Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Michelle is interested in improving the quality of the soil in her yard. She wants more water and air to penetrate the soil. What should she add to the soil to help her achieve this goal?
earthworms
Compost or sand.