PLS 671: Environmental Soil Chemistry

“Soil Lime Requirement”
Lab #5 10/25/04

I. Objective:

To measure soil lime requirement using the direct titration method, where the soil is stirred and base is added until the target pH of 6.5 is reached.

II. Background:

A variety of techniques have been used to estimate lime requirement such as soil-lime incubations, soil-base titrations, soil-buffer equilibrations, and exchangeable acidity.  The target pH has been defined as the soil pH value for optimum plant growth, however, soils may be limed for purposes other than plant growth such as converting a potentially toxic element into an insoluble form.  The direct titration method will be employed in today’s lab to estimate lime requirement.  Alkali will be added to the soil suspension to neutralize soil acidity.  A plot of pH versus alkali added will be used to estimate buffer capacity.  A measure of the soils’ buffer capacity will be determined from the slope of the linear regression of pH vs. OH- added and expressed as 1/slope.  You will also perform titrations on a sample provided in the lab labeled CECIL 

1.) Weigh 10 g of your soil in duplicate 60 mL polyethylene bottles. 

2.) Add 25 mL of deionized distilled water and stir. 

3.) Carefully insert calibrated pH electrode. 

4.) Measure initial soil pH.

5.) Add a 0.50 mL aliquot of 0.01 M NaOH, let stir until a stable pH reading is attained and record

6.) Continue the titration up to a pH of 6.5 for both replicates.

7.) Repeat the above procedure once using the soil provided in the lab labeled CECIL.

8.) Plot pH vs. base added.  Estimate the buffer capacity of your soil.