Experiment Design Analysis - Compost

John's biology class investigated the effect of recycled products on plant growth. John's lab group compared the effect of different aged grass compost on bean plants. Because decomposition is necessary for the release of nutrients, the group hypothesized that older grass compost would produce taller bean plants. Three flats of bean plants, with 25 plants per flat, were grown for 5 days. At the end of 30 days, the group recorded the height of the plants (cm). The flats were then given these treatments:

  1. Make a sketch of the three flats indicating the amounts and age of the compost:
     
     
     
     
  2. Question/Problem:
     
     
     
  3. Hypothesis with a prediction:
     
     
     
  4. Manipulated variable:
     
     
  5. Levels of treatments:
     
     
  6. Responding variable:
     
     
  7. Controlled variable:
     
     
  8. How would you improve this experiment?