Plants do best when the soil is in the neutral range, so adding liquids that are either very acidic or very alkaline is a bad idea and can lead to mineral imbalances and plant death. Plants can’t use sugars, starches, or proteins directly, and while the microorganisms in the soil (mostly outdoors) can, they can only handle small amounts. Even worse, however, is that liquids with high sugar and/or salt levels can actually pull water out of plant roots rather than allowing water to be absorbed. You also want to avoid liquids containing most cleaning products, sanitizers, or other toxic chemicals.
Leftover Or Soured Milk
Milk is neutral, but it is rich in sugars, proteins, and fats, and thus isn’t a good choice for watering plants. (Try these 7 ways to use up leftover milk instead.)
Soda, Juice, Or Sports Drinks
Most soft drinks and juices are both very acidic and very sugary—two reasons to not use them to water plants unless they are very diluted. And while they are only acidic, rather than sugary, also avoid using artificially sweetened diet drinks to water plants.
Pickle, Olive, Or Sauerkraut Juice
Vinegar is very, very acidic and the vinegar-based juice left behind in the jars of vinegar-based products is still very acidic, and sometimes quite sugary too. Juice from fermented pickles and sauerkraut is very acidic and salty. None of them are good choices for plants. Juice from naturally fermented products is loaded with beneficial bacteria however, so you may want to find ways to consume it yourself. (Here's how to make your own sauerkraut in a jar.)
Leftover Beer Or Wine
Beer and wine are both very acidic and not suitable for watering plants. If you hate to waste it, use leftovers in cooking or to make your own vinegar. (Here are 10 more things you can do with leftover beer.)
Swimming Pool Water
There's just too much chlorine and other chemicals designed to stop algae and bacteria from growing in the pool to make this liquid a good choice for plants. The exception would be if you have a “natural” pool with a biological filter system, in which case the water is great for plants.
This water has too much salt for watering anything but plants native to the dunes or salt marshes by the seashore.
Urine is very high in valuable nutrients, but you need to think of it as concentrated liquid fertilizer, not water. Caution: Straight urine kills plants and, even diluted with 5 to 10 parts of water, the salts may build up in potting soil or outdoor garden soil in climates that don’t get heavy rain at least a few times a year.
Water that has been used to flush toilets or to wash diapers or other things that have been heavily contaminated with fecal mater is called black water. It isn’t suitable for watering plants without extensive treatment; there's just too much chance of spreading bacteria that could make you sick.