This is the second most important plant nutrient after nitrogen and important in its take up. Phosphorus is an important element in many of the complex compounds in plants which they need. For example it is one of only five different elements in DNA, the others being oxygen, hydrogen, nitrogen and carbon. Even with an adequate supply of nitrogen, plants cannot take up enough nitrogen if there is insufficient phosphorus. A lot of the phosphorus in soils is in an insoluble form and plants can only take up nutrients if they are dissolved in water, in addition there is a tendency for phosphorus which is added to the soil to be converted to an insoluble form in the soil. Phosphorus is more likely to be in short supply in acidic soils.