Re: will salt affect the growth of plants

Yes, salt (table salt or sodium chloride) will harm plants if too much is 
applied. Salt makes water less available to the plant by lowering the water 
potential. Sodium and chloride can each be toxic to the plant as well. Sodium 
chloride or other ice melting salts cause a lot of damage to landscape plants. 
One symptom of salt injury is the edges of leaves dry out and die. 

Your experiment sounds reasonable. You might even want more than two pots so 
you could use several rates of salt. If you have just one rate, you might kill 
the plant right away and not have as interesting an experiment. As your 
control, you probably want to irrigate your plants with a fertilizer solution. 
Then the treatment could be the same fertilizer solution with 1, 2, or more 
teaspoons of table salt added.

References

Hershey, D.R. 1993. Evaluation of irrigation water quality. American Biology 
Teacher 55: 228-232. (your school's biology teacher may subscribe to American 
Biology Teacher)

Hershey, D.R. 1995. Plant Biology Science Projects. New York: Wiley.