Fertilizer is stuff you put on soil, or around plants, to help them grow. Farmers bag it, gardeners sprinkle it, and house-plant people mix it into pots. Plants pull food out of the soil; after a while the soil has less of that food left, and fertilizer puts some back. There are two broad kinds people talk about. Chemical fertilizer comes from a factory, often as granules or powder in a sack. Natural fertilizer is older and messier — animal waste, compost, crushed bone — and some people prefer it because it feels less harsh on the land. The word itself stays fairly literal: it is the substance, not a metaphor. You will also see the British spelling fertiliser, with an s. One more thing students notice: the ending -er often means a person (a teacher, a farmer). Here it does not. Fertilizer is the thing you use, not a person who fertilizes.