Procedure

Apparatus:

Spectrophotometer, Whatman No: 42 filter paper, Beakers, Standard Flasks, Pipettes, Funnels, etc.

Determination of Available Phosphorus in Soil

a) Preparation of Reagents:

1. 1 N Ammonium Fluoride: Dissolve 37 g NH₄F in distilled water and dilute the solution to 1 L. Store in a polyethylene bottle.
2. 0.5 N HCl: Dissolve 20.2 mL concentrated HCl and make up to 500 mL with distilled water.
3. Extracting Solution: Add 15 mL of 1N NH₄F and 25 mL of 0.5 N HCl to 450 mL distilled water. Final concentrations: 0.03 N NH₄F and 0.025 N HCl.
4. Dickman and Bray’s Reagent: Dissolve 15 g ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O) in 300 mL distilled water. Warm to ~60°C, filter, and cool. Add 34.2 mL concentrated HCl and make up to 1 L. Reaction: (NH₄)₆Mo₇O₂₄·4H₂O + 6 HCl → 7 H₄MoO₄ + 6 NH₄Cl
5. Stannous Chloride (SnCl₂·2H₂O) Stock Solution: Dissolve 2 g SnCl₂·2H₂O crystals in 8.3 mL concentrated HCl, dilute to 100 mL. Store in a brown bottle with a piece of tin metal for stability.
6. Working Solution of Stannous Chloride: Dilute 0.5 mL of the stock solution to 66 mL with distilled water. Prepare fresh before use.
7. Standard Phosphorus Solution: Weigh 0.439 g KH₂PO₄ into a 500 mL flask, add ~25 mL concentrated H₂SO₄, and make up to volume. Pipette 2 mL of this into a 10 mL flask and make up to volume. This gives a 2 ppm P stock solution.

b) Extraction:

• Weigh 5 g soil into a 100 mL conical flask.
• Add 50 mL of extracting solution.
• Shake for exactly 5 minutes.
• Filter through Whatman No. 42 paper.
• Prepare a blank using all reagents but no soil.

c) Colorimetric Estimation (Dickman and Bray’s Method):

• Take seven 25 mL standard flasks labeled: SAMPLE, BLANK, 0.2, 0.4, 0.6, 0.8, 1.
• To the SAMPLE flask, pipette 5 mL of soil extract.
• Add 5 mL of Dickman and Bray’s Reagent and 7.5 mL boric acid.
• Fill the other flasks with increasing volumes (1 to 5 mL) of standard P solution.
• Add 5 mL of Dickman and Bray’s Reagent and 7.5 mL boric acid to each flask.
• Add distilled water down the neck to rinse the molybdate adhering on the glass.
• Mix, add 1 mL SnCl₂ working solution, and make up to volume with distilled water. Mix well.
• Prepare a blank similarly.
• After 10 minutes, measure absorbance at 690 nm.
• Plot absorbance vs. concentration (ppm) and determine phosphorus in sample from the graph.
• Note: Color fades after 15–20 minutes.

d) Calculation:

Phosphorus in soil (ppm) =

ppm P in solution (from graph) × Total extractant volume (mL) ÷ Weight of soil (g)

Points to Remember while Performing the Experiment in a Real Laboratory

• Always wear a lab coat and gloves in the lab.
• Switch on the exhaust fan and check for all required chemicals and reagents.
• If reagents are missing, prepare them using the required components.
• Adjust the Bunsen burner to a small blue cone flame.
• Clean all apparatus with chromic acid and distilled water. Ensure they are dry.
• Calibrate the electronic balance before use.
• Ensure the desiccator has enough silica gel.
• Clean the crucible with chromic acid, heat it, cool it, and handle it with clean tongs or tissue—never by hand.
• Set oven temperature to 130°C and use cotton gloves when handling hot items.
• Clean the Kipp's apparatus with water and check iron sulfide and H₂SO₄ levels for H₂S gas production.
• Clean glassware with soap and distilled water.
• After the experiment, recap reagent bottles and turn off lights, exhaust fan, oven, and gas cylinder.
• Discard used gloves in the waste bin.