Milk is loaded with vitamins and minerals. Plants, including pumpkin plants, need a wide variety of minerals (micro-nutrients) to grow both plant and fruit. Among those minerals, milk is rich in calcium. Calcium is essential in helping all plants to take up other needed nutrients.
Calcium and other minerals need to be in soluble(liquid form), for plants to take them up in its roots. Calcium and other minerals in milk is in a soluble/liquid form, and is readily used by the plants.
So, some amount of milk, as a fertilizer, can actually do your plants good. However, like any fertilizer, too much of a good thing is harmful to your plants. Feed too much milk to your plants, and you have 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 micro-nutrients. They are great for plant growth and health. If you are not using them, you should be .
Applying milk to feed essential nutrients to you pumpkin plants, is definitely not cost effective. It is by far a very expensive way to feed your plants. On a cost per gallon, 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 consumption. I dilute it in water, then feed plants indoors and out.
Not all milk is equal. Ie. Whole milk has more ingredients than skim.
Like other liquid fertilizers, you can feed milk directly to your plants’ root system. Dilute it with water (10:1).
Like other liquid fertilizers, you can use milk for foliar feeding. Dilute it with water (10:1).
Tip: This author does not know a single grower of giant pumpkins, who feeds milk to their plants. Rather, they use liquid, Chelated Calcium on a regular basis.