Fertilizers are substances given to plants with the intention of promoting plant growth; they are usually applied either via the soil or by foliar spraying.
Fertilizers typically provide, in varying proportions, the three major plant nutrients (nitrogen, potassium and phosphorus), the secondary plant nutrients (calcium, sulfur, magnesium), and sometimes trace elements (or micronutrients) with a role in plant nutrition: boron, manganese, iron, zinc, copper and molybdenum.
Manure was once the dominant fertilizer, and is still used, but it's role is greatly diminished. Fertilizer can be created either from natural organic material such as manure or compost (see also organic gardening), or artificially as through the Haber-Bosch process which produces ammonia. The Haber-Bosch process uses about one percent of the Earth's total energy supply in order to provide half of the nitrogen needed in agriculture. Without this synthetic fertilizer, enough food could not be grown to support the current level of humanity on the planet. Organic material has the advantage of adding carbon compounds to the soil. A major source of soil fertility is the decomposing crop residue from prior years, though this is not considered "fertilizer."
Over-use of fertilizer can lead to algal blooms in lakes and streams that receive run-off from crop lands, and lead to long-term degradation of the soil; see in this regard eutrophication and nutrients. For these reasons, it is recommended that knowledge of the nutrient requirements of the soil vis-a-vis the crop precede applications of commercial fertilizer. In short, excess nutrient elements can cause local soil and off-site damage, as well as waste money.