In order for the soil not to be deficient in many nutrients, it will be very useful to use fertilizer. Fertilizers are classified in several ways. They can be classified by having only one nutrient (for example, potassium, phosphorus or nitrogen), in which case they are called "direct fertilizers". Inorganic or chemical fertilizers remove carbon-containing materials, except urea. Fertilizers are applied to the soil in liquid and solid form. About 90% of these fertilizers are applied in solid form. The most used solid chemical fertilizers are urea, diammonium phosphate and potassium chloride.