Nitrogen: Factors and Functions | Fertilizers | Soil Science

In this article we will discuss about:- 1. Factors Affecting the Nitrogen Content in Soil 2. Nitrogen Balance Sheet 3. Transformation 4. Nitrogen Mineralization 5. Nitrogen Supply and Plant Behaviour 6. Function.

Contents:

1. Factors Affecting the Nitrogen Content in Soil
2. Nitrogen Balance Sheet
3. Transformation of Nitrogen in Solis
4. Nitrogen Mineralization
5. Nitrogen Supply and Plant Behaviour
6. Function of Nitrogen

1. Factors Affecting the Nitrogen Content in Soil:

There are some factors that affect the nitrogen content in soil as follows:

1. Climatic Factor:

In climatic factor, rainfall and temperature influence the nitrogen content of soil. Climate plays a dominant role in determining the nitrogen content of the soil through the influence of temperature and water supply on the activities of plants and microorganism.

(i) Rainfall:

Jenny found that if temperature remains constant, the nitrogen content of soil increases with water supply. In this respect, he gave a mathematically relationship of temperature and humidity (i.e. precipitation) as follows –

N = K (1 – e^– K₂H )

where, N = Nitrogen Percentage

H = Humidity factor and also known as Mayer factor (i.e. ratio of annual precipitation in milimeters to the absolute saturation deficit of the air in milimeters of mercury. It is the measures of effective precipitation).

This equation is also applicable for virgin soil, which has not been disturbed. In other soil, this relationship is slightly changed and the equation approaches ‘O’ due to increase in temperature when humidity is constant.

He explained two factors such as – (i) Increment of vegetative growth and (ii) Decrease of microbial activities. Upto certain limit, increase in rainfall is accompanied by increase in vegetative growth and thus increases the Nitrogen in the soil. The increase of nitrogen is only due to vegetative growth of the plants. Again nitrogen increases with the decrease of microbial activity when land is under water logged condition.

Jenny has given some constant for ‘K’ for arid, semi-arid, upland soils of loam, sandy loam and silt loam soil. He gave combined formula for temperature and humidity as follows –

(ii) Temperature:

The nitrogen content of soil decreases with the increase of temperature. Jenny’s result show a marked decrease in soil nitrogen content with an increase in annual temperature from 32°F (0°C) in Canada to 70°F (21°C) in Southern United States. From his result, he gave a mathematical explanation of the relationship between temperature and nitrogen content of soil as follows –

N = Ke^– K₁T

where, N = Percentage of nitrogen of surface soil

e = base of natural logarithm

K = Constant

T = Temperature

K₁ = Another constant

This relationship holds good in virgin soil.

2. Local Factors:

(i) Type of Vegetation:

Nitrogen content of a soil depends on the type of vegetation on its surface. The nitrogen content at a particular temperature and under a given climatic regime is greater in soils developed under prairie soil (grass land) than under forest vegetation. Pairie soil has a higher organic matter content in the profile as a whole compared to the corresponding soils developed under forest vegetation.

(ii) Slope of the Land:

The plant nutrient particularly nitrogen remains on the surface of the soil. The soil erosion causes the depletion of nitrogen from surface soil due to run-off. So the soil of steep slope region contains less nitrogen due to run-off of surface soil with most of nitrogen. On the other hand, the level land contains more nitrogen than steep sloppy land as there is little chance of run-off from surface soil in level land.

(iii) Direction of Slope:

The nitrogen content of a soil is indirectly related with the direction of slope. Influence of the direction of slope is based upon the variation of temperature. The slope facing south to north hemisphere contains less nitrogen due to direct sunrays.

(iv) Water Logging Condition:

The supply and availability of nitrogen is more in water logged soils due to reduced microbial activity. Under water logged condition, the decomposition of organic matter takes place by the organisms which requires less nitrogen.

(v) Soil Texture:

Soil texture also affects the nitrogen content of soil in local areas. The clay soil contains more nitrogen than sandy soil. The nitrogen content increases as the texture becomes finer. Probably differences in water holding characteristics, aeration, fertility and tendency of mineral portion of soil to combine with organic matter are responsible in part for the change in nitrogen content with texture. The relationship of texture and nitrogen content is explained by experiment result by Walker and Brown, 1936.

3. Factors Related to Agricultural Operation:

The virgin soil gives more nitrogen after it has been cultivated. If a virgin soil is brought under cultivation and laps some year, there will be increase of nitrogen in the soil. The reduction of nitrogen content can be maintained by addition of organic matter or other land management practices.

Cultivation Index (C.I.):

Cultivation index is the measure of the quality of management practices with respect to nitrogen.

C.I = NC/NV

where, NV = Nitrogen content in virgin soil,

NC = Nitrogen content in soil after cultivation for some years.

---

2. Nitrogen Balance Sheet:

Nitrogen is added to soil and nitrogen is lost from soil through different process as follows:

Loss of Nitrogen:

(i) Crop Removal:

Plants absorb nitrogen from the soil and store them in their different parts. The crops remove large quantity of nitrogen from the soil. Four to five percent of total nitrogen is lost from the soil per acre annually through the harvested crop. On an average 50-60 kg. of nitrogen is removed from the soil due to the removal of the harvested portion of the crop. The grain crops remove larger quantities of nitrogen from the soil.

(ii) Losses of Nitrogen by Leaching:

Loss of nitrogen by leaching will occur in high rainfall areas and irrigated areas where over application of water is practiced. Usually of mineral nitrogen is lost by leaching in the form of nitrate. Collison and Mensching (1930) found that more than 99 per cent nitrogen in leachate was nitrate, less than 1 per cent in ammonium form and traces was present as nitrite. Because ammonium nitrogen converts rapidly to nitrate in soil and this is soluble in soil water and does not form insoluble compound with soil constituent.

Hence it is transported readily by water movement, in soil and nitrates reaches beneath the rootzone due to downward movement of soil water. Loss of nitrates nitrogen is greater in coarse texture soil due to its lower water holding capacity. Leaching loss of nitrogen is greater in bare soil than from cropped soil as there is no crops in the former soil to absorb water and nitrogen. Leaching losses of nitrogen vary so much with the amount and distribution of rainfall and crops present on the soil.

(iii) Losses in Gaseous Form:

Nitrogen is generally lost from the soil in the process of denitrification and other process. In addition to these, nitrogen can be lost from the soil in the form of ammonia by the process of volatilization. This loss is aggravated if sufficient ammoniacal nitrogen present in surface soil. Conditions favouring rapid volatilization are the presence of ammonia in the surface soil, soil pH above 7.0, high temperature and rapid loss of water by evaporation.

(iv) Losses of Nitrogen by Soil Erosion:

The great loss of nitrogen is brought about by soil erosion due to removal of surface soil which contains most of soil nitrogen. Loss of nitrogen by erosion is more serious than any other nutrients.

Gains of Nitrogen:

Nitrogen is most important nutrient for the plant.

Nitrogen is added to the soil through different ways as follows:

1. Fixation of Atmospheric Nitrogen:

Most soil contains nitrogen fixing microorganism.

Nitrogen can be fixed in soils in a number of ways as follows:

(a) Non-Symbiotic Nitrogen Fixation:

It is the process by which certain free living organism change atmospheric nitrogen into organic compound. Azotobacter is one of the free living organism which fix nitrogen non-symbiotically in aerobic condition. Clostridium fixes atmospheric nitrogen non-symbiotically in anaerobic condition. This organism plays a role in fixation of nitrogen in sub-soil, waterlogged soil or when the carbon dioxide concentration of soil atmosphere is high.

Blue green algae can fix nitrogen from air non-symbiotically in presence of sunlight and add 30-40 kg. nitrogen per hectare in transplanted Aman Paddy land.

20-40 kg. of nitrogen per hectare may be fixed by non-symbiotic organism.

(b) Symbiotic Nitrogen Fixation:

It is the process by which elemental nitrogen is changed to organic forms by association between certain plants and the bacteria found in nodules. Rhizobium bacteria can fix atmospheric nitrogen symbiotically. They live in the nodules of the host plant belonging to the family of leguminosae. The plants are able to use some of the nitrogen fixed by the microbes. The relationship thus established is often spoken of as ‘symbiosis’. The amount of nitrogen fixed by rhizobium bacteria depends on many factors, such as condition of soil, especially aeration, drainage, moisture, pH and amount of active calcium.

The seeds of legumes are inoculated with rhizobium culture and are sown in the field. They fix the atmospheric nitrogen which may be used by host plants. The nitrogen may pass into the soil itself, either by excretion or more probably by ploughing off of the roots and especially of their nodules and when the legumes plant is turned under the soil, some of nitrogen becomes available to the succeeding crops.

The rhizobium culture is available in the market in the name of jahawar culture, rhizobium composite, nitrogen R-H etc. Symbiotic fixation of nitrogen is confined to plants of legume family. Eight genera of Angiosperms, comprising some hundred species of widely distributed trees and shrubs, also produce nodules. They do fix appreciable quantities of atmospheric nitrogen.

2. Rainfall:

When sparking occurs, the atmospheric nitrogen combines with oxygen and form nitric oxide (NO). The nitric oxide is oxidised with oxygen and is converted to nitrogen peroxide (NO₂). The nitrogen peroxide is converted to nitrous acid and nitric acid reacting with water and comes to the soil. These two acids react with the minerals of the soil (Calcium, Potassium etc.) and forms nitrate salt such as calcium nitrate [Ca(NO₃)₂] and potassium nitrate (KNO₃).

Reactions are as follows:

Besides this, small amount of ammonia (NH₃) is formed with the reaction of oxygen and nitrogen. The ammonia comes to the soil with rain water and ammonia is converted to nitrate due to oxidation by bacteria. Five kilogram of nitrogen is added to a hectare of land by this process per year.

3. Manures and Crop Residues:

Manures and crop residues are the important sources of organic nitrogen. Manures (e.g. compost, F.Y.M. oil cakes, green manure etc.) contain nutrients and their application increases the nitrogen content of soil. The crops residues, when applied in the soil, are converted to organic matter by microbial activities and nitrogen released from them is added to the soil, thus increasing the nitrogen content of soil.

4. Fertilizers:

The fertilizers, particularly nitrogenous fertilizer, are the more important source of nitrogen. When nitrogenous fertilizers (e.g. ammonium sulphate, urea, calcium ammonium nitrate, ammonium chloride etc.) are applied to the soil, the nitrogen content of the soil increases and it compensates the nitrogen which are depleted by the crops. The nitrogenous fertilizer should be applied in schedule dose recommended for the particular crop.

---

3. Transformation of Nitrogen in Solis:

Transformation of nitrogen in soils takes place through different process as follows:

1. Amminization:

Proteins of organic substances are decomposed by the proteinase enzyme to amino acid. The process of conversion of proteins to amino acid is known as ‘amminization’.

2. Ammonification:

Amino acids are then converted to ammonia (NH₃). The transformation of organic compounds into ammonia is known as ‘ammonification’.

E. Marchal (1893), would forward this concept of ammoniacal production being nitrogen waste products in the conversion of organic matter into microbial tissue and vital energy.

The organism involved in this process are known as ammonifiers. The organisms active in this process are bacteria, fungi and actinomycetes. Ammonification proceeds best in well drained soils with plenty of basic materials.

The ammonium is readily available to microorganism and many higher plants. The young plant prefer ammonium nitrogen, although they seem to grow better if some nitrate nitrogen is also available. Paddy prefer ammoniacal nitrogen to the nitrate nitrogen in early stage of their growth.

3. Nitrification:

Ammoniacal nitrogen released may again be transformed to nitrate under suitable condition through the process of nitrification. The process of conversion of ammonium to nitrite (NO₂) and then to nitrate (NO₃) is known as ‘nitrification’. First, nitrite is formed by nitrite forming bacteria (i.e. nitrosomonas) and then to nitrate by nitrifying bacteria or nitrifier (i.e. nitrobacter).

The process of nitrification may be represented as follows:

Usually nitrites are rapidly oxidized to nitrate.

Schbesing and Munz (1877) proved that the production of nitrate (NO₃^–) from ammonium ions (NH₄⁺) which is known as nitrification was biological process.

The following points in connection with nitrification have been well established:

(i) Nitrate (NO₃) always produce from nitrite (NO₂^–).

(ii) Nitrate production can only go in the presence of oxygen. It is essentially of aerobic process.

(iii) The oxidation of ammonium ions (NH₄⁺) takes place on the surface of soil particles and it is only the exchangeable ion that is oxidised. Hence increasing the amount of exchangeable ion, the maximum rate of ammonium oxidation is increased.

(iv) The oxidation goes only on rapidly if there is a good supply of calcium (Ca) and phosphate (PO₄) and if there is proper balance of trace elements e.g. copper (Cu), iron (Fe) and zinc (Zn).

(v) The microorganisms involved in the oxidation are bacteria. Winogadsky established that it is associated with metabolism of certain chemoautotropic bacteria. Two groups are distinguished; one deriving its energy for cell synthesis by oxidation of ammonium and other by oxidation of nitrite.

Factors Affecting Nitrification:

There are some factors that affect nitrification as follows:

(i) Abundance of Ammonium (NH₄⁺) Ions:

There should be sufficient amount of ammonium ion in the soil for the process of nitrification as in this process ammonium ions are converted to nitrates. If conditions are such as to inhibit the production of ammonium ion, the process of nitrification accordingly will be reduced. The rate of nitrification will be high in a soil containing ammoniacal nitrogen in high quantity. If C : N ratio of organic matter is 15 : 1, there will be sufficient amount of ammoniacal nitrogen.

(ii) Soil Reaction:

The organisms responsible for nitrification are very sensitive to the changes in soil pH. The process of nitrification takes place at a soil pH 5.5 to 10.0, with optimum being 8.5. It has been reported that the process of nitrification has also taken place at soil pH as low as 4.5 in podzol and some grass land. At pH values even below 5.0, peat soils may show remarkable accumulation of nitrates.

(iii) Soil Aeration:

Nitrification is a process of oxidation. So free oxygen is necessary for this process for the conversion of nitrite to nitrate. Nitrification enhances due to sufficient supply of fresh air. When the concentration of oxygen is 40 per cent, the rate of nitrification is maximum. So the procedures that increases the aeration of soil upto certain limit encourages the process of nitrification. Ploughing and cultivation, especially if granulation is not unpaired, are recognized means of promoting nitrification.

(iv) Soil Moisture:

The proper rate of nitrification takes place at a moisture level which is optimum for the growth of higher plants. The process of nitrification is related by both very low and high moisture due to the organism is not in a position to take sufficient moisture in former case and decreased oxygen supply in later case. Greves and Carter found that a moisture content of about 55 per cent of water holding capacity especially favourable for nitrification. Nitrification will progress appreciably at moisture content at or even below the wilting coefficient.

(v) Temperature:

Temperature has marked influence on the rate of nitrification. The temperature most favourable for the process of nitrification is from 26.6 to 32.2°C. Low temperature slows down the process of nitrification. At freezing or below, nitrification will not take place but at about 1.6° to 4.4°C it begins and slowly increases in intensity until the optimum temperature is reached. At about a temperature of 51.6°C, nitrification practically ceases.

(vi) Exchangeable Bases:

Nitrification requires an abundance of exchangeable bases. It is very common observation that lime stimulates nitrification in soils even in those that may already contains a fare amount of active calcium.

(vii) Fertilizers:

Small amounts of many kinds of salt, even those of the trace elements stimulate nitrification. A reasonable balance of nitrogen, phosphorus and potassium has been found helpful in the process of nitrification.

4. Denitrification:

The biochemical reduction of nitrate nitrogen to gaseous compound is called ‘denitrification’. This process of nitrogen transformation in soil is not at all desirable as in this process, nitrite and nitrates are converted to oxides of nitrogen and even to gaseous nitrogen, which will escape into the atmosphere. The bacteria which are responsible for denitrification are known as denitrifying bacteria or denitrifiers. e.g. Pseudomonas, thio-bacillum sp. etc. The anaerobic condition of soil is conductive to the process of denitrification.

---

4. Nitrogen Mineralization:

Nitrogen mineralization refers to the change of organic nitrogen to mineral form, ammonium or nitrate by the soil microorganism. The ammonium or nitrate can be utilized by plants and microorganism. The term nitrification widely used in older literature, sometimes used in same sense as mineralization.

Ammonium or nitrates thus formed is utilized by another group of bacteria for their body buildup, formed into protein etc. and gets locked there in and becomes unavailable to higher plants. The process of conversion of inorganic combined nitrogen into complex organic nitrogenous form is known as ‘nitrogen immobilization’. It is thus the reverse of mineralization.

Factors Affecting Nitrogen Mineralization:

The process of nitrogen mineralization is carried out by the heterotrophic and autotrophic organism.

There are some factors that affect the decomposition by heterotrophic organism as follows:

(i) Substrate Composition:

Substrate composition has profound effect on the mineralization of nitrogen, mainly C : N ratio of organic matter which will influence the mineralization of nitrogen. If nitrogenous material is in excess, the nitrogen is mineralized and carbonaceous material is in excess, the nitrogen is immobilized. The relative availability of the two ordinarily is represented by C : N ratio of the original material.

When the C : N ratio of organic matter is below about 15 : 1 (or the nitrogen is in excess of about 2.6 per cent), the nitrogen is mineralized, which will available to plants. Richer et al. (1946) found that nitrate production decreases with increasing C : N ratio of organic matter in field experiment involving heavy organic matter addition.

(ii) Total Content of Nitrogen:

The net quantity of nitrogen mineralized usually is approximately proportional to the total quantity of nitrogen under condition of temperature and water content favourable for micro-biological activity. The mineralization will be high if there is more nitrogen in the soil. Allison and Sterting (1949) show that correlation coefficient value between total nitrogen content in soil and total nitrogen mineralized varies from 0.54 to 0.84.

(iii) Soil Reaction:

The activity of the organism responsible for decomposition of organic matter varies with soil pH. Decomposition of organic matter by heterotrophic organism can take place within a wide range of soil pH. But there is some difference. At pH below 5.5, fungi, the dominant organism, are responsible for decomposition as this organism can operate more efficiently at low soil pH. At pH above 5.5, the activity of bacteria increases and proportionately more of decomposition is brought about by the organism.

(iv) Soil Moisture:

Decomposition of organic matter takes place over a wide range of moisture condition. But it is generally very slow in extremely dry soils. The rate of decomposition will be slow at a low moisture status of the soil and water logged conditions.

(v) Temperature:

The heterotrophic organisms are able to bring about decomposition over a wide range of temperature. Generally, the activity of heterotrophic organism increases within a temperature of 35°-45°C and rapidly decreases with further rise in temperature.

Fixation of Ammoniacal Nitrogen in Soil:

Fixation means conversion of chemical elements essential for plant growth from a soluble or exchangeable form to a much less soluble or to non-exchangeable form. In this process, ammoniacal nitrogen is fixed in organic and inorganic soil fractions as they have the ability to fix ammonia in forms relatively unavailable to higher plants or even microorganism.

(i) Fixation of Ammonia by Clay Minerals:

When nitrogen present in soil in ammoniacal form, there are some soil to react with it and nitrogen is converted from exchangeable to non- exchangeable form. Several clay minerals with a 2 : 1 type structure have the capacity to fix ammonium and potassium ions. Vermiculite has the greatest capacity, followed by illite and montmorillonite. The ammonium fixing capacity of clay minerals is generally greater in sub­soil than in top soil because of higher clay content in sub-soil. The ammonium fixing capacity of a soil generally increases as the depth increases in soil profile.

The chlorite and vermiculite clay can fix ammonium both under moist and dry condition, but the montmorillonite can fix ammonium in only dry condition and not in moist condition. Standfond and Pieire (1947) found that fixation of ammonium and potassium ion are interchangeable. This can be taken as an indication that mechanism of potassium fixation and that of ammonium fixation are similar.

(ii) Fixation of Ammoniacal Nitrogen by Organic Matter:

Anhydrous ammonia or other fertilizers which contains free ammonia or which form it when added to the soil can react with soil organic matter to form compound which resist decomposition. In this sense, the ammonia can be said to be fixed by the organic matter. The fixed ammonia is subjected to subsequent slow release by mineralization.

Utilization of Ammonium and Nitrates by Plants:

Plant may absorb nitrogen in ammonium (NH₄⁺) and nitrate (NO₃^–) form. The nitrates absorbed must be reduced to ammonium by plants before it is further changed to ammino (–NH₂) form for assimilation. The cereals crops (i.e. paddy, wheat, oat etc.) absorb nitrogen in the ammonium form in their early growth stages and cotton etc. are adopted to use nitrate in their early growth stages.

During maturity stage, the plants seems to absorb a higher proportion of nitrate than ammonium ion. When plant absorbs more nitrogen than they need, some of extra nitrogen exists in the cell sap as nitrate. This makes it possible to examine plant and estimate the nitrogen status of the soil.

Alphanapthalamine is greyish white powder, which can be used to test the cell sap for nitrates. This chemical turns red in presence of nitrates in corn plant. Diphenylamine (dissolving diphenylamine in conc. Sulphuric acid; H₂SO₄) is also used to test the presence of nitrate in plant sap. The plant sap turns blue colour due to presence of nitrates.

The nitrate nitrogen of soil whether added in fertilizer or formed by nitrification may be used by plants and microorganism, lost in drainage and escapes from the nitrogen cycle in a gaseous condition. Losses of nitrate nitrogen in drainage water depends on climatic condition and cultural practices. In humid areas and where irrigation is practiced, losses of nitrate nitrogen by leaching are significant. On the other hand, the losses are minimal in arid and semiarid region as the water loss by leaching is low of those areas.

---

5. Nitrogen Supply and Plant Behaviour:

1. Deficiency Symptoms:

(i) The plant becomes yellowish or light green in colour and remains stunted. The leaves and young fruits tend to drop prematurely.

(ii) A nitrogen starved plant ripens prematurely and crop gives poor yield. The kernels of cereals and the seed of other crops do not attain their normal size and become shrivelled and light in weight.

(iii) It delays reproductive growth but enhances profuse vegetative growth. The flower bud often turns pale and shed prematurely.

(iv) Root growth is severely affected.

2. Nitrogen Supply and Carbohydrate Utilization:

If the utilization of carbohydrates does not keep pace with the formation i.e. photosynthesis, there will be the accumulation of carbohydrate within the plant. With abundant supply of nitrogen, the tendency is for carbohydrates to be utilized to produce nitrogenous compound i.e. protein. Gardner, Robertson (1942) and others found that the sucrose percentage decreases with increasing the nitrogen in sugar crop like sugarcane and sugarbeet.

3. Nitrogen Supply and Root Growth:

The growth of above ground portion i.e. shoot is relatively more than that of the growth of the root when there is abundant supply of nitrogen in the soil. One of explanation given for this is that with the increasing supply of nitrogen, the proportion of carbohydrate used in aerial portion increases and the proportion of carbohydrate translocated to the root decreases. According to other theory, more shoot growth and less root growth is due to growth regulating substances known as auxin which increases with the supply of nitrogen.

The principal plant auxin, IAA (Indole acetic acid) contains nitrogen and apparently is formed by alteration of tryptophane, an amino acid found in plant. The concentration of tryptophane increases with the supply of nitrogen. The critical level of auxin for the growth of shoot is more than that required for the growth of root.

4. Nitrogen Supply and Succulency:

Nitrogen promotes vegetative growth and brings succulence in plant when nitrogen present in excess amount. This is because of the fact that when there is an excess supply of nitrogen, more protein i.e. protoplasm is synthesized at the cost of carbohydrates. So there is less deposition of carbohydrate on the cell wall and accordingly cell wall cannot be thickened. This is due to more formation of protoplasm, large cell with thin walls are formed.

About 80 per cent of protoplasm is water. That is why, the plant, growing with excess nitrogen, becomes watery and succulent. The fibre cell of sannhemp (Crotolaria juncea) becomes long and thin due to high doses of nitrogen supply, which is of low mechanical strength. Nitrogen application tends to produce succulency, a quality particularly desirable in crops such as spinach, lettuce, radish and fodder crops.

5. Nitrogen Supply and Fruiting:

Nitrogen delays reproductive growth. Krans and Kraybill (1918) found in tomato that the plant becomes generally unfruitful due to excess supply of nitrogen. In case of cereal crops (i.e. paddy, wheat etc.), the straw is more responsive to nitrogen supply than the grain and the straw and grain ratio is increased.

6. Nitrogen Supply and Maturity:

Nitrogen delays ripening by encouraging more vegetative growth.

In considering the effect of nitrogen on crop maturity four factors must be taken into account such as:

(i) The degree of nitrogen deficiency,

(ii) The quantity of nitrogen applied,

(iii) Time of nitrogen application and

(iv) The nature of the crop.

7. Nitrogen Supply and Cold Injury:

Nitrogen promotes vegetative growth which result in reduction of carbohydrates and sugar. Cold injury of the plant is correlated with carbohydrate content of the plant. Plants having high percentage of carbohydrate are more resistant to cold injury than the plant with low carbohydrate content. This behaviour of the plant has got a special significance in extremely cold countries.

8. Nitrogen Supply and Lodging of Plants:

Lodging is the inelastic displacement of the plant from vertical or initial position. In cereal crops, the straw becomes weak and the crop very often lodges as the nitrogen increases the plant height, and weight of leaf area. Nitrogen enhances the tillering of the cereal crops and lodging may occur.

9. Nitrogen Supply and Incidence of Insect Pests and Diseases:

Due to excess supply of nitrogen plant becomes soft and sappy. The plants become more liable to attack of certain fungi and its resistance to diseases is lowered. The insect-pest attacks the plant which is more succulence. Susceptibility being associated with succulency and succulency is associated with nitrogen supply.

---

6. Function of Nitrogen:

(i) Nitrogen tends primarily to encourage above ground vegetative growth and it imparts dark green colour to plants.

(ii) Nitrogen is a regulator that governs to a considerable degree the utilization of potassium, phosphorus and other constituents.

(iii) It promotes vegetative growth and improves the quality of produce including fodder, leafy vegetable and food crops. It increases the tillering of cereal crops.

(iv) When nitrogen is present in sufficient quantities in soil, plants acquire healthy green colour which is neither too dark green nor to light, growth of the plant is fairly rapid and crop matures normally and gives high yield.

(v) Nitrogen application tends to produce succulency, a quality particularly desirable in crops such as spinach, lettuce, radish and fodder crops.

(vi) It increases the protein content of food and fodder. It also increases the plumpness of grains in cereal crops.