Evaluation and utilization of the apatite-phosphorus in pyroclastic flow deposits in Abashiri district of eastern Hokkaido, Japan

In Abashiri district, in the eastern part of Hokkaido, much arable land that has been leveled for agricultural use has subsoil layers that consist of Kutcharo pyroclastic flow deposits. Kutcharo pyroclastic flow deposits in subsoil were generally compacted by land leveling, so that penetration resistance is too high to allow the root development of arable crops. However, the available phosphate (Truog-P) content is high (70-170 mg P2O5/kg). Thus, the chemical forms of phosphorus (P) in the pyroclastic flow deposits were evaluated using the selective extraction of each form of phosphorus, i.e. Al-bound, Fe-bound and Ca-bound P (Al-P, Fe-P, Ca-P). The utilization of the phosphorus in the pyroclastic flow deposits was also investigated in a field cultivation experiment. Sugar beet was grown in a field with pyroclastic flow deposits in subsoil. Two years ago at the start of the experiment, trenchers were used to thoroughly mix pyroclastic flow subsoil with top soil in a part of the field to improve the root development (subsoil mixed plot) and the rest of the field was used as a control plot. It was determined that most of the P in the pyroclastic flow deposits was in the Ca-P form. It is well known that volcanic tephra contains the mineral apatite. Therefore, the high Truog-P content of Kutcharo pyroclastic flow deposits should be due to the presence of apatite. The field cultivation experiment showed that subsoil mixing clearly improved root development in the subsoil. The phosphorus, uptake was 50% higher for the deep cultivated plot than the control plot. This suggested that the sugar beet roots utilized the apatite-P in their root development. Although the presence of the pyroclastic flow deposits has been a limiting factor for agricultural productivity, the apatite-P should be an important resource which can improve agricultural activities in Abashiri district.