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.