Vertical distribution and accumulation of phosphate in virgin soils and arable soils of Tokachi district, Hokkaido

TANI Masayuki, MIZOTA Chihiro, YAGI Tetsuo, KATO Taku, KOIKE Masanori

doi:10.20710/dojo.81.4_350

Heavy application of phosphate fertilizer and subsequent phosphate accumulation in arable soils, especially Andosols, are critical issues for sustainable agriculture in Japan. The effects of fertilization and plant nutrient management on phosphate accumulation and its speciation in uncultivated virgin soils and cultivated upland soils of Tokachi district, Hokkaido, were investigated. Three pairs of virgin and arable soil profiles were collected from adjacent sites to evaluate the vertical distribution of total, sequentially extractable, and available phosphates in the soils. Net accumulated amounts of phosphate in the arable soils were calculated while accounting for bulk density and differences in the thickness of layers between upland and adjacent virgin soils. In soils classified as Low-humic Andosols (Typic Hapludands), Cumulic Andosols (Pachic Melanudands), and Haplic Brown Lowland soils (Fluventic Dystrudepts), surplus phosphates were accumulated at depths of 0〜40, 0〜30, and 0〜35cm, respectively, of the arable soil profiles. Phosphates were distributed homogeneously in the plow layer due to physical disturbance and mixing. The vertical distributions of phosphate in the paired virgin and arable soils indicated that most of the surplus phosphate accumulated without leaching in the plow layer of the Andosols, whereas some surplus phosphate leached out from the surface layer of the Brown Lowland soils. Net accumulated amounts of phosphate in the cultivated Andosols were remarkably large, with estimated annual accumulation rates of >130kg ha^<-1>. Most of the accumulated phosphate may be specifically adsorbed by active aluminum and iron, which could be extracted by fluoride through a strong ligand-exchange reaction. The net accumulated phosphate in the cultivated Lowland soil was relatively small. Most of the phosphate may accumulate through precipitation processes as calcium hydrogen phosphate and amorphous aluminum phosphate, which could be dissolved by weak acids and extracted by a weak ligand-exchange reaction, respectively.

Vertical distribution and accumulation of phosphate in virgin soils and arable soils of Tokachi district, Hokkaido

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