In general, water is the best fluid with which to water plants. Milk contains a higher concentration of sodium, which would inhibit plants from efficiently taking up nutrients from the soil. Coca-Cola or any other sugary, carbonated beverage contain large quantities of sodium. Additionally, sugared beverages attract insects and encourage rotting.
About the Author
Michelle Z. Donahue has worked as a journalist in the Washington, D.C., region since 2001. After several years as a government and economic reporter, she now specializes in gardening and science topics. Donahue holds a bachelor's degree in English from Vanderbilt University.