To assess the long-term impat of a rnage of P fertiliser rates on crop yield and economic returns.

After 8 years of trial work on a Mallee clay loam with subsoil limitations:

• In only 2 years did yields increase after applying P fertiliser,
• the site has remained responsive at nil and 5P treatments, but maintaining rates >5kg P/ha did not produce any extra yield,
• in 2010 there as a 25% increase in wheat yield between nil P and where P had been applied (at rates 5kg P/ha or higher),
• the 5kg P/ha treatment provided the highest net income, and
• soil Colwell P together with PBI accurately estimated the response seen in 2010.