tsunami deposit and the top of the original soil. The boundary between the tsunami
deposit and the original soil was evident because the bottom of the tsunami deposit
had a coarse texture, because sands settle faster than finer particles in a soil
suspension, and the top of the original soil had a comparatively finer texture
(Fig. 6.4c).
After air-drying the tsunami deposit and original soil samples, the samples were
ground gently in a porcelain mortar and passed through a 2 mm sieve to prepare a
fine earth fraction. As a result, the samples were mostly passed through the 2 mm
sieve. Using these fine-earth fractions, total C, N, and S content, pH(H 2 O) and pH
(KCl) at the soil:water ratio of 1:2.5, electric conductivity at the soil:water ratio of
1:5 (EC(1:5)), the contents of water-soluble Na, K, Ca, and Mg in the 1:5 water
suspension, and NH 4 -exchangeable Na, K, Ca, and Mg were determined. The
contents of NH 4 -exchangeable Na, K, Ca, and Mg were determined by extracting
the sample obtained after extraction of water-soluble Na, K, Ca, and Mg with
1 mol L
À1 NH 4 acetate (pH 7) twice (Thomas 1982). Entrained solution volume
after centrifugation and decantation of water extract was obtained by weight measurement, and the amount of each cation in the entrained solution was subtracted
from each total cations found in the NH 4 acetate-extract for the calculation of NH 4
exchangeable cations.
The tsunami caused severe damage to buildings. However, the physical damage
to farmland was not very severe compared to that of buildings, probably because the
tsunami affected only the surface of the farmland and a direct hit between tsunami
and farmland soil was mostly avoided.
Intensively eroded sites were found on the inland side of the road running along
the coast (Fig. 6.3a). The distance between the road and the shoreline was less than
1 km. When the tsunami dropped from the slightly higher road, paddy field soil was
excavated and removed. At these sites, the soil removal was greater than several tens
of centimeters from the original surface of the paddy fields. Similar soil erosion
occurred along ridges (30–40 cm high), as shown in Fig. 6.3b, although the intensity
of the erosion at these sites was lower than that along the roads and the plow sole
mostly remained. The intensity of soil erosion over the wide surface of farmland was
dependent on whether or not the farmland had been plowed. If the soil had not been
plowed, erosion was limited and the rice stubble after harvest mostly remained
(Nanzyo 2012). In contrast, plowed soil was removed by the tsunami, especially in
farmland near the coast. Soil removal was lower in the farmland distant from the
coast, even if the land had been plowed.
These interactions between tsunami and farmland are summarized in Fig. 6.3c.
Erosion occurred at sites ①, ②, and ③, and deposition occurred at sites ④ and ⑤.
Ion exchange and precipitation reactions occurred as chemical interactions at sites ④
and ⑤ (Fig. 6.3c). If there was a muddy (sometimes containing sulfides) and/or
sandy deposit beneath the shallow seawater or in the nearshore zone (①) including
the Teizan canal, the deposits might have been transported to farmland and deposited
at sites ④ and/or ⑤. Moreover, the A p horizon soil, after tilling, was at least partly
lost. Thus, the deposits on the farmland also contain the eroded A p horizon soil. The
chemical reactions at sites ④ and ⑤ include the exchange reaction between Na
+ in
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6 Role of Inorganic Soil Constituents in Selected Topics
deposit and the original soil was evident because the bottom of the tsunami deposit
had a coarse texture, because sands settle faster than finer particles in a soil
suspension, and the top of the original soil had a comparatively finer texture
(Fig. 6.4c).
After air-drying the tsunami deposit and original soil samples, the samples were
ground gently in a porcelain mortar and passed through a 2 mm sieve to prepare a
fine earth fraction. As a result, the samples were mostly passed through the 2 mm
sieve. Using these fine-earth fractions, total C, N, and S content, pH(H 2 O) and pH
(KCl) at the soil:water ratio of 1:2.5, electric conductivity at the soil:water ratio of
1:5 (EC(1:5)), the contents of water-soluble Na, K, Ca, and Mg in the 1:5 water
suspension, and NH 4 -exchangeable Na, K, Ca, and Mg were determined. The
contents of NH 4 -exchangeable Na, K, Ca, and Mg were determined by extracting
the sample obtained after extraction of water-soluble Na, K, Ca, and Mg with
1 mol L
À1 NH 4 acetate (pH 7) twice (Thomas 1982). Entrained solution volume
after centrifugation and decantation of water extract was obtained by weight measurement, and the amount of each cation in the entrained solution was subtracted
from each total cations found in the NH 4 acetate-extract for the calculation of NH 4
exchangeable cations.
The tsunami caused severe damage to buildings. However, the physical damage
to farmland was not very severe compared to that of buildings, probably because the
tsunami affected only the surface of the farmland and a direct hit between tsunami
and farmland soil was mostly avoided.
Intensively eroded sites were found on the inland side of the road running along
the coast (Fig. 6.3a). The distance between the road and the shoreline was less than
1 km. When the tsunami dropped from the slightly higher road, paddy field soil was
excavated and removed. At these sites, the soil removal was greater than several tens
of centimeters from the original surface of the paddy fields. Similar soil erosion
occurred along ridges (30–40 cm high), as shown in Fig. 6.3b, although the intensity
of the erosion at these sites was lower than that along the roads and the plow sole
mostly remained. The intensity of soil erosion over the wide surface of farmland was
dependent on whether or not the farmland had been plowed. If the soil had not been
plowed, erosion was limited and the rice stubble after harvest mostly remained
(Nanzyo 2012). In contrast, plowed soil was removed by the tsunami, especially in
farmland near the coast. Soil removal was lower in the farmland distant from the
coast, even if the land had been plowed.
These interactions between tsunami and farmland are summarized in Fig. 6.3c.
Erosion occurred at sites ①, ②, and ③, and deposition occurred at sites ④ and ⑤.
Ion exchange and precipitation reactions occurred as chemical interactions at sites ④
and ⑤ (Fig. 6.3c). If there was a muddy (sometimes containing sulfides) and/or
sandy deposit beneath the shallow seawater or in the nearshore zone (①) including
the Teizan canal, the deposits might have been transported to farmland and deposited
at sites ④ and/or ⑤. Moreover, the A p horizon soil, after tilling, was at least partly
lost. Thus, the deposits on the farmland also contain the eroded A p horizon soil. The
chemical reactions at sites ④ and ⑤ include the exchange reaction between Na
+ in
136
6 Role of Inorganic Soil Constituents in Selected Topics