involving use of nitrate (NO₃^–) as an electron acceptor in the absence of O₂.

The rate of de-nitrification of added nitrate nitrogen to submerged soils is dependent upon the carbon content in soils, being slower in soils low in carbon than in soils rich in carbon.

However, the rate and magnitude of N loss through de-nitrification are influenced by various following factors:

(i) Ammonia Volatilization:

This process usually operates in alkaline and calcareous soils, but it may occur from flood water on a soil having moderate to slight acidic reaction.

Ammonium yielding or containing fertilisers when applied to soils react with CaCO₃ to form (NH₄)₂ CO₃ and calcium precipitates as the following general equation.

X(NH₄)₂Y + NCaCO_3(S)DN(NH₄)₂ CO₃ + Ca_n Y_x

where Y represents accompanying anion and N, X and Z are dependent on the valencies of the anion and cation.

The final reaction product, (NH₄)₂ CO₃, is most unstable and decomposes as follows:

The amount of NH₄OH formed in a particular time will depend on the solubility of Ca_n Y_x and its rate of formation. If Ca_n Y_x is insoluble, the reaction will proceed towards the right hand direction producing more (NH₄)₂ CO₃ and consequently more NH₄OH to be formed. If no insoluble compounds of calcium is formed, no appreciable amount of (NH₄)₂CO₃ will present.

When (NH₄)₂ CO₃ decomposes as the above equation, CO₂ is lost from the solution at a faster rate than NH₃ and thereby increases the concentration of OH^– ions. Consequently, higher amount of NH₄⁺ in the solution becomes electrically balanced by OH^– which would favour NH₃-volatilisation loss as follows:

NH₄⁺ + OH^–DNH₄OH DNH₃ ↑ + H₂O

If Ca_n Y_x is soluble, then loss of N through NH₃ volatilisation will be dependent upon the resultant pH of the soil. The NH₃—NH₄⁺ equilibrium is pH dependent with lower pH values producing NH₄⁺ forms.

Further, in case of submerged rice soils where volatilisation loss of NH₃ is very likely from flood water through diffusion from the soil, the photosynthetic and respiratory activity of submerged aquatic biota, their biomass, and factors affecting their growth play a significant role in controlling rice flood water pH by the following reaction:

Aquatic organisms control flood water pH over a wide range primarily with aqueous carbonate equilibrium system.

CO₂ (Atmos.) + H₂O = H₂CO₃DH⁺ + HCO₃^–DCO₃^2-+ H⁺

When flood water pH exceeds 7.4, the loss of N through NH₃ volatilisation may be more. Flood water pH increases by mid-day to as high as pH 9.5-10 and decreases as much as 2-3 units during the night time. This increase and decrease in pH during the mid-day and night may be due to utilisation of liberated CO₂ for carrying out photosynthesis of the aquatic biota and release of CO₂ during respiration of the biota respectively.

Factors affecting Ammonia Volatilisation:

Various research investigations have shown that ammonia volatilisation is influenced by different soil factors like pH, CaCO₃ content, temperature, amount of NH₄⁺ fertilizers applied, CEC etc. and by other parameters including flood water, atmospheric etc.

However, the following physico-chemical and biological parameters in soils including submerged soil-water-atmosphere system are appended below:

(i) Soil factors

a) Soil pH and pE

b) Soil salinity and alkalinity

c) Calcium carbonate content

d) CEC

e) Buffering capacity

f) pCO₂

g) Microbial activity

h) Temperature

(ii) Flood water parameters

a) pH

b) pCO₂

c) Concentration of NH4—N

d) Alkalinity (CO^2- and hco3 concentration)

e) Temperature

f) Depth

g) Activity of aquatic biota

h) Phosphorus concentration

i) Use of pesticides

(iii) Atmospheric parameters

a) Air temperature

b) Solar radiation

c) Wind velocity particularly near the canopy height

d) pNH₃

Therefore, the volatilisation of ammonia from a soil is a function of various soil properties, flood water characteristics and other atmospheric parameters.

(ii) Soil pH:

Soil pH affects the NH₃-volatilisation and with an increase in pH, the loss of N through ammonia volatilisation increases. Up to about pH 9, ammonia concentration increases by a factor of 10 per unit increase of pH.

CEC:

The CEC of the soil affects the intensity of ammonia volatilisation, being lower with higher the CEC.

CaCO₃ content:

The magnitude of NH₃-volatilisation is affected by the CaCO₃ content of the soil, being highest loss of N through NH₃-volatilisation from ammonium fertilizers forming insoluble compounds of calcium due to reaction with CaCO₃.

Temperature and Atmospheric Conditions:

The high temperature and solar radiation in the summer or dry season increase the ammonia volatilisation than that in the wet or kharif season where lower solar energy and temperature prevail. In addition, wind velocity near the canopy height affects the ammonia volatilisation, usually being more with higher wind velocity.

Minimisation of N Losses:

Nitrogen is required by most of the crops especially high amounts in rice. It is evident that the recovery of fertilizer nitrogen hardly exceeds 30-40% under field conditions. Ironically, the soil and different climatic conditions favouring crop growth adversely affect the recovery of nitrogen from the soil and are responsible for its rapid loss.

Such rapid losses of N through de-nitrification, leaching, ammonia volatilisation etc. can somewhat be minimized through efficient management techniques of nitrogenous fertilizers like use of slow-release N-fertilizers, combined application of inorganic and organic sources of N in different ratios, placement of fertilizer N, split application, time of application etc.

Slow release N fertilizers. There are a number of proved or potential advantages of fertilizers that release N slowly throughout the growing season. These slow release fertilizers offer potential for increasing efficiency of N fertilizers by reducing the above losses.

However, the materials and processes that have been developed or evaluated for controlled availability of nitrogen can be grouped as follows:

1. Compounds of low water solubility that must undergo chemical and microbial decomposition for the release of N, e.g. Urea-Formaldehyde, Urea-Z(UZ), Oxamide, CDU and IBDU.

2. Sparingly soluble compounds e.g. magnesium ammonium phosphate (Mag Amp).

3. Soluble or relatively water-soluble compounds undergoing slow decomposition e.g. guanyl urea sulphate (GUS), guanyl urea phosphate (GUP).

4. Traditional water-soluble products treated with different synthetic or natural chemical compounds to impede dissolution e.g. sulphur coated urea (SCU), lac-coated urea (LCU) etc.

5. Nitrification and urease inhibitors etc. N-serve, ATC (4, amino-1, 2, 4-triazole hydrochloride), ST (Sulphathiazole), MAST (2-amino-4-methyl-6-trichloro methyl triazine; MT (3-mercapto-1, 2, 4-triazole), etc.

Important characteristics and behaviour of some of the slow release N fertilizers are discussed below:

Urea form (Urea-Formaldehyde):

Urea-forms are white, odorless solids containing about 38% N which are made by reacting urea with formaldehyde in the presence of a catalyst. Urea-forms range from short-chain, water soluble molecules to long-chain, water- insoluble molecules depending on the mole ratio of urea to formaldehyde and on the pH, time and temperature of reaction.

The completely insoluble urea forms are of no agronomic significance but those of higher solubility are of great importance.

An activity index is used by the Association of Official Agricultural Chemists to evaluate the suitability of urea-formaldehyde compounds as follows:

Activity index (AI) =%CWIN – %HWIN/%CWIN × 100

where, CWIN = Per cent nitrogen insoluble in cold water (25°C),

HWIN = Per cent nitrogen insoluble in hot water (98-100°C)

Crotonylidene diurea (CDU):

This slow release nitrogenous compound is formed by the acid catalysis of urea and acetaldehyde. It was first manufactured in Germany and is now a commercial product in Japan. It contains about 30% nitrogen. The microbial transformation of chemically bound nitrogen in CDU is known to be temperature dependent.

Iso-butylidene diurea (IBDU):

It is a condensation product of urea and isobutyl aldehyde in the 2: 1 mole ratio (2, urea and 1 isobutyl aldehyde) and was developed in Japan. In pure form, it contains 32% nitrogen and has the solubility in water of only 0.1 to 0.01% depending on pH and temperature. However, nitrogen release from IBDU is usually regulated by granule size.

Sulphur Coated Urea (SCU):

It is a controlled release nitrogen fertilizer and the basic principles involve formation of a sulphur shell around each urea granule which is accomplished by spraying molten sulphur on urea particles. Then molten sealant (a mixture of polyethylene and bright stock oil) is applied to block capillary pores and cracks in sulphur coating.

The release rate of nitrogen is regulated by varying thickness of sulphur and sealant coating. Urea granules must have about the same thickness of coating on them regardless of their size to give the uniform rate of dissolution. Urea granules should be roundish with smooth surface otherwise it will be very much difficult to coat effectively with sulphur. Nitrogen content of SCU varies from 36-38%.

Of the coated fertilizers, SCU has been extensively tested on various crops. In many countries the crop response to SCU was superior to the response of prilled urea in a single (basal) application and often superior to split application at the same nitrogen rate.

Besides these, various combinations with urea N have proved beneficial for increasing rice yield by reducing the losses of nitrogen through different channels especially ammonia volatilisation.