Many types of animal manure can provide plant nutrients---if you use them correctly. Dairy cow manure is very rich in nitrogen, phosphorus and potash, which are essential plant nutrients. If you had one cow, she would produce 27,000 pounds of this natural substance each year, according to the Arlington (Virginia) Organic Gardening Club. You must compost cow manure or mix it with water to make "manure tea" because fresh, uncomposted manure can burn your eggplants.
Fresh Cow Manure Can Be Too Strong
Fresh cow manure is very strong. It can burn the roots of eggplants and many other plants if you spread it around the base of your plants without composting it first. Although the N-P-K ratio of cow manure looks small compared to chemical fertilizers (it's about .25-.15-.25, according to GardenMonsters.org), it contains no fillers or other ingredients that plants cannot use. It's wise to compost cow manure with other plant materials such as fallen leaves and grass clippings for at least two months before you apply it to your eggplants.
Best to Plan Ahead
The Arlington Organic Gardening Club advises that you harvest all of your eggplants in fall and pull the spent plants. Then cover the area where you will be growing next summer's eggplants with six inches of fallen leaves or compost and about 3/4 inch of cow manure on top. Later, in winter or early spring, as soon as you can work your soil, dig the manure/compost mixture into your soil. The manure will compost right in your garden over the winter and finish its composting process in the soil where you will later plant next summer's eggplants, giving them valuable nourishment without burning the plants.
Eggplants Can Get Too Much Nitrogen
Texas A&M University reports that eggplants and other vegetables can become "tall, spindly and unproductive" if you give them too much of the plant nutrient nitrogen. This can happen if you use large amounts of cow manure to fertilize your eggplants. If you notice that your plants are leggy, and they are not producing flowers and fruit, cut back on the amount of cow manure compost you feed them. If you have dug lots of cow manure into your planting area, it could be causing this to happen. The only way to remedy the situation is to dig your eggplants out of the ground and transplant them to an area that does not contain any cow manure, or that contains only a small amount of it.
Helps Eggplants' Soil
When you add cow manure to your eggplants' growing area, it helps the plant by making the soil hold water that the plants need, and it also gives the soil a good structure, resulting in light, fluffy soil that doesn't become compacted, according to the Plantea.com. Heavy clay soils can benefit greatly when you add cow manure. Eggplants need well-drained soil that is full of nutrients, and this is what cow manure provides.