CHANGES OF PHOSPHORUS FRACTIONS UNDER CONTINUOUS CORN PRODUCTION IN ATEMPERATE CLAY SOIL

Limited efficiency of fertilizer P may be improved through an understa nding of soil P fraction changes with time. This study examined sequen tial changes in soil organic P (Po) and inorganic P (Pi) ina Ste. Rosa lie clay (Humic Gleysol; fine, mixed, frigid. Typic Humaquept) under c ontinuous corn with and without P fertilization. Soil P was fractionat ed into Bicarb-Pi and Po, NaOH-1-Pi and Po, HCl-Pi, NaOH-Pi and Po, an d Residue-P. In the non-P fertilized plots, soil total extractable Po declined by 14% of the initial value over five years of corn productio n, whereas soil Pi fractions were unchanged. The losses of soil Po wer e mainly from NaOH-1-Po. Added, fertilizer P increased NaHCO3-Pi and N aOH-1-Pi in plots receiving 44 and 132 kg P ha(-1) yr(-1) and increase d Residue-P in plots receiving 132 kg P ha(-1) yr(-1). Although NaOH-1 -Po decreased slightly in the plots receiving 44 kg ha(-1) yr(-1) P fe rtilizer total soil extractable Po was maintained in P fertilized plot s. Mineralization of from 16 to 29 kg P ha(-1) yr(-1) Po was needed to account for soil Po losses. Bicarb-Pi and NaOH-1-Pi appeared to be mo st important for assessment of soil P fertility changes in long-term f ertilized soils.