Phosphorus is an important macronutrient required by the plants. It is taken up by the plants in the form of orthophosphate ion. The pH of the soil determines the amount of phosphorus absorbed by the plants. Since the mobility of phosphorus in the soil is very low, it is taken up by the plant roots only from its immediate surroundings. The phosphorus concentration is very low in the soil solution as compared to the roots. Therefore, it is taken up by the plants against the concentration gradient.