Plants use water and light to create sugar, which they use to nourish themselves. It is common knowledge that sodas, like Coke and Pepsi, are liquids that are full of sugar, but the effect they have on plants is actually far more harmful than helpful.
Negative Effects of Sugared Sodas
Standard Coke or Pepsi will have adverse effects on plant growth and will eventually kill the plant. Though these drinks are packed with sugar, the plant does not need any more sugar; it makes all of the sugar it needs through photosynthesis. The extra sugar from the sodas will just get packed down in the soil, where it will inhibit the plant's ability to absorb water through its roots.
Effects of Diet Sodas
Diet Coke and Diet Pepsi do not use sugar as a sweetener, instead relying on a chemical known as aspartame. Because there are no sugars, there will be no adverse effects on the plant. Pouring diet sodas into your plant's pot will not cause any major problems for the plant, with the exception of possibly drowning it.
Additional Negative Effects of the Sugar
Pouring soda into your plant's soil not only has the adverse effects like constricting water intake and excessive sugar build-up, but it also attracts bugs. Bugs will be attracted to the scent of the sugar; and before you know it, they'll be all over the plants, most likely eating it. Soda on plants also promotes bacterial and fungal growth, which can also eventually kill your plants.