Empty milk jugs should be rinsed with tap water before recycling. Rather than pouring the rinse water down the drain, I use it as houseplant fertilizer. The small amount of milk in the water has useful amounts of three main plant nutrients: nitrogen (N), phosphorous (P), and potassium (K).
Because these nutrients are mostly present in organic forms, such as proteins, the nutrients are released slowly through the action of soil microorganisms. This slow release of nutrients provides for steady growth of houseplants.
I also use the rinse water from orange or apple juice containers. Juice is especially rich in potassium (K). The combination of nutrients from milk and juice provides a balance of N-P-K from natural sources. Since I’ve adopted this practice, I have found it unnecessary to use any other fertilizer on my houseplants.
Joseph R. Heckman, Ph. D.
Extension Specialist — Soil Fertility, Rutgers University
New Brunswick, NJ