Physiological Science Gakujutsu or Rigaku Dept. Evaluation 3 Submit necessary documents. Example: For the academic year [1st Semester Expected to confer a Degree in September, ] Those who withdrew on September 30, or later. Publication of Doctoral Thesis A doctoral thesis will be published on the Internet.
Application for the Preliminary Evaluation FormP Contents of Doctoral Thesis Form2. The CEC of the studied soils was very low to moderately low, with values ranging from 2. The average value of the CEC for all samples was 4. The sandy nature of the soils, with low clay and OM contents, was responsible for the soils in all areas having a low CEC.
CEC values increased with depth Table 5. All of these results indicate that soils were more fertile in irrigated areas compared to rain-fed areas. Most of the arable soils in the study area had a sandy texture, while an accumulation of clay content was observed in the subsoil layers 10—40 cm , with a simultaneous decrease in sand content. Ranges of 0. Therefore, multicollinearity was not a serious problem in this study. Major rice season yields ranged from 1. Second rice season yields ranged from 2. This is because a second rice crop was only planted at one site site R14 , resulting in the sample size being too small for rain-fed areas.
For the whole crop year, the average rice yield was 2. SOC values ranged from 3.
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There were also significant relationships between rice yield and the application rates of N, P, and K Fig. There are two reasons for no manure application. Firstly, some farmers do not feed their own cattle animals, so they do not have the manure for application. Secondly, other farmers have less manure in their houses, so they need to store more manure and apply in the soil together once having enough manure.
Meanwhile, they neither gather up the manure scattered outdoors nor gather manure from their neighbors. Comparing rice yield and SOC under the period of manure application, the highest rice yield and SOC were found with manure application every year, with average of 4. Relationship between rice yield and SOC at all sites. Figure 3.
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Figure 4. Relationship between rice yield and N, P, and K application rates: A irrigated areas; B rain-fed areas.
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Figure 5. Figure 6.
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The results of a stepwise multiple regression analysis for SOC and rice yield using manure, N, P, and K fertilizers, taking into consideration the application periods of manure, are shown in Table 8. For all sites in irrigated areas, rice yield was significantly correlated with N and P fertilizers, while SOC was significantly correlated with manure. When manure was applied every year, rice yield was significantly correlated with N fertilizer alone, while SOC was significantly correlated with manure alone. When manure application was applied every other year, rice yield was significantly correlated with N and P fertilizers, while SOC was significantly correlated with N fertilizer alone.
For all sites in rain-fed areas, rice yield was significantly correlated with application rates of manure, N and P fertilizers, while SOC was significantly correlated with application rates of manure and N fertilizer.
If manure was applied every year, rice yield was significantly correlated with manure application rate only, while SOC was significantly correlated with application rates of manure and N fertilizer. When manure was applied every other year, rice yield and SOC were significantly correlated with N application rate only.
SOC content is influenced by various regional factors, such as soil type, texture, topography, land use type, and management practices Hao and Kravchenko Hao XY , Kravchenko AN : Management practice effects on surface soil total carbon: differences along a textural gradient.
Slightly higher SOC was found in the surface soil compared to the subsoil in this study. Ecological Appl. Furthermore, the accumulation of manure on the soil surface enriches SOM in the surface layer Mathew et al. This result clearly showed that SOC accumulates in the surface soil, which is a normal phenomenon in tropical soils Wong et al.
http://ecolearning.org.uk/171.php Soil Res. Soil texture is also an important variable affecting SOC levels, because aggregation processes are favored by small particle sizes that provide high surface areas. Many studies Konen et al. J , 67, — J , 71, — Forest Ecol Manag , , — Geoderma , 3—4 , — Biol , 19, — This is because the high sand content in soil results generally a low CEC due to the particles having no charge Islam et al.
Kasetsart J. Moreover, the main clay mineral in sandy soils in Northeast Thailand is kaolinite Dur et al. In : Proceedings of Management of Tropical Sandy Soils for Sustainable Agriculture: A holistic approach for sustainable development of problem soils in the tropics.
Geoderma , 99, 27 — A few studies were consistent with our findings. For instant, Laopoolkit et al. Islam et al. They explained that there might be difference in the type of clay mineral in other studies and could be other factors, such as a high salt content and limited rainfall that result in a high CEC but a low SOC. Furthermore, Hassink Hassink J : Effects of soil texture and grassland management on soil organic C and N and rates of C and N mineralization.
This issue suggests that adding manure is more important than soil texture for improving SOC. Thus, SOC increases or decreases in parallel with increasing or decreasing finer soil particles, respectively. Therefore, the fraction of finer soil particles of a given soil type represents an important predictor of SOC in the soil Parton et al. J , 51, — Hassink Hassink J : Preservation of plant residues in soils differing in unsaturated protective capacity. J , 60, — A highly significant positive correlation between EC and SOC was obviously seen Table 6 because salinization of soil in Northeast Thailand is frequently evident as salt patches in the surface soil during the dry season Maeght et al.
Moreover, the soil samplings in this study were collected in the dry season, and rain-fed areas dry land were mostly covered in this study area, resulting in higher EC in the surface soil than the subsoil layers. Iwai et al. Maeght et al. Soil pH is an important chemical property that is primarily attributed to low rice yield causing low nutrient return by organic materials.