What could possibly be the shortest experiment to find out the best pH range for Plant Growth ?

Make up solutions at several different pH levels - I would
suggest 3,5,7,9,and 11. For each of the different pH solutions, fold 10 seeds of the
same type (you can buy them as seeds, or used dried peas, beans, or whole mustard seed
from the grocery store) into a paper towel, dampen the towel with the solution, and
place it into a zipper-type plastic bag. Label the bag with the pH, and repeat for each
of the solutions. Each day record germination of the seeds; when most have sprouted you
can measure the length of the first root to see
growth.

Solutions can be mixed form hydrochloric acid and
sodium hydroxide if you have access. If you are doing this at home, you can try using
vingar for the acid and baking soda for the base. Use distilled or purified
water.

You could extend this by actually growing the plants
and watering them with the different pH mixtures. I'd suggest the mustard seeds, because
they grow really fast. It would be important to treat every plant exactly the same way -
same container size, same sun exposure, same amount of water - and vary ONLY the pH you
are watering with. Also it would be best to not use soil, which can buffer, or change,
the pH of the liquid you are adding. You could use plain sand, or water crystals (see
link below) to support the seedlings. After they have grown for a while, you can measure
them, and you could also carefull pull them up, dry them, and weigh them to get data on
dry weight, which is a good indicator of plant growth.

The
links below tell you more about why pH affects plant growth, and the lab has nice blank
data tables that will help you see how to write this experiment up if you need to do
so.