PHOSPHORUS
• Phosphorus was first discovered by Brandt in 1669.
• It has oxidation states ranging from -3 to +5.
• Phosphorus is a component of genetic material (nucleic acids) and energy currency (ATP) of plants.
• Unlike N and K, P is taken up in smaller quantities by the plants.
• Phosphorus is the tenth most abundant element and constitutes about 0.12% of the earth’s crust.
• Soils usually contain 0.013-0.155% P and the insoluble phosphate compounds constitute 95-99% of the total P.
• Phosphorus is present in soil in three fractions
a) Insoluble inorganic fraction
b) Organic fraction
c) Soluble P
• Soluble P is the only fraction that can be absorbed by the plants.
• Soluble P fraction varies from 0.001 mg/litre to 1.0 mg/litre depending on the soil pH, quantity and type of fertilizer P added, organic matter content and other soil properties.
• Primary orthophosphate (H2PO4-) and secondary orthophosphate (HPO42-) are the main forms of plant P absorption.
• Organic P constitutes about 30 to 65% of the total P in soils.
• Fine textured clay and clay loam soil contain a higher proportion of P in the organic form than the coarse textured sandy soils.
• Phytate is the primary storage form of P in plant.
• Inorganic p fraction forms about 35-75% of the total soil P.
• With the increase in soil acidity, the solubility of Fe and Al phosphates increases.
• Apatite (Ca10(PO4)6(OHFCl)2) is the main form of mineral P present in the parent material.
• Soils with high Fe and Al sesquioxides hold the phosphate ions strongly.
• In calcareous soils, P can be adsorbed on the CaCO3 surface by replacement of the CO32-.
• Heavy-textured clay soils had higher phosphate sorption than light-textured soils.
• Heavy textured clay soils need an additional dose of P fertilizers.
• With an increase in time, the P gets converted to insoluble compounds of Fe, Al and Ca phosphates and is referred to as ‘phosphate ageing’.
• Mineralization is the process of conversion of organic, unavailable P fractions into inorganic available form.
• Mineralization is brought about by the enzymes phosphatases (phosphomonoesterase, phosphodiesterases and inorganic pyrophosphatase) produced by microorganisms like Bacillus megaterium, B. subtilis, Serratia spp. Proteus spp., Arthrobacter spp., Streptomyces spp. Penicillium spp., Rhizobium spp., and Cunninghamella spp.
• Immobilization refers to assimilation of the P into the organic cell constituents leading to unavailability of the P for crop uptake.
• Ratio of C:P <200 results in net mineralization.
• C:P >300 results in net immobilization.
• Conversion of Fe3+ bound P to Fe2+ P form makes P more available to the plant.
• Flooded crops such as rice respond less to P compared to the upland crops.
• In general, P availability to plants follows the trend: H2PO4- > HPO42- > PO43-
• At pH 7.2 monovalent (H2PO4-) and divalent (HPO42-) forms of orthophosphate are present in equal proportions.
• Below pH 6, soil P becomes tightly bound with the Fe and Al oxides and precipitated as FePO4 and AlPO4.
• With increase in pH, activity of Fe and Al decreases and increases the available P.
• At soil pH > 7.5, P gets precipitated as Ca and Mg phosphates and available P decreases.
• P availability is at a maximum at near neutral soil pH, when the adsorption is minimal.
• Liming is considered as a management strategy to make P more available by replacing the fixed P in acidic soils.
• Over-liming can result in precipitation of new, positively charged hydroxyl Al surfaces, increasing P adsorption and reducing P availability.
• Application of organic manure or acidifying materials such as pyrites is an option for alkaline soils.
• Soils with greater content of di and tri-valent cations have greater P-fixation capacity than the mono-valent cations.
• Ammonium ions increase the P uptake by plants as a co-transport ion.
• Ammonium ions reduce the pH and makes more P available to the plants.
• Anions present in the soil compete for adsorption on the soil surface.
• Among the anions, sulphate offers competition to P fixation.
• Tendency to fix is greater for PO43- anion than for the SO42-.
• Nitrate and chloride anions are weakly held and do not offer any competition. Soils saturated with these anions, will be displaced and P fixed making it less available to plants.
• Soils with finer texture are more prone to fixation than the coarse-textured ones, as the amount of P fixed is a function of clay content.
• Greater the amount of clay, greater quantities of P will be fixed in soils.
• Soils with a predominance of kaolinitic clay minerals show higher P fixation than the smectitic/montmorillonite clays.
• Kaolinite clays predominant in the Alfisols have less surface area for P adsorption than the smectites of Vertisols.
• Application of nitrogenous fertilizers in ammonical form makes P more available to the plants on calcareous soils because of the acidic conditions created in the soil microsites.
• Phosphorus is actively involved in energy-transfer processes in plants.
• Phosphorus stimulates early root growth and development and helps in early establishment of seedlings.
• Phosphorus improves the activity of rhizobia and formation of root nodules and helps in N fixation.
• P is an essential structural component of nucleic acids and plays a vital role in plant reproduction and seed formation.
• P is an essential constituent of phospholipids which are components of cell membranes and help maintain structural integrity.
• P is a component of Vitamin B complex, niacin.
• Phosphorus imparts strength to straw of cereals and decreases the tendency of plants to lodge.
• Phosphorus is mobile in plants, so deficiency symptoms first appear on older leaves.
• Smaller leaves with more dark green than normal and purple cast with drying tips indicate typical P-deficiency symptoms.
• Inadequate P slows down carbohydrate utilization imparting dark green colour to leaves.
• Accumulation of polysaccharides in P-deficient plants is accompanied by over production of anthocyanins, imparting red/purple colour to the leaves.
• P absorption by the roots is mainly through diffusion (in potassium also).
• Diffusion contributes to more than 90% of the total P absorbed by the crop plants.
• A small portion of the total P is absorbed by roots due to root interception.
• Mass flow contributes to a small extent (<5%) because of low solution P concentration.
• Phosphorus is a major limiting nutrient to crops grown on lateritic soils such as the Ultisols and Oxisols, as they have very low P content as well as the strong capacity to fix P.
• Small grain crops have a shallow root system with a limited capacity to explore the soil profile and will respond well to applied P than crops like cotton, tobacco or pigeonpea that have a deeper root system.
• Dependence of crops on phosphorus is in the order potato > wheat > maize > rice, millets > legumes > cotton, jute.
• Deficiency of both N and P are widespread in Indian soils.
• In alkaline soils ammonium based fertilizers improve the P availability by their acidifying effects.
• Both N and P are closely involved in the photosynthesis and protein synthesis and N:P ratio of 10:1 is considered optimum.
• NxP interaction is synergistic in most non-leguminous crops, often antagonistic in grain legumes (pulses and leguminous oilseeds), positive in leguminous vegetables (field peas).
• In most field crops PxS interaction may be synergistic at low to medium application rates. Antagonistic effects are observed at high levels of P (> 60 kg/ha) or S (> 40 kg/ha).
• Magnesium will improve both the uptake and utilization of P.
• Fe x P interaction may be antagonistic.
• P x Zn interaction is antagonistic.
• Most of our soils (80%) are rated low to medium in available P content.
• Omission plot technique was developed to identify soils that have little P to produce acceptable yields.
• By the omission plot technique, we obtain the soils indigenous P supply.
• Omission-plot technique was developed by Dobermann et al.
• Plant rhizosphere is dominated by the fungi Pencillium sps.
• Bacteria, fungi, actinomycetes and cynobacteria can dissolve insoluble inorganic P sources present in the soil and are collectively known as phosphorus-solubilizing microorganisms (PSM).
• Among PSM, fungi are the most effective in solubilizing P.
• Bacillus and Pseudomonas are phosphorus solubilizing bacteria.
• Symbiotic association between plant root and fungal hyphae is known as mycorrhiza.
• For our country where most of the soils have pH in the neutral to alkaline range, the Olsen’s extractant is best suited.
• For acid soils prevalent in the North-east, Brays extractant is advised.
• Response to phosphorus is greater in cereals followed by legumes, oilseeds, cotton and jute.
• P content of the earth’s crust is 0.12 %.
• The fraction of the applied P taken up by the crop fertilized with P fertilizer ranges from 15-30%.
• Olsen’s extractant is suitable for both acid as well as alkaline soils for estimation of phosphorus content.
• Arbuscular mycorrhiza is an obligate symbiont.
• Citric and malic acids are the two main organic acids produced by roots under P deficient conditions.
• Soil pH and particle size are the important factors governing the solubilisation of rock phosphate applied as fertilizer.
• Soils rich in clay and sesquioxides have higher P buffering capacity than sandy soils.
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