Phosphorus Availability and Transformation as Affected by Repeated Phosphorus Additions in an Ultisol

Phosphorus (P) solubility and transformation in soils determine its availability to plants and loss potential to the environment, and soil P dynamics is impacted by fertilization and soil properties. A Ultisol sample was interacted with 20 mg L^?1 P solution from one to ten times. The P-reacted soils were then analyzed for water-soluble P (0.01 M calcium chloride (CaCl₂)–extractable P); plant-available P (Olsen P); ammonium chloride P, aluminum P, iron P (NH₄Cl-P, Al-P, Fe-P, respectively); and occluded P (Oc-P). The degree of P saturation (DPS) was calculated from ammonium oxalate–extractable Al, Fe, and P. The amount of P sorbed by the soil was highly correlated with the frequency of P addition with high percentage of P adsorbed initially and gradually decreased as the P addition continued. The relative abundance of the five P fractions in the P-reacted soil was in the order of Fe-P (36.5 percent) > Al-P (35.6 percent) > Oc-P (22.8 percent) > Ca-P (2.7 percent) > NH₄Cl-P (2.3 percent). Both Olsen P and CaCl₂-P were significantly increased by the repeated P addition process and highly correlated in an exponential function. The DPS was increased above the so-called critical point of 25 percent after the first P saturation process and kept increasing as the P addition continued. The P availability and adsorption in the soil were controlled by soil free and amorphous Al and Fe. The results suggest that repeated P application will build soil P to an excessive level, and consequently result in poor P-use efficiency and high P-loss potential to surface and groundwater.