土壌中の粘土量の簡易推定法による土壌調査―小学校・中学校における環境教育・防災教育での活用を目指して―

抄録

Clay has a strong cation exchange ability. If a soil sample is put into a plastic tube with a methylene blue (blue color cation) aqueous solution and the mixture is well mixed, the top layer is colored with blue. We can measure the layer length as an indicator of the amount of clay in the sample soil. We collected 24 sample soils from Mt. Handa in Okayama city, where a big landslide occurred in the Nishinihon torrential rainfall disaster (July 2018). Our methylene blue method for determining the clay amount shows that the clay amount of the sample collected from a point close to the landslide was the highest in 24 samples. The amount of clay in a soil sample can thus be a possible indicator of a landslide. Chemical weathering of rock minerals generates clay minerals with alkali and alkaline earth metal cations as byproducts. Actually, the adsorbed blue layer length is proportional to the soil conductivity caused by chemical weathering. This “methylene blue adsorption method” can be conducted with just a plastic tube or a PET bottle, a ruler, methylene blue solution, and sample soil, which are inexpensive, simple and easy procedures, and results can be confirmed by the visual blue colored layer. Therefore, this approach is suitable for education on disaster prevention at primary and junior high schools.