|MadSci Network: Botany|
Full Question: For my science fair project, I watered 1 inch tall plants with orange, apple, and grape juice, milk and tea. I used 1/4 c of liquid diluted with 3/4 c. water. I also watered with plain water and Miracle Gro. The plants watered with juice died immediately and developed a white mold on the soil. The ones watered with milk died in five weeks. Tea did the best. What in tea would make them grow well? Could the antioxidants in tea get into vegetable plants as they grow and make them healthier? Response: Fruit juices contain sugars that make water less available to the plant. Milk contains sodium, milk sugar, and proteins which also make water less available to the plant. Basically, anything dissolved in water will make it more difficult for the plant to absorb water. So the plant may be dying of thirst when you give it fruit juices, soda, or milk. Sugar-free sodas would probably not be nearly as harmful as regular sodas because they contain low concentrations of artificial sweeteners instead of high concentrations of sugar. If you look at the labels, you can calculate how much sugar or sodium you have per liter. My skim milk bottle label says 125 mg sodium per 240 ml serving which is 520 mg/liter sodium. From a plant perspective, that is a harmful concentration of sodium. Hershey (1993) describes how irrigation water is rated based on its electrical conductivity (EC). An irrigation water with an EC above 1.5 dS/m is considered poor quality. Just the 520 mg/liter sodium in skim milk would translate to an EC of 2 dS/m. Hershey and Sand (1993) found that 1% milk had an EC of 5.2 dS/m. Even diluting by a factor of 4, as you did, leaves an EC of 1.3 dS/m which rates only fair as an irrigation water. Excellent irrigation water has an EC below 0.25 dS/m. Another factor with fruit juices, milk, and soda is that the sugars promote an unnaturally rapid growth of soil microbes as indicated by your white mold. The soil microbes do at least three things that can hurt plants. Microbes produce waste products that may harm plant roots, they absorb mineral nutrients from the soil that plant roots require, and they use soil oxygen, which plant roots require. Tea is made from dried leaves and boiling water. Tea leaves do not contain large amounts of sodium or sugars thus tea is not as bad for plants as milk, fruit juices or sodas because it is not as concentrated. Some of what dissolves from tea leaves consists of mineral nutrients that plants require. Tea may thus act as a fertilizer so may be better than plain water for plant growth. It seems unlikely the antioxidants in tea would benefit plants. Plants make all their own organic compounds, including antioxidants. Plants do not "eat" in the way animals do. All plants need to absorb from the soil are inorganic mineral nutrients and water. How much better did the plants watered with tea do compared to the control plants watered with Miracle-Gro or tap water? How did you measure plant growth? Height is not the best measure of plant growth (Hershey, 1995). References Hershey, D.R. 1995. Plant Biology Science Projects. New York: Wiley. Hershey, D.R. 1993. Evaluation of irrigation water quality. American Biology Teacher 55:228-232. Hershey, D.R. and Sand, S. 1993. Electrical conductivity. Science Activities. 30 (1):32-35.
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