Abstract
In order to understand the present condition of chemical properties of paddy soil in Hokkaido, we sampled plowed soil from paddy field (1,578 points) in Hokkaido. We drew a digitized map with 1 km grid, and estimated the soil chemical property for each mesh. The results are summarized as follows. 1) pH ranged 4.5-6.9, and the average was 5.5. Nearly half of the points investigated were below the lower limit (pH 5.5) according to the soil diagnosis standard of Hokkaido. 2) Incubation nitrogen ranged from 1 to 293 mg kg^<-1> ; the average was 99 mg kg^<-1>. 3) The average available phosphate (Bray No.2) was 500 mg kg^<-1>, which is five times as much as the lower limit of the soil diagnosis standard (100 mg kg^<-1>). 4) Exchangeable Ca ranged from 510 to 5,400 mg kg^<-1> ; the average was 1,985 mg kg^<-1>. Exchangeable Mg 80-3,320 mg kg^<-1> ; the average was 574 mg kg^<-1>. The low pH was thought to be due to the low level of Ca. 5) Incubation silica ranged from 34 to 290 mg kg^<-1> ; the average was 103 mg kg^<-1>. 51% of the points were lower than the lower limit of the soil diagnosis standard (100 mg kg^<-1>). In particular, Brown lowland soil and Upland soil types seemed to have a tendency to be lower. 6) Free iron oxide ranged from 4.2 to 106.2g kg^<-1> ; the average was 18.6 g kg^<-1>. Easily reducible manganese ranged from 15 to 2,272 mg kg^<-1> ; the average was 278 mg kg^<-1>. In the case of free iron oxide, 46% of the points were lower than the limit (15 g kg^<-1>) and a remarkable difference between regions and soil types was observed. Results mentioned above suggest that paddy soil in Hokkaido has some problems that are awaiting solutions. Those problems include (1) low pH, (2) a lack of silica-supplying ability and (3) a lack of oxidation capacity (free iron oxide).
