Ideas for a high school investigative research project on plants include examining the effects of various chemicals and nutrients on plant growth. Key nutrients to consider are nitrogen, phosphorus, and potassium, with suggestions to compare different sources of nitrogen, such as ammonia versus nitrates. Instead of using potentially hazardous pesticides, exploring different plant foods with varying nutrient ratios is recommended. It's essential to establish controls for the experiment, including a positive control known to promote growth and a negative control expected to have no effect. Consistency in other variables, such as water, soil type, and light exposure, is crucial to ensure that the experiment accurately measures the impact of the chosen fertilizer or nutrient variation.