After studying about recycling, members of Miguel’s

biology class investigated the effect of various

recycled products on plant growth. Miguel’s lab

group compared the effect of different aged grass

compost on bean plants. Because decomposition is

necessary for release of nutrients, the group

hypothesized that older grass compost would produce

taller bean plants. Four flats containing the same

species of bean plants (25 plants/flat) were grown for

5 days. The plants were then fertilized with compost

as follows: (a) Flat A: 450 grams of two-month-old

compost; (b) Flat B: 450 grams of four-month-old

compost; (c) Flat C: 450 grams of six-month-old

compost; (d) Flat D: 0 grams of compost. The flats

of beans were planted in the same type of soil. The

plants received the same amount of sunlight and

water each day. At the end of 30 days, the group

recorded the height of the plants in centimeters. What is the flaw in this experiment?