Distribution of phosphorus forms in soil following long-term continuous and discontinuous cattle manure applications

Better information on soil P and its distribution in various forms is needed to assess their bioavailability and environmental impact. This study investigates the distribution of P forms following long‐term manure application to a calcareous clay loam soil in a semiarid region of Canada. The manure was applied and incorporated annually each fall at 30, 60, and 90 Mg ha−1 yr−1 under rain‐fed and at 60, 120, and 180 Mg ha−1 yr−1 under irrigated conditions, continuously for 30 yr and as well as for 14 yr followed by 16 yr of no further application. After 30 yr, soil samples were collected from depths 0 to 15 and 15 to 30 cm for all treatments. Soil P was partitioned into different forms through sequential chemical fractionation. In comparison with the non‐manured control, manure application significantly increased the levels of all P forms. The largest increase occurred for soil test P (STP). The increase was greater in irrigated than in rain‐fed blocks and greater in the 15‐ to 30‐cm than in the 0‐ to 15‐cm depth. The STP/ sum of all P forms in the soil (TP) ratios of 0.42 to 0.47 in manured soil were significantly higher than in the non‐manured control (<0.20), but were not significantly different from the cattle manure applied (0.38). In both soil depths, most P was inorganic P (Pi) with organic P (Po) accounting for ≤5% P. The STP (877 mg kg−1) and TP (2555 mg kg−1) levels in the topsoil of the plots where manure application had been discontinued were higher than in the non‐manured control, showing that the effects of manure application in excess of plant use are still evident even 16 yr after the last manure application. Thus, manure application has long lasting and substantial residual effects, still measurable in all soil P forms.