Is it a fertilizer?
Milk contains calcium (Ca). Calcium is an important plant macronutrient. Macronutrients are foods that help plants grow and function. Calcium helps build plant cell walls which in turn allow the transport of other plant nutrients. Also calcium level in the soil controls the soil pH (pH is a chemical balance that allows plants to operate, or not). If calcium is washed out of the soil, the soil will become more acid and can affect plant growth. (Agricultural lime such as dolomite lime contains calcium.)
If you feed plants milk–whole milk or powdered milk–you are feeding plants calcium.
So milk can be a tomato plant fertilizer: Sprinkle a quarter to a half cup of powdered milk on top of the soil after planting, and repeat every two weeks throughout the growing season.
What else does milk do for plants?
Milk is a fungicide.
Old-time garden wisdom AND recent scientific plant research say that milk contains fungicidal properties. If you spray milk on plants, it will control the growth of bad fungi.
Fungi are microscopic organisms. A handful of soil contains thousands and thousands and thousands of fungi. There are good fungi and bad fungi. Good fungi help build the soil by breaking down organic matter into nutrients plants can use. Bad fungi are parasites that feed on plants. Bad fungi include mildews (downy mildew, powdery mildew), rusts, rots (root rot, damping off, fruit rot), canker, scab, spot (black spot and anthracnose), wilts (fusarium and verticillium) and smuts (and black sooty molds caused by black sooty fungi spores).
Fungi are spread by spores. They germinate and root much like plants. When a fungi takes root as a parasite on plant tissue it feeds and begins to grow. A fungus can be prevented from rooting and can be removed (prune off the infected plant tissue and throw it away in a paper bag so that the fungi spores do not spread). Fungi allowed to grow will spread via spores floating on the wind or swept along in a drop of water.
Fungicides rarely kill fungi. They are most useful as a preventive, not as a cure. Fungicides cover plant tissue and do not allow fungi to root.
Plant researches in Brazil and Australia have recently used milk as a fungicide on vegetable crops, grapes, and flower crops. They found that spraying a dilute mix of 1 part milk and 9 parts water prevented fungi from growing. (The researchers also believe that the potassium phosphate in milk helps boost the plant's immune system and may also work as an antibiotic.)
Once again, milk–like other fungicides–does not cure fungal diseases but helps to prevent them. Milk keeps fungi from growing and spreading.
A note: skim-fat milk works best; the fat in whole milk may clog up your sprayer. As well, reconstituted powdered milk will work.
Tomato disease fighter formula: combine 1 part skim milk and 9 parts water. Spray the plant every two to three weeks until mid-summer (most fungal diseases have run their course by mid-summer, except where the weather stays warm and humid).