Re: Why did the liquids kill the plants? Why did the tea do well?

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.