Re: Pumpkin plants are growing better in Diet Coke than water , Why?

I assume you used the same potting soil in each pot and the same size pots. 
Did you also have the same number of plants per pot?

There are several possible explanations. 

1. It might simply be that the Diet Coke plants receive more light than the 
plain water plants. It is not always easy to assure indoor plants get the same 
amount of light, even if they are on the same windowsill.

2. The water used to make the Diet Coke might have more mineral nutrients, 
such as potassium, magnesium, calcium and sulfate, than your plain water 
source. If you used city water, you should be able to get a copy of your tap 
water analysis from the water company.

3. It might be that your tap water source is just not a good quality 
irrigation water for plants. Irrigation water quality varies widely.

4. Diet Coke contains potassium benzoate and phosphoric acid. They provide 
potassium and phosphate, two essential mineral nutrients for plants. The USDA National Nutrient 
Database indicates diet cola with aspartame contains 110 mg/liter of 
phosphorus, which is more than adequate for optimum plant growth. It also 
contains 60 mg/liter of potassium, which is a substantial amount and more than 
most tap waters.

5. Diet Coke might also lower the soil pH (make soil more acid) because it 
contains phosphoric acid and citric acid. Low soil pH makes certain mineral 
nutrients, such as iron, manganese, zinc and copper, more available to the 
plant. 

Ammonia contains ammonium ions which provide a source of nitrogen for the 
plants. However, too much ammonium can be toxic to plants, which might explain 
why the leaves are smaller. One effect of lots of ammonium fertilizer is that 
the soil pH drops.

References 


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


Re: Why will plants grow in Diet Coke and water and not other sodas?


Re: Are there plants that can be grown in both acidic AND basic soil?

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.