Nitrogen, phosphorus, and potassium also referred to as N, P, and K are primary nutrients plants need for growth. Grasses require much more nitrogen and potassium than they do phosphorus.
Very little phosphorus is removed by cattle grazing. While more phosphorus is removed with a hay crop, the amount is still small compared to nitrogen or potassium.
From both an economic standpoint and an environmental stewardship perspective phosphorus should not be applied to pastures or hayfields without a soil test. A soil test from a few years make is generally sufficient to establish if any phosphorus should be applied.
