The effects of long term fertilization on soil phosphorus status in manural loessial soil

The investigation of phosphorus accumulation under various management regimes is essential to crop production, and as well as to minimize the potential impact of P on environment. Changes of soil total phosphorus (TP), organic P (OP), Olsen-P and the relationships between Olson-P and TP were studied under a 12-year long term fertilization experiment in manural lossial soil. The results indicated that soil TP and Olsen-P contents of plough layers were gradually increased by the application of chemical phosphorus fertilizer, while soil OP contents were not increased. Soil OP contents were decreased on plots where no fertilization or only P and K fertilizers were applied, and soil TP and Olsen-P contents were increased rapidly on plots where 2 different rates of dairy manure were applied, which were recommended on the basis of its N content, in addition to the inorganic N, P and K fertilization. OP contents of the manure treatments were elevated to some extent as well, and the ratios of OP to TP were dropped in the beginning years and then are maintained at a stable level. These results showed that accumulation of soil phosphorus is mainly in the form of inorganic even on the treatments with organic manure application. A significant linear correlation between soil TP and Olsen-P or their increments were observed. Olsen-P increased amount was about 20.8 mg/kg when soil TP are increased by 100 mg/kg and the increased ratio of Olsen-P to TP were gradually decreased with the history of the fertilization. When soil Olsen-P reaches to a certain level, such as 16 mg/kg, which was a critical value for most of cereal crops, the decreased rate of phosphate application was strongly recommended. As for fields of applying organic manure, the application rate which based on its phosphorus contents is suggested, and this recommendation will help to effectively decrease the rapid build-up of phosphorus pool caused by organic manure application.