To fertilize means to add nutrients to soil to help plants grow stronger and produce more food. Farmers fertilize their fields by spreading substances like composted manure, ground-up fish, or specially made chemicals that contain nitrogen, phosphorus, and other elements plants need. A gardener might fertilize tomato plants to get bigger, juicier tomatoes, or fertilize a lawn to make the grass greener and thicker.

Plants are like people: they need food to grow. While they make some of their own food through photosynthesis using sunlight, they also absorb nutrients from the soil through their roots. After seasons of growing crops, soil can become depleted, like a pantry running low on supplies. Fertilizing replenishes what's been used up.

The word also has a scientific meaning related to reproduction. In biology, to fertilize means to combine a male reproductive cell with a female reproductive cell to create new life. When a bee carries pollen from one flower to another, it helps fertilize the flower so it can produce seeds and fruit. This is why farmers sometimes keep beehives near their crops.