Inorganic soluble phosphorus (orthophosphate) is taken up by living things and incorporated into their cells and tissues (reaction 2: immobilisation).  When they die, the phosphorus is converted back to orthophosphate (reaction 2: mineralisation).  Orthophosphate is also removed from water by adsorption to soil particles and by precipitation as calcium, iron and aluminium salts (reaction 3: fixation).  Under anaerobic conditions, fixed mineral phosphorus is converted to soluble orthophosphate (reaction 4: reduction release).