Interference of Soil‐Extractable Phosphorus in Measuring Soil Microbial Biomass Phosphorus

Soil microbial biomass contains a labile phosphorus (P) fraction and is an important source of phosphorus for plant nutrition. It is therefore essential to estimate soil microbial biomass P accurately. Two calcareous alluvium soils with low (soil 1, 8.9 mg kg−1) and high (soil 2, 460 mg kg−1) Olsen P were collected. Soil 1 was amended with a series of dipotassium phosphate (K2HPO4) solutions (0, 37.5, 75, 150, 375, 750 mg P kg−1 soil). The objective was to investigate the interference and removal methods of soil‐extractable P when measuring soil microbial biomass P. The results showed that the soil microbial biomass could not be determined in those soils where Olsen P was greater than 60 mg kg−1. This part of the soil P should be removed by either anion resin or 0.5 M sodium bicarbonate (NaHCO3) solution (pH 8.5) prior to fumigation. The recovery rates of the added phosphate in the two soils tested were greater than 150%. Calibration was not necessary when calculating soil microbial biomass P. The microbial biomass P values of the two soils were 12.42±1.02 and 15.00±1.06 mg kg−1, respectively.