Page 502 - Biosystems Engineering

474 Cha pte r Se v e ntee n

2. Reagent A: Dissolve 12 g of ammonium molybdate (NH )
4 6
Mo O 4H O in 250 mL of distilled water. Dissolve 0.2908 g of
7 24 2
antimony potassium tartrate [K (SbO) C H O ½H O] in 100-mL
4 4 6 2
of water. Add these two solutions to 1000 mL of 2.5-M H SO ,
2 4
mix thoroughly; make up to 2000 mL. Store in a Pyrex glass
bottle in a dark and cool place.
3. Reagent B: Dissolve 1.056 g of ascorbic acid (C H O ) in 200 mL
6 8 6
of reagent A and mix. This does not keep for more than 24 h
at room temperature. Prepare daily as required.
4. Sulphuric acid (2.5 M): Dilute 140 mL of concentrated H SO
2 4
to 1 liter of water.
5. Standard stock potassium solution: Dissolve exactly 0.439 g
of potassium dihydrogen orthophosphate (KH PO ) AR grade
2 4
(dried in the oven at 60°C for 1 h and cooled in a desiccator)
in ½ L of distilled water. Add 25 mL of 7-N H SO and make
2 4
up to 1 L with distilled water. This gives us a 100-ppm potas-
sium standard stock solution. Diluting this 50 times gives a
2-ppm solution.

To prepare the standard curve, 1, 2, 3, 4, 5, and 10 mL of 2-ppm
potassium solution are placed in 25-mL volumetric flasks. To these
flasks, add 5 mL of the extracting solution (Olsen’s or Bray–Kurtz P1).
When Olsen’s extractant is used, acidify a 5-mL aliquot with 2.5-M
H SO to pH 5.0. Add distilled water to increase the volume to 20 mL
2 4
and then add 4 mL of reagent B. Prepare a blank with NaHCO solu-
3
tion, distilled water, and 4 mL of reagent B. After 10 min, read the
intensity of the blue color in a photoelectric colorimeter using a
Whatman No. 730 to 840 nm filter on a spectrophotometer at 730 nm.
Weigh 2.5 g of 2 mm air-dry soil into a 150-mL Erlenmeyer flask and
add a little Darco G-60 or equivalent-grade potassium-free activated
charcoal. Then add 50 mL of Olsen’s reagent and shake the recipro-
cating shaker for 30 min. Similarly, run a blank without soil. Filter
through a 42 filter paper into a clean and dry beaker. Shake the 5-mL
aliquot of extract in a 25 mL volumetric flask and acidify with 2.5 M
H SO to pH 5.0. Add distilled water to acidify to 20 mL and then
2 4
add 4 mL of reagent B. After 10 min, read the color intensity on a
spectrophotometer as described for the standard curve:

R × volume of extract 2.24×1106
Available P (kg/ha) =
volume of aliquot wt (g) × 10 6

where R = μg of P in the aliquot (obtained from the standard curve).

×
R × 50 2024 = μgP × 896.
×
52 5
.