Milk is loaded with vitamins and minerals. All plants, including pumpkin plants, need a wide variety of minerals and micronutrients to grow both the plant and the fruit. Among those minerals, milk is rich in calcium. Calcium is essential in helping all plants to take up other needed nutrients.
Importantly, calcium and other minerals need to be in soluble, or liquid form, so plants can take them up through their roots. Calcium and other minerals in milk are in a soluble liquid form. As a result, it is readily used by the plants.
So, some amount of milk, as a fertilizer, can indeed do your plants good. However, like any fertilizer, too much of a good thing is harmful to your plants. If you feed too much milk to your plants, the soil becomes a rotting, stinking mess, with no additional benefit to the plant.
Liquid fertilizers (like Miracle-Grow, fish fertilizer, seaweed fertilizer, and compost or manure teas) contain calcium and a wide range of micronutrients. They are great for plant growth and health. If you are not using them, we recommend you do so.
Applying milk to feed essential nutrients to your pumpkin plants is definitely not cost-effective. Rather, it’s a very expensive way to feed your plants. On a cost per gallon basis, any of the liquid fertilizers are far less expensive. The only time this author uses milk on plants, is when it is past its expiration date for human consumption. And, I dilute it in water to feed plants indoors and out.
Not all milk is equal. For example, whole milk has more ingredients than skim.