Fertilizers are natural or synthetic materials added to soil or plant tissues to supply one or more nutrients essential to plant growth. They help remedy nutritional deficiencies in the soil. Fertilizers may be classified based on their nutrient composition, e.g., those containing only one nutrient (such as potassium, phosphorus, or nitrogen) are called straight fertilizers. Inorganic or chemical fertilizers typically do not contain carbon, except for urea. Fertilizers are applied in liquid or solid form, with about 90% applied as solids. The most widely used solid chemical fertilizers are urea, diammonium phosphate, and potassium chloride.