DETERMINATION OF SOIL AVAILABLE PHOSPHORUS Forms of soil

DETERMINATION OF SOIL AVAILABLE PHOSPHORUS

Forms of soil P Soil P Organic P Inositol P Nucleic acid Inorganic P Phospholipids Ca- P HPO 4 -2 H 2 PO 4 - Fe, Al - P

Soil P Immobilization Organic P Fixation Soil solution P Mineralization Inorganic P Solubilization • Plants absorbs P as H 2 PO 4 - or HPO 42 • Total P content of a soil is of little agronomic value • Available P is the component of total P that is available to plants

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. 3 NH 4 F + 3 HF + Al. PO 4 H 3 PO 4 + (NH 4)3 Al. F 6 3 NH 4 F + 3 HF + Fe. PO 4 • H 3 PO 4 + (NH 4)3 Fe. F 6 Inclusion of acid (HCl) helps to release P held by Ca ions Ca 3(PO 4)2 + 3 H+ H 3 PO 4 + Ca 2+

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

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 H 3 PO 4 + (Soluble P in soil extract) H 3 P(Mo 3010)4 12 H 2 Mo 04 (Molybdic acid) Reduction Ascorbic acid H 3 P(Mo 3010)4 + 12 H 20 (Molybdo- Phosphoric acid) Blue colour

Method Determination of P in the soil extract – Pipette 10 ml of filterate into a 25 ml 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.

Results Table of values Standards (ug P/ml) %T 0. 8 1. 6 2. 4 3. 2 4. 0 Soil extract y Blank z Log % T 2 - log%T

Calculations Calibration curve

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

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 2 g Conc of P in soil (ug/g) = Conc P in soil extract (ug/ml) x 10