Distribution of phosphorus forms in soil amended with poultry litter of different ages and application rates: Agronomic and environmental perspectives

Poultry litter (PL) can be used as a viable alternative to phosphate fertilizers. However, there is a lack of information about phosphorus (P) distribution in inorganic (P_i) and organic (P_o) forms and its transformation in soils amended with PL of varying age (based on litter clean-out frequency) and application rate. This study aimed to determine the effect of PL age and application rate on soil P forms and their bioavailability. Soils were amended with 5 and 10 Mg ha⁻¹ PL using 6-, 18-, and 30-month-old litter and incubated for 6 months. Soil P fractionation was performed following the Hedley protocol. Soil P availability and soil P storage capacity (SPSC) were determined using Mehlich 3 (M3) extraction. Results indicated that P transformation from labile to stable P forms occurred over 150-day incubation. Litter age had no significant effect on the distribution of soil P forms. However, the highly reactive P_i (HRP_i) form was higher for treatments with 10 Mg ha⁻¹ PL on Day 0, indicating a risk for P loss, which was also revealed by negative SPSC for those treatments. At Day 0, M3-P was positively correlated to HRP_i. However, from Day 30 to 150, M3-P was strongly correlated to both HRP_i and moderately reactive P_i (MRP_i) forms, indicating MRP_i contribution to soil P availability. The negative relationship between HRP_i and SPSC further confirms that high HRP_i on Day 0 may be an environmental concern.