Planning your Science Fair Experiment
After choosing your topic, you will have to design your science fair experiment. Research your subject on-line and in the library to find the available information.
Make a list of the variables that will influence your experimental outcome. In the case of the tomatoes, this would include soil, light, water, temperature and, of course, fertilizer. Your experiment will have two or more similar parts. All of the variables, except the one you are testing, must remain the same for each of these individual tests. If more than one variable changes, then you will not know which one caused the results you observe.
In the tomato example, all of the plants must get identical amounts of water, light, heat and soil. Insect damage must be minimized. The only difference would be in the amount of carefully measured fertilizer each plant would receive. One set of plants should get no fertilizer; these plants are called the "control group."
You must also decide ahead of time what you will measure to determine the outcome of the experiment. In this case, it could be the number and/or size (circumference, weight) of the tomatoes.
Write down the procedure for your science fair experiment, and include a schedule for completing all the steps and a list of materials you will need.