For best accuracy, fill several containers with growing medium and plant seeds so that all of the plants begin at the same point. Keep one or more control containers so as to have an untreated specimen for comparison. Clearly identify the test material used in each container. Bush green beans and marigolds make good test subjects, but any fast-growing plant will serve.
Treat each test container with a different commercial fertilizer, following the label directions. Compare the results over time to determine if one gives better results than another and which of them give better results than the control container.
The numbers in a fertilizer description tell the balance of nitrogen, phosphorus and potassium in the mix. Treat test containers with fertilizers of different ratios and measure results. Report on the optimum fertilizer ratio for the test plant.
Compare the effects of the amount of fertilizer on the plant by treating specimens with varying amounts of it. Leave one untreated. Treat one with the prescribed amount from the directions. Treat other containers with a fourth, a half, one and a half, two and three times the directed amount. Report on which amount performed best and which proved deleterious to the plant.
Test liquid, pellet and solid fertilizers to determine which format produces the best results. Record observations on a regular schedule to catch interim variations.
Test the theory that fertilizers can improve soil by using different growing media. Try sand, common yard dirt, commercial potting soil and peat moss, for example. Leave one set of containers untreated and treat the other with a commercial fertilizer. Prepare additional sets of containers to test more than one fertilizer. Identify the best combination of medium and fertilizer.
Apply the same fertilizer to several types of plants and compare the results to a control group of the same types. Observe the growth variations of each type to determine whether a fertilizer labeled for fruits and vegetables also benefits flowers to the same degree. Determine if a flower fertilizer helps or hinders vegetables.
Show the effects of fertilizer in different points in the plant's life cycle. Plant a group of containers. Leave one untreated. Add fertilizer to one at the start of the project. Each week, add fertilizer to another plant, marking the date on the container. Report on the best time to fertilize for optimum results.