Nutrient elements

Plant nutrients A plant nutrient is said to be essential if: A deficiency of the nutrient makes it impossible for the plant to complete its life cycle. Such deficiency can be prevented or corrected only by supplying this element. The element is involved directly in the nutrition of the plant.

MACRO NUTRIENTS Carbon Hydrogen Oxygen Nitrogen Phosphorus Potassium Calcium Magnesium Sulphur These are obtained from atmospheric CO2 and soil water. These are obtained by plants from the soil

Plant nutrients Micronutrients Iron Manganese Boron Molybdenum Copper Zinc Chlorine These are obtained by plants from the soil

Nitrogen

Nitrogen N is extremely important for Agriculture N should be managed carefully Plants need it in large amounts It is fairly expensive to supply It is easily lost from the soil

NITROGEN Atmospheric N2 N fert Immobilization Nitrogen cycle

Nitrogen Transformations Mineralization Immobilization Nitrification Fixation Denitrification Volatilization

NITROGEN N – Mineralization Amino acid NH4+ NO3- N – Immobilization The microbial conversion of organic N to mineral-N Amino acid NH4+ NO3- immobilization Nitrosomonas / nitrobacter Bacteria/ fungi mineralization N – Immobilization This is the conversion of inorganic N to organic form in microbial tissue.

Mineralization of N-compounds The process takes place in essentially 3 steps:- aminization, ammonification and nitrification. Aminization Conversion of proteins and allied compounds to amines and amino acids by heterotrophic bacteria and fungi. Proteins R–NH2 + CO2 + Energy Heterotrophic Fungi/bacteria

Ammonification It is conversion of amines and amino acids to NH4+ by heterotrophic bacteria and fungi R – NH2 + H2O NH3 + R –OH + Energy Heterotrophic Fungi/bacteria R-NH2 NH3 + H+ NH4+

Ammonification Fates of NH4+ 1) fixed by clay minerals, 2) may be converted to NO2- or NO3- by nitrification 3) used by plants (NH4+), 4) volatilization High pH Soils > 7.5

Soil conditions affecting Ammonification 1) C:N ratio Material C:N ratio FYM (composted) 20 FYM (Fresh) 100 Saw dust 400 2) Aeration: well – drained aerated soil 3) pH :Generally fungi act best in pH < 5.5; Bacteria in pH >5.5

Nitrification Nitrification is the biological oxidation of ammonia to nitrate. 2 NH4+ + 302 → 2NO2- + 2H2O + 4H+ + Energy 2 NO2- + O2 → 2NO3- + Energy

Nitrification 2 - step process 1. 2NH4+ + 3O2 ---> 2NO2- + 4H+ + 2H20 + E Nitrosomonas/Nitrosococcus 2. 2NO2- + O2 --> 2NO3- + E Nitrobacter Process is acid causing due to release of 4 H+

Nitrification Reactions require molecular oxygen Reaction releases H+ ions resulting in the acidification of the soil. Reactions involve microbial activity and therefore soil conditions (moisture supply, temperature & pH) should be optimum for microbial activity.

Gaseous Losses of Soil N Gaseous losses of N occur mainly as N2 N2O, NO and NH3. Mechanisms for the losses are: Denitrification Chemical reactions involving nitrites under aerobic conditions Volatilisation of ammonia

Gaseous Losses of Soil N a) Denitrification: is the biochemical reduction of nitrates under anaerobic conditions to gaseous N-compounds. NO3- NO2- NO N2O N2 O O O O Factors Affecting Denitrification i) Soil moisture and oxygen supply ii) Availability of organic substrate iii) Soil pH and temperature

Gaseous Losses of Soil N Reaction of nitrites under aerobic conditions Nitrites in a slightly acid solution will evolve gaseous N when brought in contact with certain ammonium salts, with simple amines such as urea, and even with Non-nitrogenous sulphur compounds and carbohydrates. Possible reaction: 2HNO2 + CO(NH2)2 → CO2 + 3H2O + 2N2 ↑

Biological nitrogen fixation Conversion of N2 in the soil atmosphere into NH4+ by specialized groups of micro-organisms. Non-Symbiotic (free living microbes): Clostridium – anaerobic Azotobacter – aerobic, blue-green algae

2. Symbiotic N Fixation Bacteria invades host plant root Rhizobium Bacteria invades host plant root Response of host plant root is to grow a nodule for the bacteria to live in. Bacteria takes N2 from the air and converts it into R-NH2 in bacteria and some is in the form of NH4+ Fate of N Fixed by Rhizobium: 1) used by host plant, 2) leaks out of root to become available to surrounding plants, Cowpea Nodulation Host plant-bacteria complex

Non-Biological Fixation 1) Atmospheric electrical discharge (lightning) Oxidation of N N2 + 3O2 2NO-3 2) Industrial process 10-20% of NO3- via atmospheric deposition

N - balance Available soil N Commercial fertilizers immobilization Non Symbiotic fixation immobilization Crop removal Symbiotic fixation Available soil N Rainfall Crop residues and manure Atmosphere Gaseous losses Erosion losses Soil organic matter Leaching losses Fixation by clay minerals

Distribution of N in soils 1) Drainage : Under poor drainage, decay of O.M. is very slow Organic N is high, Mineral N is low 2) Topography: On slopes, N content is lost faster than that on summit or valley bottom.

Distribution of N in soils 3) Texture: N, content decreases as texture becomes coarser. 4) Seasonal effect: Nitrate level slowly increases during the dry season due to low leaching. Nitrate levels increase rapidly at the beginning of the rainy season due to increased bacteria activity and mineralization in the presence of low leaching. During the rainy season level of N falls and remains fairly constant until the next dry season due to leaching of N compounds from the soil.

N in Plant Nutrition is an essential constituent of all living matter. important in the formation of protein. forms an integral part of chlorophyll molecule. an adequate supply of N is associated with vigorous vegetative growth and deep green colour.

N deficiency Symptoms A stunted yellowish appearance Yellowing or chlorosis, usually 1st appearing on the lower leaves, the upper leaves remaining green In severe shortages – leaves will turn brown and die.

P -cycling

Phosphorus Discovered by Hening Brand in 1669 Name Origin: phôs (light) and phoros (bearer) “phosphorus = light-bearer” Atomic number 15 Atomic weight 30.974 One isotope 32P

Soil Phosphorus Lithosphere is the main source and reservoir of P Lithosphere – 0.12% = 1200ppm Soil solution – 0.2 to 0.3ppm Total P content of soil depends O.m content Parent material Degree of weathering

Forms of soil P

Inorganic P Insoluble or nearly insoluble P compounds Acid soils Free Fe and Free Al combine with phosphate Al3+ + H2PO4- + 2H2O H+ + Al(OH)2H2PO4 (soluble) (insoluble) Alkaline soils Free Ca and Mg combine with Phosphate 2PO43- + 3Ca2+ Ca3(PO4)2 (soluble) (Insoluble )

Inorganic P 2) Insoluble phosphate-clay complexes 4 5 6 7 8 Al3+ + Al (OH)2 + 2H+ Precipitated form Dissolved form Fixation by Al and Fe Fixation by Ca Reaction with clays 4 5 6 7 8 Soil pH

Soluble phosphate forms H3PO4 = phosphoric acid, H2PO4- = 1º orthophosphate, HPO4-2 = 2º orthophophate, PO4-3 = phosphate

Calcium Phosphate Compound â Ca(H2PO4)2 â monocalcium phosphate â CaHPO4 â dicalcium phosphate â Ca3(PO4)2 â tricalcium phosphate â Ca5(PO4)3OH2 â hydroxyapatite â Ca5(PO4)3F â Fluorapatite 6 pH 8

P Fixation

P Fixation P “Fixation” – refers to the processes whereby available phosphorus forms combinations with other soil constituents and thus becomes unavailable to plants.

Mechanisms for P fixation 1) Precipitation reaction between P and other ions in soil solution to form hydroxy-phosphates Al(OH)2+ + H2PO4- Al(OH)2H2PO4 + 2H+ (soluble) (insoluble) Ca2+ + HPO42- CaHPO4 (soluble) (Insoluble ) 2) Adsorption reaction between soluble P and insoluble soil components Al2O3.3H2O + 2H2PO4- 2Al(OH)2H2PO4 + 2OH- (soluble) (insoluble)

Most Available P between pH 5.5 - 7 40

Factors affecting P fixation Type of clay minerals and amount of clay in soil Presence of OH- of Fe, Al and Mn Soil reaction Presence of organic matter

The presence of Organic matter in the soil Om reduces P fixation; the formation of phospho-humic complexes which are more easily assimilated by plants anion replacement of the phosphate by the humate ion. the coating of sesquioxide particles by humus to form a protective cover.

Secondary Macronutrients Ca Mg and S are secondary macronutrients They are required by plants in lesser amounts than NPK but in larger quantities than the micronutrients. They are essential for correcting the reaction of soil. Ca,Mg for reclaiming acid soils S for reclaiming alkaline soils They are prone to leaching

Magnesium Mg is the only mineral element in the chlorophyll molecule

Magnesium It is absorbed by plants as the ion Mg 2+. Sources of soil Mg Weathering of rocks such as: Dolomite , Ca Mg (CO3)2 or Olivine , MgFeSiO4 Biotite , K (Mg Fe)3 Al Si3 O10 (OH)2

Calcium Weathering of rocks such as: Ca is absorbed by plants as Ca2+ Source of Soil Ca Weathering of rocks such as: Dolomite Ca.Mg (CO3)2 Calcite CaCO3 Apatite Ca5(PO4)3·(Cl, F, OH) Calcium feldspars

Sulphur S is an essential element particularly associated with protein synthesis. Plant available form of S = SO42-. Sulphur fertilization is becoming important because of changes in the use of some fertilizers SSP (18% P2O5; 8-10% S) to TSP (45% P2O5; <3% S) AS (NH4)2SO4 (21%N; 24%S) to CO(NH2)2 (urea) (46%N)

Micronutrients Nutrient elements required by plants in relatively small amounts. Iron Manganese Boron Molybdenum Copper Zinc Chlorine These are obtained by plants from the soil

Micronutrients Source of micro nutrients soil parent material 2) organic matter In uncultivated lands there is a greater concentration of micronutrients in the surface soil where O.M. levels are high.

Micronutrients Plant available forms of micro nutrients Nutrient Iron Fe2+ or Fe3+ Manganese Mn 2+ Boron BO33- Copper Cu 2+ Molybdenum MoO42- Zinc Zn 2+ Chlorine Cl-

Nutrient mobility and deficiency symptoms For nutrients which are mobile in plants deficiency symptoms 1st appears on the lower and older leaves. Eg: N, P, K, Mg, and Zn. Mg deficiency

Nutrient mobility and deficiency symptoms Nutrients with limited mobility in plants produce symptoms on new leaves or growing points. Eg: calcium, sulphur, boron, iron, copper, and Mn. Fe deficiency

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