Plants mostly take up nitrogen as nitrate. Animals can only absorb organic nitrogen, mostly as amino acids and protein. Phosphorus is a limiting nutrient in many situations. The only important form for plants is PO 4 3- . This mostly comes from weathering of rocks.
Plants convert phosphate into organic forms.
Animals and decomposers convert organic forms of phosphorus into inorganic forms. Phosphate gets bound to soil particles and humus. Phosphorus may cycle for many years without being lost. Without disturbance, these losses are usually small. Storms and fires, in particular, cause loss of particulates.