Expired milk is not a dependable houseplant fertilizer. Avoid pouring it into pots where residues and decomposition can complicate watering, hygiene and root care.
Milk is not a practical routine feed
Expired milk adds an unmeasured organic mixture to a container. Its nutrients are not supplied in the same controlled form as a labeled plant feed. Pouring it into an indoor pot can create residue and hygiene problems.
Fermentation is not a guarantee
A fixed storage period does not establish that a homemade milk mixture is mature, balanced or appropriate for a particular root system. Do not judge it only by smell or assume that dilution solves every issue.
Dispose of it appropriately
Follow local food-waste guidance for unwanted dairy products. Many simple backyard compost systems are not suited to dairy additions, so check the system's instructions. Water plants with a suitable source and use a measured feed when needed rather than turning a disposal problem into a root treatment.
Related reading: Why Milk Is a Poor Routine Watering Choice.