From the Illinois Agronomy Handbook....The impact of
eliminating phosphorus or potassium fertilizer on
yield and soil-test level will depend on the initial soil
test and the number of years that applications are
omitted. In a recent Iowa study, elimination of phosphorus
application for 9 years decreased soil-test
levels from 136 to 52 pounds per acre, but yields were
not adversely affected in any year as compared to
plots where soil-test levels were maintained (Figure
11.17). In the same study, elimination of phosphorus
for the 9 years when the initial soil test was 29 resulted
in a decrease in soil-test level to 14 and a
decrease in yield to 70 percent of the yield obtained
when adequate fertility was supplied. Elimination of
phosphorus at an intermediate soil-test level had little
impact on yield but decreased the soil-test level from
67 to 26 pounds per acre over the 9 years. These as
well as similar Illinois results indicate little if any potential
for a yield decrease if phosphorus application
was eliminated for 4 years on soils that have a phosphorus
test of 60 pounds per acre or higher.

http://iah.ipm.uiuc.edu/pdf/Agronomy_HB/11chapter.pdf