Milk expires when bacteria begin to grow and multiply in large numbers in it. This, if ingested, can have toxic effects on people. I do not know if this holds true for plants, but I would first test this on a 'sacrificable' plant, because the introduction of forgein bacteria into a plants medium may rot the plants roots (and make it smell like rotting milk!).
הדמיון הוא יותר חשוב מאשר ידע