Nutrients are inorganic and organic compounds from which plants obtain the elements they need to build their structures. The elements themselves are often referred to as nutrients. Depending on the location of the plant (terrestrial or aquatic), nutrients are taken from the air, water or soil. These are usually simple inorganic compounds (e.g. water (H2O) or carbon dioxide (CO2)) and ions (e.g. nitrate (NO3), phosphate (PO43-) or potassium (K+)). The availability of a nutrient depends on its chemical behavior and local conditions. Since the nutrient elements are required in a certain quantitative ratio, insufficient availability of an element often limits the growth of plants. On the other hand, increased growth can be achieved by supplying nutrients (fertilization).