Plants need water to create carbohydrates via photosynthesis to grow. Furthermore, about 80 to 95 percent of growing tissues in plants are made up of water. While milk may provide some nutrients to plants, watering them with it exclusively results in wilting, fungal growth and rot.
Plants require a huge amount of water for growth and to prevent wilting due to transpiration, which is why plant nurseries manage water for optimum growth. Milk also contain bacteria and fat that will spoil, which in turn cause the plant to wilt and suffer poor growth and as well as making the soil smell bad.