5.1.3 Water Management and Characteristics of a Paddy
Field Soil Profile
The area percentage of irrigated paddy fields is approximately 60% of the total
paddy-field area in the world (Global Rice Science Partnership 2013). Figure 5.3
shows an example of seasonal changes in an irrigated paddy field in Miyagi
Prefecture, Japan. In spring, farmers till the drained paddy field (Fig. 5.3a) and
apply basal fertilizers (Fig. 5.3b). During this season, paddy fields are partially dried.
After introducing water, puddling, and transplanting in late spring, rice plants are
grown under submergence (Fig. 5.3c) until the maximum tillering stage in early
summer. Following midseason drainage for 2 weeks, the paddy field is irrigated
intermittently or submerged again (Fig. 5.3d) until the end of August. Then, irrigation is stopped and the paddy fields are dried for harvest (Fig. 5.3e).
The Eh value in the plow layer soil of paddy fields responds to the paddy field
water management. Changes in Eh value were monitored using a platinum electrode
connected to a reference electrode and a voltmeter. Figure 5.4 shows examples of Eh
change with paddy field water management. Three water management schemes were
carried out: midseason drainage and intermittent irrigation (Fig. 5.4a), midseason
drainage and submergence (Fig. 5.4b), and continuous submergence (Fig. 5.4c).
At about 50 days after transplanting and submergence, the Eh values ranged
around À0.1 to À0.2 V, suggesting that all three of the plow layer soils were nearly
fully reduced due to continuous submergence. The Eh values of plots (a) and
(b) increased with midseason drainage. Subsequently, the Eh value of plot
(a) tended to be higher than those of the other plots due to intermittent irrigation,
whereas the Eh value of plot (b) decreased to the same level as that of plot (c), which
was continuously submerged. One hundred and ten days after transplanting, all the
Fig. 5.3 Seasonal changes in a paddy field. (a) After spring tillage and basal fertilizer application,
(b) close-up view of applied fertilizers, (c) initial growth with submergence, (d) heading stage, (e)
rice plant harvest time
102
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions
Field Soil Profile
The area percentage of irrigated paddy fields is approximately 60% of the total
paddy-field area in the world (Global Rice Science Partnership 2013). Figure 5.3
shows an example of seasonal changes in an irrigated paddy field in Miyagi
Prefecture, Japan. In spring, farmers till the drained paddy field (Fig. 5.3a) and
apply basal fertilizers (Fig. 5.3b). During this season, paddy fields are partially dried.
After introducing water, puddling, and transplanting in late spring, rice plants are
grown under submergence (Fig. 5.3c) until the maximum tillering stage in early
summer. Following midseason drainage for 2 weeks, the paddy field is irrigated
intermittently or submerged again (Fig. 5.3d) until the end of August. Then, irrigation is stopped and the paddy fields are dried for harvest (Fig. 5.3e).
The Eh value in the plow layer soil of paddy fields responds to the paddy field
water management. Changes in Eh value were monitored using a platinum electrode
connected to a reference electrode and a voltmeter. Figure 5.4 shows examples of Eh
change with paddy field water management. Three water management schemes were
carried out: midseason drainage and intermittent irrigation (Fig. 5.4a), midseason
drainage and submergence (Fig. 5.4b), and continuous submergence (Fig. 5.4c).
At about 50 days after transplanting and submergence, the Eh values ranged
around À0.1 to À0.2 V, suggesting that all three of the plow layer soils were nearly
fully reduced due to continuous submergence. The Eh values of plots (a) and
(b) increased with midseason drainage. Subsequently, the Eh value of plot
(a) tended to be higher than those of the other plots due to intermittent irrigation,
whereas the Eh value of plot (b) decreased to the same level as that of plot (c), which
was continuously submerged. One hundred and ten days after transplanting, all the
Fig. 5.3 Seasonal changes in a paddy field. (a) After spring tillage and basal fertilizer application,
(b) close-up view of applied fertilizers, (c) initial growth with submergence, (d) heading stage, (e)
rice plant harvest time
102
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions