Composting and psychology projects are good science projects for fifth graders. A fifth grade science project is the culmination of the elementary school curriculum. Fifth graders have the knowledge and skills to do longer and more complicated projects, as well as more sophisticated analysis.
One example of a psychology project determines whether the color of food affects a person's perception of its taste. Use three containers of apple juice. Put green food coloring in one container, red food coloring in another and leave the third container uncolored. Ask your test subjects to drink 1 cup each of the red, green and uncolored apple juice, and then ask them to guess the flavor of the juice in each cup. Record your findings. You may find that some people realize the cups are all filled with the same juice, but some may think the flavors are different because the colors are different.
Composting is a good long-term project in ecology. Fill a bin with layers of soil, sawdust, leaves, grass clippings and newspapers. Wet each layer before applying the next one. Let this compost sit for six weeks. Then, plant lettuce seeds in two pots, one filled with soil mixed with compost and one filled with soil only. Record which plant grows more quickly, and learn how the growing medium affects the growth rate of plants.