Lime Requirement (SMP)

Slightly Modified Shoemaker-McLean-Pratt (SMP) Single Buffer Method

By Jared L. DeForest

Reagents

SMP Buffer – Part A

In 1 liter flask

• Fill one-third the flask with DI water
• 1.8 g p -nitrophenol (HO · C ₆ H ₄ NO ₂ )
• 3.0 g potassium chromate (K ₂ CrO ₄ )
• 53.1 g calcium chloride dihydrate (CaCl ₂ 2H ₂ O)

Shake vigorously.

SMP Buffer – Part B

In 250 ml flask

• 100 ml DI water
• 2.0 g Calcium acetate (Ca(OAc) ₂ )

Mix until fully dissolved.

Add part B to part A in the 1 liter flaskand shake vigorously when they are combined. Place on an orbital shaker for 2 hours.

SMP Buffer – Part C

In the same 1 liter flask from above

• 2.5 ml triethanolamine (TEA)

Shake mixture periodically until it is completely dissolved.

Adjust to pH 7.50 ± 0.02 with 4 M NaOH or 4 M HCl

Transfer mixture into a 1 liter volumetric flask.

Bring to volume with DI water.

Purge the solution with N ₂ and store in a dissector. CO ₂ or water vapor will contaminate the buffer.

Procedure

In a 125 ml sample cup

• 5.2 g Fresh sieved soil
• 10 ml DI water
• 10 ml SMP buffer

Place on orbital shaker (250 rpm) for 10 minutes.

Let sit for 30 minutes.

The times of shaking and standing are of prime importance. Therefore, stagger samples, at most, in sets of six to keep close to these times as possible.

Gently swirl the solution before measuring pH. Record to nearest 0.01 pH unit as soil-buffer pH.

Adapted from Sim (1996) Chapter 17 “Lime Requirement”           

SSSA Methods of Soil Analysis. Part 3. Chemical Methods               

Created: 09/15/10