It would be possible to grow plants on Mars only in technologically advanced, controlled environments that could keep the plants warm and give the plants enough atmosphere, light, and water to live. Scientists anticipate that humans would be able to use the Martian soil as a substrate for plant growth. Based upon results from the Phoenix Mars Lander mission, the soil pH and salt content are not detrimental to plants. Human visitors would likely have to add a little fertilizer, but scientists still do not know enough about the chemistry of the Martian soil to determine what additional fertilizers plants may need. Studies of Martian soil characteristics will help future scientists develop ways to grow plants for human communities on Mars. You can read more about these results from the Phoenix Mars mission at http://www.jpl.nasa.gov/news/phoenix/release.php?ArticleID=1766