Vinegar as a Herbicide?

Prior to the widespread use of chemical herbicides, cultural controls, such as altering soil pH, salinity, or fertility levels, were used to control weeds.  Sixth graders have designed an experiment where they are testing vinegar solutions to see if the vinegar will alter the pH of the soil enough to kill weeds.  One meter square plots have been developed to test and observe the plant reactions to solutions containing differing amounts of vinegar.  

Both qualitative and quantitative data will be taken.  Most students are predicting that the solution with a 9:1 ratio of vinegar will work.  What do you think?