Soil Lab MethodsBuffer pH (Lime requirement)
A routine buffer pH or lime requirement pH is made on all samples. The Moore-Sikora buffer method is used to calculate the lime requirement of ground, agricultural limestone required to raise the soil pH of the surface 8 inches to the target pH suggested in EC 476 (Nutrient Management for South Carolina Based on Soil-Test Results).
Components of the Moore-Sikora Buffer
For every liter of solution, the following quantities of chemicals are dissolved.
MES hydrate, 99% (4-morpholinoethanesulfonic acid (C6H13NO4S.xH2O, fw without water=195.23)): 7.43 g
MOPS (3-(N-Morpholino) propanesulfonic acid (C7H15NO4S fw=209.26)): 27.40 g
Boric Acid (H3BO3, fw=61.83): 13.10 g
Potassium Chloride (KCl fw=74.56): 74.00 g
Potassium Hydroxide (KOH fw=56.11): 11.07 g
Quantitatively add each component above to a container that will hold the appropriate volume. Use deionized water to rinse out the weighing containers. Add deionized water to 80% of the final intended volume and stir the solution overnight. Dilute the solution with deionized water to the intended final volume and stir thoroughly.
Calibrate a pH meter to pH buffers of 7.00 and 4.00. After calibration, a pH 00 buffer is checked to ensure the electrodes are accurately measuring pH above 7.00. The pH 00 buffer should read 00 +- 0.01.
Fifteen mL deionized water is added to a 15 mL aliquot of the solution and the pH of the 1:1 mixture is measured. If the pH of the 1:1 mixture is not 8.00 +- 0.01, the pH of the original solution is adjusted using dropwise additions of concentrated KOH or HCl. The pH of the full strength buffer should be 7.92+-0.01 to ensure a pH of 8.00+-0.01 for a 1:1 mix of buffer with water.
- To each soil-water mixture from the pH determination (15 g soil + 15 mL H20) add 15 mL of the buffer solution by automatic pipette.
- Stir thoroughly and allow to stand a minimum of 30 minutes.
- Standardize pH meter with buffer 7.0 and 4.00. Read buffer blank (buffer blank =15 mL buffer + 15 mL deionized water) and set buffer offset.
- Verify values of check samples before leaving analyser.
- If the Check soil readings are not within tolerance, check standardization and/or seek assistance. Verify all check values at end of run.
- Save data to a text file and print.
- Dump cups then autoclave samples from Marion and Horry.
- Store electrodes in storage solution or buffer standard solution with meter in the Standby mode.
Lime requirement for fish pond samples
The same procedure for buffer pH on fish pond and shrimp pond samples is followed.