磷在土壤中有效性的衰减

培育试验表明，磷与红壤中有效性的衰减可分为快反应（3小时）阶段和慢反应阶段，前者有效磷以直线关系下降，后者则为一渐减性曲线。快反应阶段磷有效性的衰减量和土壤活性铝含量显著相关，但与活性铁和交换铝相关不显著。用红壤、潮土、板浆白土三种性质不同的土壤进行的生物试验表明，产量也随着土－磷作用时间延长而下降，而且有一个终止时间，中性水稻土在8个月培育后产量即可不再下降，石灰性潮土为16个月，而酸性红壤在28个月以上，影响产量衰减的可能和磷在土壤中的双核化（转化为环状双核结构）和吸收作用有关，它们在程度上和延续时间上不同土壤不同，在水稻土上最弱，延长时间最短，而在红壤上作用最强，延长时间最长，潮土处于两者之间。

DECLINE OF PHOSPHORUS AVAILABILITY WITH TIME IN SOILS

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.