Along with nitrogen and potassium, phosphorus is an essential element for plant and animal growth and important during cell division and growth. The role of phosphorus in cell division and expansion means crop establishment and early growth is highly dependent on sufficient sources of the nutrient. Trials have shown significant agronomic penalties from applying phosphorus more than 10 days after germination. Most of these phosphorus timing trials indicate the optimum time for P-fertiliser application is before or during seeding.

A naturally deficient soil with a moderate to high PBI will tend “lock-up” phosphorus. A long term regime of applying P fertiliser decreases the capacity of the soil to do this, giving increased effectiveness of subsequent fertiliser applications.