Fertilizer is a substance which is added to the soil to improve plant growth. The soil contains many chemicals, such as potassium, nitrogen and phosphorus, which plants need for healthy growth. As crop after crop is harvested, these chemicals are used up. Fertilizers contain these chemicals, and are put in the soil to replace them. Most fertilizers are manufactured, though other natural substances, such as manure and bone meal, are fertilizers. Fertilizers allow farmers to grow far more food, and millions of tons of fertilizers are used each year around the world.