When planted in the ground, the roots of a plant will take up water and in the water you will also find dissolved molecules of fertilizer. These molecules are used as building blocks to create plant tissue, flowers, fruits, and even foodstuffs for the plants themselves. A potato for example is a storage of plant energy (carbohydrates) which the plant has created from air, water, and fertilizer (molecules) using the sun as an energy source. The fertilizer can end up in the ground in one of two ways:
1) Either animals have left their waste (manure) which rots, or plant materials have died (cut grass, fallen leaves, etc). As these plant materials, or the manures, decompose fertilizer molecules are released. For example, manure, orange peel, corn trash, dead flowers all contain fertilizer molecules which can become available to other plants (recycled) by the microbes in the soil. 2) A garden may manually add synthetic fertilizer (e. g. man-made fertilizer) to the plants. When the fertilizer comes in contact with water, the materials dissolve and the fertilizer molecules are released.
This process does not require microbes to break down the waste material like in the example above, so it often acts faster. So what is the advantages and disadvantages of fertilizers ? Most plants, especially modern varieties, will pull more fertilizer molecules out of the ground than what is naturally re supplied. In a forest, the decomposing leaves and bark will feed the soil and give ferns (growing below the trees) enough fertilizer to live on. However, in a garden where you grow vegetables or flowers, these plants typically consume large quantities of fertilizer..
higher amounts than nature can naturally re supply. in order for the plants to grow properly, they need the right amount of fertilizer available in the soil. If there is too little, growth may be stunted and flowers and fruits may not develop properly. The plants become susceptible to attack by insects and disease. A plant which is fed well is often able to survive better. This is the same with humans: people who are starving is often plagued by disease compared to people who get all the nutrients they need.
Therefore the gardener must manually fertilize the plants to ensure they have enough food to grow on. Plants which are fertilized often are bigger, stronger and produce more fruits and flowers than plants which are not fertilized. Fertilized plants are generally often also more disease resistant than unfed plants. There really is no disadvantages to fertilizing, and most home owners should fertilize plants such as vegetables, fruits and flowers because these presumably modern hybrids tend to consume much more fertilizer than nature can provide..
This also goes for lawns, for example, which has individual grass plants growing very close together much closer than grass plants would in nature and therefore have to compete with each other for nutrients. However, if you give TOO MUCH fertilizer the plants can suffer just as they would if they get too little. For example, both magnesium and calcium molecules are needed by the plants, but given too much calcium, for example, can interfere with the plants ability to take up magnesium and the other way around, too.
If you add fertilizer to certain plants when the soil is dry or the temperature is very hot, you also risk damaging the plants. While there are no specific disadvantages to fertilizing if doing it correctly, you do need to know approximately which type of fertilizer (which nutrients) the plant require, how much to add, and when to add it. It also needs to be watered in to avoid that the plant dies from thirst (plants which grow in fertilizer rich soil without water will cause water to exit the plants and kill them) There could again say why use fertilizer when you already have dung.