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DETERMINATION OF SOIL AVAILABLE PHOSPHORUS

DETERMINATION OF SOIL AVAILABLE PHOSPHORUS. Forms of soil P. Soil P. Immobilization. Fixation. Organic P. Soil solution P. Inorganic P. Plants absorbs P as H 2 PO 4 - or HPO 4 2- Total P content of a soil is of little agronomic value

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DETERMINATION OF SOIL AVAILABLE PHOSPHORUS

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  1. DETERMINATION OF SOIL AVAILABLE PHOSPHORUS

  2. Forms of soil P

  3. Soil P Immobilization Fixation Organic P Soil solution P Inorganic P • Plants absorbs P as H2PO4- or HPO42- • Total P content of a soil is of little agronomic value • Available P is the component of total P that is available to plants Mineralization Solubilization

  4. Available Soil P (Bray 1) A high degree of correlation between Acid flouride extraction procedure and Plant P uptake. The efficiency of acid flouride extraction is due to unique complexing ability of P. 3NH4F + 3HF + AlPO4 H3PO4 + (NH4)3AlF6 3NH4F + 3HF + FePO4 H3PO4 + (NH4)3FeF6 • Inclusion of acid (HCl) helps to release P held by Ca ions • Ca3(PO4)2 + 3H+ H3PO4 + Ca2+

  5. Method Step 1. Flouride Extration of Available P • Weigh 2 g of soil into a 50ml shaking bottle. • Add 20ml of the extracting solution and shake for 1 minute. • Filter into a suitable container.

  6. Method Step 2. Determination of P in the soil extract The orthophosphate in the soil extract is converted to a blue complex (molybdo-phosphoric acid). The color intensity measured with spectrophotometer H3PO4 + 12H2Mo04 H3P(Mo3010)4 + 12H20 (Soluble P in soil extract) (Molybdic acid) (Molybdo- Phosphoric acid) H3P(Mo3010)4Blue colour Reduction Ascorbic acid

  7. Method Determination of P in the soil extract • Pipette 10ml of filterate into a 25ml vol. flask. • Add 1 ml of molybdate reagent followed by 1 ml of the dilute reducing agent. • Shake well and allow the solution to stand for 15 minutes. • Measure the percent transmission at 520 nm wavelength on a spectrophotometer.

  8. Results Table of values

  9. Calculations Calibration curve

  10. Calculations Equation for a linear curve y = mx +c From the calibration curve ; y = mx But y = 2- log %T x = Conc (ug/ml) Hence 2- log %T = m x (conc ug/ml) Conc P (ug/ml) in soil extract = 2-log %T m

  11. Calculations Conc of P in soil (ug/g) = Conc P in soil extract (ug/ml) x volume of extractant (ml) mass of soil (g) Conc of P in soil (ug/g) = Conc P in soil extract (ug/ml) x 20 ml 2g Conc of P in soil (ug/g) = Conc P in soil extract (ug/ml) x 10

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