Phosphorus fractions in an acid soil continuously fertilized with mineral and organic fertilizers

The effect of different treatments on the fate of applied P was investigated in a long-term field experiment started in 1972 - 1973 following a maize - wheat sequence. The soil samples were collected after 29 years of continuous addition of mineral fertilizers and amendments such as farmyard manure (FYM) and lime. The total P content of all the treatments increased compared to the original soil; NaOH-inorganic P (P-i) (NaOH-P-i) representing Fe and Al-bound P was the dominant P-i fraction. At the beginning of the experiment ( 1972 - 1973), the various P pools could be quantitatively ranked in the following order: residual P> NaOH- organic P (P-o)> NaOH- P-i> NaHCO3- P-o> NaHCO3-P-i> HCl- P> H2O-P. As a result of continued P fertilization and cropping, the order changed as follows: residual P> NaOH-P-i> NaOH- P-o> NaHCO3- P-i> NaHCO3- P-o> HCl- P> H2O-P. Compared to the imbalanced mineral fertilizer application, the balanced as well as integrated application of nutrients resulted in significantly lower P adsorption capacity of soils. The Olsen extractable-P fraction (plant-available P) increased from about 12 mg kg(-1) soil in 1972 to about 81 mg kg(-1) soil in the treatments receiving P for the last 29 years.