Phosphorus leaching in a calcareous soil treated with plant residues and inorganic fertilizer

Column experiments were conducted over 45 d to determine the degree of P mobility. The sandy loam soil was spiked with 200 mg P kg^–1 and 5% organic residues. The treatments included: control without any water‐soluble P and plant residues, potato, wheat, water‐soluble P fertilizer, wheat + water‐soluble P, and potato + water‐soluble P. Each column was leached with distilled water, and leachates were collected and analyzed for P, K⁺, Ca²⁺, Mg²⁺, along with pH and EC. Sequential extraction was performed on soil samples at the end of leaching column experiments. The relatively high initial concentration of P in the leachates decreased to more stable values after 15 d which can be attributed to the colloid‐bound P. The P concentrations in the leachates fluctuated between 8 and 220 mg L^–1 in the water‐soluble–P fertilizer treatment, between 0.80 and 230 mg L^–1 in the potato + water‐soluble‐P treatment, and between 0.90 and 214 mg L^–1 in the wheat + water‐soluble P treatment. Leaching loss of P mainly occurred in the 15 d of leaching, accounting for 94%, 88%, and 65% of total P leached in wheat + water‐soluble‐P, potato + water‐soluble‐P, and water‐soluble‐P treatments, respectively. Maximum amount of P leached was found from an exponential kind model and was in the range 0.45 mg kg^–1 to 125.4 mg kg^–1 in control and potato + water‐soluble‐P treatments, respectively. Sequential extraction results showed that in control and amended soils the major proportion of P was associated with Ca. The leachate samples in all treatments were saturated with respect to hydroxyapatite, β‐tricalcium phosphate, and octacalcium phosphate up to 20 d of leaching, whereas they were undersaturated with respect to Mg‐P minerals through the entire leaching experiment.