Phosphorus desorption from calcareous soils with different initial Olsen-P levels and relation to phosphate fractions

PurposeCalcareous soils are characterized by high pH and phosphorus (P) fixation capacity. Increasing application of P fertilizer recently has significantly improved soil P concentration, especially available P (Olsen-P) and inorganic phosphate (P-i) fractions. However, there are few data available on the ability of soils with different initial Olsen-Plevels to continuously supply P (i.e., P desorption capacity) to crops without additional P fertilization and on which P-i fraction exerts the greatest influence on P desorption capacity.Materials and methodsFive soils with different initial Olsen-P levels (0.5, 14.3, 38.4, 55.4, 72.3mgkg(-1), hereafter refer as OP1, OP2, OP3, OP4, and OP5) but similar other soil properties were selected to evaluate the capacity of P desorption and its relationship with P-i fractions. Soil P was sequentially extracted once daily for 16 consecutive days using Olsen solution.Results and discussionThe content and proportions of dicalcium phosphate fraction (Ca-2-P), octacalcium phosphate fraction (Ca-8-P), aluminum phosphorus fraction (Al-P), and iron phosphorus fraction (Fe-P) in P-i increased significantly with the increase of initial Olsen-P (P<0.01). Applied P fertilizer was mostly stored as Ca-8-P in the soil. Soil P desorbed reached an equilibrium after 16 extractions for all soils, and P desorption capacity (12-358mgkg(-1)) showed a significant linear relationship with initial Olsen-P (P<0.01), with an increase of 4.2mgkg(-1) desorbed P per 1mgkg(-1) increase of initial Olsen-P. Ca-2-P exerted the conclusive effect on P desorption in the first four extractions, but Ca-8-P played a more important role in the 16 extractions.ConclusionsCa(8)-P was the greatest potential pool for P desorption after Ca-2-P was depleted. P desorption capacity was significantly linearly related to initial Olsen-P (P<0.01). Different fertilizer use strategies were developed based on P desorption capacity for soils with different initial Olsen-P levels. The present study provided basic data on how to reduce effectively the application amount of chemical P fertilizer.