Agricultural Utilization of the Apatite-Phosphorus in Pyroclastic Flow Deposits in Subsoil Using a Trencher.

In Abashiri District, much arable land leveled for agricultural use has subsoil layers that consist of Kutcharo pyroclastic flow deposits. These deposits in subsoil are generally firm and they inhibit the root development of arable crops. However, their available phosphorus (Truog-P) content is high (70-170 mg P₂O₅ kg^-1). Sources of phosphorus (P) in the pyroclastic flow deposits were previously evaluated as apatite. In the present study, the utilization of the apatite-P in pyroclastic flow deposits in the subsoil was tried in a field cultivation experiment with sugar beet. Before the experiment, trenchers were used to thoroughly mix the pyroclastic flow subsoil with topsoil in a part of the field to promote root development (subsoil mixed plot) and the rest of the field was used as a control plot. During the cultivation, the changes of the chemical forms of soil P and P uptake of sugar beet were measured. Subsoil mixing clearly improved root development. For the subsoil mixed plot, the yield of sugar was 30% higher, and the phosphorus uptake was 50% higher than the control plot. Under cultivation, it was observed that Truog-P, Al and/or Fe bound P were increased in the subsoil mixed plot. These results suggested that the weathering of apatite-P in the pyroclastic flow deposits was enhanced in the subsoil mixed plot by the reaction with soil reactive Al and/or Fe. The weathering of apatite-P should increase the P uptake and yield of the sugar beet. Thus the apatite-P should be an important P resource for agriculture.