This document discusses methods for measuring phosphorus concentration in soil samples using a UV-Visible Spectrophotometer. It describes preparing a standard curve by taking aliquots of a phosphorus solution and measuring their absorbances. It then provides a formula for calculating the available phosphorus concentration in a soil sample based on the absorbance reading and standard curve, along with a rating scale for classifying soil phosphorus levels.

4. Log I0/It=KC
Where Io is the intensity of the incident radiation and It is the transmitted
radiation.
C: Concentration of solution
k: Constant

5. UV-Visible Spectrophotometer

7. 6. Activated charcoal - Darco G-60.
7. 2.4 p- nitrophenol (0.25%)
5 N
(5 N) 148 ml

8. (5 N)

9. Preparation of std. curve
Prepare a series of standards by taking 0, 1, 2, 3, 4, 5 and 10 ml of 2
ppm P solution in 25 ml volumetric flask .
Add 5ml NHCO3 sol. To each flask and adjust pH as above.
Develop colour and record the reading as above.
Construct a graph by plotting readings on X-axis and P on Y-axis.

10. Available P Total vol. of extract 1 2.24 X 106
(Kg/ha) = R X -------------------------- X --------------- X --------------------------
Vol. of aliquot taken Wt. of soil 106

11. Result:
Sr. No. Particulars Rating of soil (kg/ha)
1. Very low Less than 7
2. Low 7.1- 14
3. Medium 14.1- 21
4. Moderately High 21.1- 28
5. High 28.1 - 35
6. Very high More than 35
Rating of soil for phosphorous :
The available phosphorous status of the soil is ------------
kg/ha, therefore soil is ------------ in phosphorous.