Potted plants generally grow bigger when watered with water rather than milk. Milk is more viscous than water, and the nutrients in milk do not benefit plants the same way they benefit humans. Milk can also lead to the stunted growth or death of the plant.
Water is the most essential substance that plants extract when watered with either fresh water or milk. Water transports nutrients from the soil to the plant and is an ingredient in the photosynthetic reaction that plants use to create energy. Proteins in milk make it thicker and harder to absorb than water, however, milk contains enzymes, which help stave off the growth of harmful root fungi. Milk can halt the development of leaf mildew and aid in regulating soil pH, and it is also rich in calcium, which is necessary for the growth of fruits from the tomato family and leafy green vegetables such as spinach.
Diluting the milk before usage is recommended by home-growers to gain the fungicidal and nutritional benefits of this substance without choking the roots. Using undiluted milk can often kill plants, because the high protein, fat and nutrient content encourages the growth of other bacteria and fungi detrimental to plant growth even as it halts the growth of the original microorganisms.