Decline of available P (Olsen-P) in red earth with time may be devided into two stages: fast and slow stages. The former stage can be described with a straight line equation, y=a+bx, the second stage with a curve equation, y=a+blogx,where y=available P still remained, and x=incubation periods (month). Available P decline has a significant correlation with the amount of active Al content in soils but not with the amounts of active Fe and exchangeable Al. Pot experiment with three different types of soils: acidic red earth, neutral paddy soil, and calcareous fluvo-aquic soil, using wheat and rye grass as test plant, showed that the yield decline with soil and P reaction periods and ceased at different time with different soils. For the neutral paddy soil, the decline in yield ceased after 8 months' incubation, for fluva-aquic soil, after 16 months, but for red earth the decline ceased after more than 28 months. This may be due to different in extent of the formation reactions of P chelate ring structure and diffusing into the deeper layer under the surface of R₂O₃ and calcite in the soils. In neutral paddy soil this two reactions were weaker and led to higher available P remained after incubation period, while in acidic red soil and calcareous fulvo-acquit soil they were much more stronger especially in red soil, and lasting longer time.