Nutrients Required by Plants
- Plants require 16 essential elements for their normal growth and development.
- The essential elements exist as structural components of a cell, maintain cellular organizations, function in energy transformations and in enzyme reaction.
- Carbon, Hydrogen and Oxygen are three naturally occurring nutrients and form about 94 per cent of the dry weight of plants. These are the major components of carbohydrates, proteins and fats. Besides their structural role, they provide energy required for the growth and development of plants by oxidative breakdown of carbohydrates, proteins and fats during cellular respiration.
- Nitrogen, Phosphorus and Potassium are three major or primary nutrients which are to be made available in larger quantities.
- Nitrogen is an essential constituent of metabolically active compounds such as aminoacids, proteins, enzymes and some non-proteinous compounds. When nitrogen is a limiting factor, the rate and extent of protein synthesis are depressed and as a result plant growth is affected. The plant gets stunted and develops chlorosis.
- Phosphorus is a structural component of all membranes, chloroplasts and mitochondria and a constituent of sugar phosphates, viz., ADP, ATP, nucleic acid, Phospholipids and phosphatides. Phosphorus plays an important role in energy transformations and metabolic processes in plants. It stimulates root growth.
- Potassium plays an important role in the maintenance of cellular organisations by regulating permeability of cell membranes and keeping the protoplasm in a proper degree of hydration. It activates the enzymes in protein and carbohydrate metabolism and translocation of carbohydrates and imparts resistance to plants against fungal and bacterial disease.
- Calcium, magnesium and sulphur are secondary nutrients which are required in relatively smaller but in appreciable quantities. Calcium, a constituent of the cell wall, an activator of different plant enzymes and is essential for the stability of cell membranes.
- Magnesium is a constituent of chlorophyll and chromosome. It is known to play a catalytic role as an activator of a number of enzymes, most of w.hich are concerned with carbohydrate metabolism.
- Sulphur is required to synthesize the sulphur containing amino acids and proteins, activity of proteolytic enzymes and increases oil content in oil bearing plants.
- Iron, zinc, manganese, copper, boron, molybdenum and chlorine are required by plants in small quantities for their growth and development. Hence they are known as micronutrients or trace elements. The very fact that the micronutrient elements are required by plants in very low concentration suggests that they all function as catalysts or at least closely linked with some catalytic processes in plants. Manganese, zinc and copper are components of certain biological oxidation-reduction systems. Manganese performs some function in photosynthesis, acts as regulator to the intake and state of oxidation of certain elements. Zinc is concerned with the functioning of Sulphydryl compounds such as cystein, in the regulation of oxidation - reduction potential within the cells. Copper is a constituent of cytochrome oxidase and component of many enzymes like ascorbic acid oxidase, phenolase and lactase. Molybdenum is a constituent of nitrate reductase and nitrogenase enzyme and is associated with nitrogen utilization and in nitrogen fixation.Chlorine stimulates the activity of some enzymes and influences carbohydrate metabolism.
- Boron helps in cell development by its influence on polysaccharide formation. It regulates translocation of sugars across membranes and polyphenolase activity. Iron is a constituent of cytochromes, haem and non haem enzymes. Perhaps the best known role of iron is its catalytic role in enzyme activity.
Diagnosis of Fertilizer Requirement
- For obtaining maximum crop yields with maximum benefit to the cultivators, it is most essential that the crop plants should be fed properly with all nutrients. Soils deficient in particular nutrients must be supplied with fertilizers containing those plant nutrients.
- Thus it is important to know which plant nutrients are lacking in a soil. Simple and elaborate tests have been developed by the agricultural scientist to estimate the nutritional requirements of soils and crops. These methods are known as diagnostic techniques. Fertilizer requirement is known by different diagnostic techniques and they are as follows ;
By Plant Observation
- This is one of the method to know the fertilizer need of plants by means of the hunger signs of plants which can be detected by the eye.
- The basis of the method is the fact that the plant suffering from severe deficiencies and excess of mineral nutrients usually developed well-defined and typical sign of disorders in various organs, particularly in the leaves. Usually, specific abnormal colours are developed in the leaves due to deficiency of plant nutrients.
- Although the hunger signs in plants are easily observed, it is not easy to recognise the particular nutrient deficiency in nature due to various field conditions. This requires experience and practice in the field.
By Plant Analysis
- The use of plant analysis as a tool to diagnose fertility status mainly consists of :
- Plant tissue tests or rapid tests,
- Total analysis,
- Biochemical methods.
- The basis of plant analysis for diagnostic purposes is that the amount of a given nutrient in a plant is an indication of the supply of that particular nutrient and is directly related to the quantity present in the soil. The normal growth of a plant is determined by the supply of the nutrients. However, there is one disadvantage with this method, that is, while the shortage of one nutrient can limit the growth, other nutrients may show higher contents in the cell sap irrespective of the supply.
- The use of plant tissue tests as a means to diagnose soil fertility status has been found to be important. This is a rapid test of the cell sap of the growing plants. The sap from the ruptured cells is tested for unassimilated nitrogen, phosphorus, potash and other nutrients. Tissue tests are getting popular because of the convenience of handling and the small number of equipment needed for the test. The test can be made in a few minutes.
- Total analysis is used extensively in research work as this gives a quantitative indication of the level of nutrients in plants. However, it should be remembered that the determination of total analysis gives both the assimilated and unassimilated nutrients. Many nutrients such as N, P, K, Ca, Mg, Mn, Zn, Cu, Fe, Mo and B can be determined by this method. Usually, the mature plants are selected for this testing.
- Biochemical methods to determine the soil fertility require costly equipments, but offer good opportunities for research work. Two methods are recognised amongst biological tests. They are, use of higher plants, Microbiological methods.
By Fertilizer Experiments
- In India, simple field experiments on farmers fields as well as complex field experiments are very popular.
- Simple Field Experiments - In well managed state farms, the level of soil fertility is usually higher than in the farmers fields. This is due to the use of manures, fertilizers, good management practices, etc. Many experiments conducted on farmers fields have revealed the deficiency of nutrients at various levels. These experiment have to be simple in nature with N, P, K, NP, NK, PK, NPK as the treatments.
- These simple field experiments on farmers fields are very educative and effective for the farmers, as they themselves see the deficiencies and the response of the nutrients. These trials are useful for advising the correct type and amount of fertilizer.
Complex Field Experiments
- Complex field experiments allow the testing of many factors at a time and permit a study of interaction among various nutrients. Complex fertilizer trials helps in determining the correct kinds of fertilizer, amount and the method of application for each of the soil zone. These experiments are complicated, expensive and can be done only by experienced people.
By Soil Testing
- Soil testing is one reliable diagnostic tool whose value in evaluating soil-fertility conditions has been recently recognised in India. Soil testing is multipurpose in nature. Its purposes are :
- To group soils into classes relative to the levels of nutrients for suggesting fertilizer practices.
- To predict the probability of getting a profitable response to the application of fertilizers.
- To help evaluate soil profitability and To determine specific soil conditions i.e., alkalinity, salinity, acidity, that limit crop yields and can be improved with soil amendments and other management practices.
Organic Fertilizers and Manures
- Organic fertilizers include both plant and animal bi-products. They are slow acting. Organic nitrogen fertilizers include oil cakes, fish manure, dried blood from slaughter houses etc., where as organic phosphorus from bone meal and organic potassium from cattle dung ash, wood ash, leaf mould, tobacco stems and water hyacinth.
- Manures are organic or inorganic substances applied to the soil to supply one or more nutrients to plants to obtain increased yields.
- Manures are classified as follows
|Bulky (Slow acting with large quantities of organic matter) Eg: Cattle, Sheep Poultry, Pig, Goat,, Horse manures, Compost, Green Manures, Sewage.Sludge.
||Concentrated(Quick acting with small quantity of organic matter.Eg: Groundnut cake, Castor cake, Bonemeal, Blood meal, Horn meal, Wood ash, Cotton and Linseed Meal.
||(Artificial manures,Chemical fertilizers very quick acting with No organic matter.Eg: Nitrogenous, Ammonium,Phosphatic, Potassic and Sulphate fertilizers.
- Nitrogen is the first fertilizer element of the macronutrients usually applied in commercial fertilizers. Nitrogen is very important nutrient for plants and it seems to have the quickest and most pronounced effect.
Role of Nitrogen In Plants
- Nitrogen is of special importance in the formation of protein in plants,
- It forms a constituent of every living cells in the plants,
- It is also present in chlorophyll,
- It is involved in photosynthesis, respiration and protein synthesis,
- It plays an important role in vegetative growth and it imparts dark green colour to plants.
- If excess nitrogen is applied it delays ripening by encouraging more vegetative growth. The leaves acquire a dark green colour, become thick and leathery and in some cases crinkled. The plants become more liable to attack of pests and diseases. In case of cereal crops, the straw becomes weak, and the crop very often lodges and straw and grain ratio is increased. Excess nitrogen deteriorates the quality of some crops such as potato, barley and sugarcane. It delays reproductive growth and may adversely affect fruit and grain quality.
- The deficiency of Nitrogen leads to formation of yellowish or light green coloured leaves and plant become stunted. The leaves and young fruits tend to drop prematurely. The kernels of cereals and the seed of other crops do not attain their normal size, and become shrivelled and light in weight.
- Phosphorus is the second fertilizer element and it is an essential constituent of every living cells and for the nutrition of plant and animal. It takes active part in all types of metabolism of plant. It is an essential constituent of majority of enzymes and also structural component of membrane system of cell, chloroplasts and the mitochondria. It is intimately associated with the life process.
- Phosphorus stimulates root development and growth in the seedling stage and there by it helps to establish the seedlings quickly. It hastens leaf development and encourages greater growth of shoots and roots. It enhances the development of reproductive parts and thus bringing about early maturity of crops particularly the cereals. It increases the number of tillers in cereal crops and also strengthen the straw and thus helps to prevent the lodging. It stimulates the flowering, fruit setting and seed formation and the development of roots, particularly of root crops. Phosphorus has a special action on leguminous crops. It induces nodule formation and rhizobial activity.
- Excess phosphorus leads to profuse root growth, particularly of the lateral and fibrous rootlets. It leads to some trace element deficiencies particularly iron and zinc.
- Deficiency of phosphorus leads to restricted root and shoot growth, leaves may shed prematurely, flowering and fruiting may be delayed considerably. In case of potato tubers phosphorus deficiency leads to formation of rusty brown lessions.
- Potassium is the third fertilizer element. Potassium acts as a chemical traffic policeman, root booster, stalk strengthener, food former, sugar and starch transporter, protein builder, breathing regulator, water stretcher and as a disease retarder but it is not effective without its co-nutrients such as nitrogen and phosphorus.
- Potassium is an essential element for the development of chlorophyll. It plays an important role in photosynthesis, i.e., converting carbon-dioxide and hydrogen into sugars, for translocation of sugars, and in starch formation. It improves the health and vigour of the plant, enabling it to withstand adverse climatic condition. It increases the crop resistance to certain diseases. Potash plays a key role in production of quality vegetables. Potassium is an enzyme activator and increases the plumpness and boldness of grains and seeds. It improves the water balance. Promotes metabolism and increases the production of carbohydrates.
- Potassium deficiency causes stunting in growth with shortening of internodes and bushy in appearance, brings about chlorosis, i.e., yellowing of leaves and leaf scorch in case of fruit trees. It is also responsible for the 'dying back tips' of shoots. Its deficiency leads to reduction in photosynthesis, blackening of tubers in case of potato, tips or margin of lower leaves of legumes, maize, cotton, tobacco and small grains are either scorched or burnt.
- Secondary nutrients include calcium, magnesium and sulphur, which play an important role in plant growth and development. The details of these nutrients are given below.
- Calcium as calcium pectate is an important constituent of cell wall and required for cell division. It is a structural component of chromosomes. It includes stiffness to straw and there by tends to prevent lodging.It enhances the nodule formation in legumes, helps in translocation of sugars, neutralizes organic acids which may become poisonous to plants. It is an essential co-factor or an activator of number of enzymes.It improves the intake of other plant nutrients, specially nitrogen and trace elements by correcting soil pH. Excessive amounts of calcium can decrease the availability of many micronutrients.
- Deficiency of calcium lead to 'Die back' at the tips and margins of young leaves. Normal growth of plants is arrested i.e., roots may become short, stubby and bushy, leaves become wrinkled and the young leaves of cereal crops remain folded. The acidity of cell sap increases abnormally and it hampers the physiological function of plant. As a result of which plant suffers and causes the death of plant at last.
- Magnesium is an essential constituent of chlorophyll. Several photosynthetic enzymes present in chlorophyll requires magnesium as an activator. It is usually needed by plants for formation of oils and fats. It regulates the uptake of nitrogen and phosphorus from the soil. Magnesium may increase crop resistance to drought and disease.
- Deficiency of magnesium leads to yellowing of the older leaves known as chlorosis. Acute deficiency of magnesium also causes premature defoliation. In case of maize the leaves develop interveinal white strips, in cotton they change to purplish red, veins remain dark green, in soybean they turn yellowish and in apple trees, brown patches (blotches) appear on the leaves.
- Sulphur has specified role in initiating synthesis of proteins. Sulphur is an important nutrient for oil seeds, crucifers, sugar and pulse crops. It is an essential constituent of many proteins, enzymes and certain volatile compounds such as mustard oil. It hastens root growth and stimulates seed formation. It is essential for the synthesis of certain aminoacids and oils. It can be called as master nutrient for oilseed production.
- The deficiency of sulphur leads to slow growth with slender stalks, nodulation in legumes may be poor and nitrogen fixation is reduced. The young leaves turn yellow and the root and stems become abnormally long and develop woodiness. In case of fruit trees, the fruits become light green, thick skinned and less juicy. Sulphur deficient plant produces less protein and oil.
- Micronutrient elements are required by plants in very low concentration suggests that they all function as catalyst or atleast closely linked with some catalytic process in plants. Micronutrient elements include boron, copper, zinc, iron, manganese, molybdenum and chlorine.
- Boron helps in cell development by its influence on polysaccharide formation. It regulates translocation of sugars across membranes and polyphenolase activity. Iron is a constituent of cytochrome, haem and non-haem enzymes. Perhaps the best known role of iron is its catalytic role in enzyme activity.
- Copper, zinc and manganese are components of certain biological oxidation-reduction systems. Manganese performs some function in photosynthesis, acts as regulator to the intake and state of oxidation of certain elements.
- Zinc is concerned with the formation of Sulphydryl compounds such as cystein in the regulation of oxidation-reduction potential within the cells. Molybdenum is a constituent of nitrate reductase and nitrogenase enzyme and is associated with nitrogen utilization and in nitrogen fixation. Chlorine stimulates the activity of some enzymes and influences carbohydrate metabolism.
- Inserting or drilling or placing the fertilizer below the soil surface by means of any tool or implement at desired depth to supply plant nutrients to crop before sowing or in the standing crop is called placement.
- With placement methods, fertilizers are placed in the soil irrespective of the position of seed, seedling or growing plants before sowing or after sowing the crops. The following methods are most common in this category.
Plough - Sole Placement
- In this method, the fertilizer is placed in a continuous band on the bottom of the furrow during the process of ploughing. Each band is covered as the next furrow is turned. No attempt is usually made to sow the crop in any particular location with regard to the plough sole bands.
- This method has been recommended in areas where the soil becomes quite dry up to a few inches below the soil surface during the growing season, and especially with soils having a heavy clay pan a little below the plough-sole. By this method, fertilizer is placed in moist soil where it can become more available to growing plants during dry seasons.
Deep Placement of Nitrogenous Fertilizers
- This method of application of nitrogenous and phosphatic fertilizers is adopted in paddy fields on a large scale in Japan and is also recommended in India. In this method, ammonical nitrogenous fertilizer like ammonium sulphate or ammonium forming nitrogenous fertilizer like urea, is placed in the reduction zone, where it remains in ammonia form and is available to the crop during the active vegetative period.
- Deep or sub-surface placement of the fertilizer also ensures better distribution in the root zone and prevents any loss by surface drain-off. Deep placement is done in different ways, depending upon the local cultivation practices. In irrigated tracts, where the water supply is assured, the fertilizer is applied under the plough furrow in the dry soil before flooding the land and making it ready for transplanting. In areas where there is not too much of water in the field, it is broadcast before puddling. Puddling places the fertilizer deep into the root zone.
Sub - Soil Placement
- This refers to the placement of fertilizers in the sub-soil with the help of heavy power machinery.
- This method is recommended in humid and sub-humid regions where many sub-soils are strongly acidic. Due to acidic conditions the level of available plant nutrients is extremely low. Under these conditions, fertilizers, especially phosphatic and potassic are placed in the sub-soil for better root development.
- This method refers to the application of fertilizers into the soil close to the seed or plant.
- Localised placement is usually employed when relatively small quantities of fertilizers are to be applied. Localised placement reduces fixation of phosphorus and potassium.
- It is the process of mixing two or more different fertilizers varying in physical and chemical composition without any adverse effects.
- For this formulation certain additional materials called 'Fillers' and 'Conditioners' are used to improve the physical condition of the mixed fertilizer. This mixed fertilizer should be applied as top dressing.
- The use of liquid fertilizers as a means of fertilization has assumed considerable importance in foreign countries. Solutions of fertilizers, generally consisting of N, P2O5, K2O in the ratio of 1 : 2 : 1 and 1 : 1 : 2 are applied to young vegetable plants at the time of transplanting. These solutions are known as 'Starter Solutions'.
- They are used in place of the watering that is usually given to help the plants to establish. Only a small amount of fertilizer is applied as a starter solution. The starter solution has two advantages.
- The nutrients reach the plant roots immediately,
- The solution is sufficiently diluted so that it does not inhibit growth.
- As such a starter solution helps rapid establishment and quick early growth. There are two disadvantages of starter solution, if watering is not a part of the regular operation-extra labour is necessary and the fixation of phosphate may be greater.
Direct application of liquid fertilizers to the soil need special equipment. Anhydrous ammonia (a liquid
- under high pressure upto 14 kg per square cm. Or more) and nitrogen solutions are directly applied to the soil. This practice is very popular in the United States of America. Plant injury or wastage of ammonia is very little if the material is applied about 10 cm below the seed. If the application is shallow, nitrogen from ammonia will be lost. This method allows direct utilisation of the cheapest nitrogen source.
- Straight and mixed fertilizer containing N, P and K easily soluble in water, are allowed to dissolve in the irrigation stream. The nutrients are thus carried into the soil in solution. This practice of fertilization is called "Fertigation". This saves the application cost and allows the utilization of relatively in expensive water-soluble fertilizers. Usually nitrogenous fertilizers are most commonly applied through irrigation water.
- This refers to the spraying on leaves of growing plants with suitable fertilizer solutions. These solutions may be prepared in a low concentration to supply any one plant nutrient or a combination of nutrients.
- It has been well established that all plant nutrients are absorbed through the leaves of plants and this absorption is remarkable rapid for some nutrients. Foliar application does not result in a great saving of fertilizer but it may be preferred under the following conditions.
- When visual symptoms of nutrient deficiencies observed during early stages of deficiency.
- When unfavourable soil physical and chemical conditions, which reduce fertilizer use efficiency (FUE).
- During drought period where in the soil application could not be done for want of soil moisture.
- There are certain difficulties associated with the foliar application of nutrients as detailed below,
- Marginal leaf burn or scorching may occur if strong solutions are used.
- As solutions of low concentrations (usually three to six per cent) are to be used, only small quantities of nutrients can be applied in single spray.
- Several applications are needed for moderate to high fertilizer rates, and hence
- Foliar spraying of fertilizers is costly compared to soil application, unless combined with other spraying operations taken up for insect or disease contro
Soil Fertility and its Importance
- Soil fertility may be defined as the inherent capacity of soil to supply plant nutrients in adequate amount and in suitable proportion and free from toxic substances. There are two types of soil fertility viz.
Inherent or Natural Fertility
- The soil, as a nature contain some nutrients, which is known as inherent fertility. Among plant nutrients nitrogen, phosphorus and potassium is essential for the normal growth and yield of crop. The inherent fertility has a limiting factor from which the fertility is not decreased.
- The fertility develops by application of manures and fertilizers, tillage, irrigation, etc., is known as acquired fertility.
- The acquired fertility has also a limiting factor. It is found by experiment that the yield does not increase remarkably by application of additional quantity of fertilizers.
Factors Effecting Soil Fertility
- The factors that are effecting soil fertility may be of two types, i.e.,
- Natural factors and
- Artificial factors
- The natural factors are those which influences the soil formation and the artificial factors are related to the proper use of land.
- The factors effecting the fertility of soil are parent material, climate and vegetation, topography, inherent capacity of soil to supply nutrient, physical condition of soil, soil age, micro-organisms, availability of plant nutrients, soil composition, organic matter, soil erosion, cropping system and favourable environment for root growth.
Maintenance of Soil Fertility
- Maintenance of soil fertility is a great problem of our farmers. Cultivation of particular crop year after year in the same field decreases the soil fertility. To increase the soil fertility, it is necessary to check the loss of nutrient and to increase the nutrient content of soil.
- The following things must be properly followed for increasing the fertility of soil.
- Proper use of land,
- Good tillage,
- Crop rotation,
- Control of weeds,
- Maintenance of optimum moisture in the soil,
- Control of soil erosion,
- Cultivation of green manure crops,
- Application of manures,
- Cultivation of cover crops,
- Removal of excess water, (drainage)
- Application of fertilizers,
- Maintenance of proper soil reaction
- It is well known fact that in high rainfall areas, due to the leaching of bases, acids soils are formed, while in low rainfall regions, on account of arid and semi arid conditions, saline and alkali soils occur.
- Thus soil vary in acidity or alkalinity. The soil reaction is indicated by pH scale. When Ca(OH)2 or lime is added to the soil, it will become alkaline.
Liming of Acidic Soils
- Liming means addition of any compound containing Calcium alone or both calcium and magnesium, that is capable of reducing the acidity of the soil. Lime correctly refers only to Calcium oxide (CaO), but the term as applied in agriculture is universally used to include various other materials also, like Calcium carbonate, Calcium hydroxide, Calcium - magnesium carbonate (marl) and Calcium silicate slags.
- The effects of liming on the soil and plants are as follows :
- Lime neutralizes soil acidity,
- Beneficial soil bacteria are encouraged by adequate supplies of lime in the soil,
- Lime makes phosphorus more available,
- Liming helps the availability of potash and molybdenum,
- Lime furnishes two essential elements, namely calcium and magnesium (if lime is dolamitic) for plant nutrition,
- Lime reduces toxicity of Al, Mn and Fe,
- Improves soil physical condition
Fertilizers and Environmental Pollution
- Fertilizers are relatively safer than pesticides which exhibit toxic properties on living systems. However, all the quantities of fertilizers applied to the soil are not fully utilized by plants. About 50 per cent of fertilizers applied to crops are left behind as residues. Though, inorganic fertilizers are not directly toxic to man and other life forms, they have been found to upset the existing ecological balance. The nutrients escape from the fields and are found in excessive quantities in rivers, lakes and coastal waters.
- Algae blooms occur when the nutrient load is high, and these smother other aquatic vegetation and also interfere with the oxygen regulation in the water bodies. This phenomena may lead to loss of fish. Among the major synthetic plant nutrients, nitrogenous fertilizers cause most harm. Contamination of the environment arises because not all the fertilizer applied is taken up by the crop and removed at harvest. In tropical climate the maximum recovery in dry land crops is 50 to 60 per cent and 40 per cent in rice because much of nitrogen is lost as ammonia into the atmosphere.
- Eutrophication of water bodies due to higher nitrate and phosphate concentrations, increasing levels of nitrates in drinking water sources, accumulation of heavy metals such as lead and cadmium in soils and water resources are the principal causes of environmental concerns due to fertilizer use in agriculture. In the a national wide survey it was found that many streams and more than 20 % of wells contain 10 to 50 mg or even more of nitrates per litre of water. The contamination is caused by domestic sewage leaking to the ground water. The nitrates in drinking water can lead to several ailments. Blue - baby syndrome in infants and gastric and other forms of cancer have been related with nitrates in drinking water or diet.
- Another hazard associated with excessive use of fertilizers is the gaseous loss of nitrogen, into the atmosphere. High doses of carbon dioxide and ammonia that escape into the atmosphere both from fertilizer manufacturing plants and soils affect human health. Further the oxides of nitrogen have been reported to adversely affect the ozone layer, which protects the earth from UV radiation and heating up of earth.
- The oxides of nitrogen cause respiratory diseases like asthma, lung cancer and bronchitis. Arsenic, ammonia are waste stream components of nitrogen manufacturing plants while fluoride, cadmium, chromium, copper, lead and manganese are waste stream components of phosphatic fertilizer industry. If these waste stream of components are not properly disposed they cause harm to human beings and animals with contamination of air and water.
- The keeping quality of perishables like vegetables and fruits get declined with excess use of fertilizers particularly nitrogenous fertilizers.