Plants need liquid to absorb nutrients from the soils. However, gardeners and plant enthusiasts may wish to experiment and see just what kind of liquid works best. There are a number of common options, such as milk, juice and water, and though each one has its merits, pure water is still the most reliable liquid for optimal plant growth.
Milk is full of protein and vitamins, so it may seem like a good choice for watering plants. However, it has several drawbacks. It is more viscous than water and is more difficult for the plants' roots to absorb. It also will clump in the soil and promote bacterial and fungal growth, both of which are harmful to plants.
Saltwater and Mineral Water
Saltwater and mineral water contain minerals. Saltwater contains salt while mineral water contains extras such as calcium and magnesium. Salt and the other minerals are not used by the plant and will build up in the soil over time. This may lead to root blockages, meaning the plant will be unable to efficiently absorb nutrients. Be careful with tap water; it often contains excess minerals such as fluoride, which could also be harmful to plants.
Juices and Colas
Juice and colas are typically very acidic. The acid could corrode the waxy coating of the plant, making it sick and vulnerable to diseases and bacteria. They both also contain sugars, which will build up in the soil and attract insects and potentially harmful bacteria.
Pure and Distilled Water
Pure and distilled water are the best choices for watering plants. They both contain few, if any, extra elements that the plant doesn't need, as well as a neutral pH (acidity/alkalinity) that many plants desire. However, do not use "softened" water, which is mineral water that has undergone a process to remove the excess minerals. "Soft water" has had its calcium and magnesium replaced with sodium, which can be just as harmful to plants.