Ecycling of nitrogen from dead to living plants is often the only source of nitrogen in the soil.
Fertilizer is the kind of material to help the plant grow.The essential plant nutrients are nitrogen, phosphorus and potassium. These elements are required for plants to complete their life cycle. Secondary nutrients are calcium, sulfur and magnesium. These nutrients are normally present in the soil but need to be replenished periodically. Trace elements might be added, such as boron, copper and cobalt. Compound fertilizers are used around the world with several variations.
To make plants grow faster, what you need to do is supply the elements that the plants need in readily available forms. That is the goal of fertilizer. Most fertilizers supply just nitrogen, phosphorus and potassium because the other chemicals are needed in much lower quantities and are generally available in most soils. Nitrogen, phosphorus and potassium availability is the big limit to growth.
The numbers on a bag of fertilizer tell you the percentages of available nitrogen, phosphorus and potassium found in the bag. So 12-8-10 fertilizer has 12-percent nitrogen, 8-percent phosphorous and 10-percent potassium. In a 100-pound bag, therefore, 12 pounds is nitrogen, 8 pounds is phosphorous and 10 pounds is potassium. The other 70 pounds is known as ballast and has no value to the plants.
So why don't people need fertilizer to grow? Because we get everything we need from the plants we eat or from the meat of animals that ate plants. Plants are factories that do all of the work to process the basic elements of life and make them available to us.