The study looked at how different fertilization methods affect the levels of phosphorus in red paddy soil over a long period. They tested five fertilization treatments and found that using fertilizers with phosphate increased both total and available phosphorus in the soil. Without phosphate, the soil's total phosphorus decreased slowly over time. The study also showed that higher levels of available phosphorus led to higher rice yields, but the increase in yield slowed down when the available phosphorus was above 20 mg/kg.