Acid sulfate soils and salt-affected soils are widely distributed in the Mekong Delta of Viet Nam. As well as the improvement of chemical properties, it is es-sential to minimize soil structural degradation. However, relatively little research has been conducted on behaviors of aggregates in the Mekong Delta. In the present study, saline and sodic soil (SS), alluvial soil (AS), and acid sul-fate soil (ASS) were collected from the Mekong Delta. First, aggregate stability tests were conducted on them. Base on the aggregate stability test, dry clods of 2–5 mm in size under fast wetting process were used to evaluate aggregate stability of the soils after incubation with soil amendments. Chicken manure was used as a compost and eggshell-CaCO3 was an alternative for lime. Soil and amendments were mixed and incubated in a glass bottle for 45 days at a constant temperature of 25 ◦C. During the in-cubation, CO2 concentration in the headspace of the glass bottle was periodically measured. After incubation, soil pH and aggregate stability were evaluated. Mean weight diameter (MWD) was used to evaluate aggregate stabil-ity. Eggshell application increased the pH of all three soils. Compost raised the pH of SS and AS but it was less effect-tive at raising pH of ASS. The combination of compost and eggshell was the most effective at increasing soil pH. Rapid increases in CO2 emission rate after eggshell application suggested that the CaCO3 rapidly reacted with the H+ in the soils. Correlations between soil pH and CO2 emission suggested that microbial activity increased with increasing in soil pH. Eggshell application alone destabilized soil ag-gregates, whereas the compost and the compost-eggshell combination improved aggregate stability.
Iron, one of the essential elements, is a very common element, since the total weight of the iron is ap-proximately 30 % of the weight of the Earth. However, concentration of iron in oceans is less than nmol, which is extremely low. Thus, iron supply from the terrestrial area through air dusts and river flows is significantly im-portant for ocean ecosystems. In this sense, understanding the mechanism of iron behavior on lands is essential. Reviewing published articles on dissolved iron genera-tion mechanisms suggests that soil organic matter and iron in the soil particles are behaving simultaneously. Under the condition that freezing and thawing processes is im-portant, thawing depth might be the key factor to determine dissolved iron concentration. In addition, analysis on ab-normal dissolved iron concentration peak in the late 1990s’ in the Amur River is indicating that the effect of thawing of permafrost have a significant effect on dissolved iron pro-duction mechanisms. Based on this hypothesis, we con-ducted field observations combined with the analysis of long-term evaluation of thawing depths. The result indi-rectly supports the hypothesis. For the further understand-ing, classification of organic matter contributing the chela tion with iron, and the monitoring of iron and dissolved organic matters during freezing and thawing processes are critical.
Recently, large scale farmland consolidation and upsizing of farming machinery have been progressing in the rice cultivation. In addition to the rice for staple food, various rice cultivations such as forage rice using a high-yield variety are conducted. On the other hand, the air temperature in summer has been increasing and it has great influence on the quality of rice. In this paper, the improve-ment of plowing practice and introduction of the under-ground irrigation system (FOEAS) mitigated the decline in quality of rice under high air temperature condition by maintaining the favorable rhizosphere environment. More-over, the nutrient balance of the soil was clarified. The sili-cic acid balance was negative in all cultivation. Silicic acid was effective for improvement of rice quality under high air temperature. Besides, this paper showed the measures to promote the aerobic decomposition of rice straw derived from cultivation of high-yield rice varieties. Finally, this paper proposed that the way of “soil-making” is not lim-ited to the application of soil amendments and soil-making fertilizers, but also comprehensive management including the machine work for the purpose of improving the rhizo-sphere environment.
Multiple-intertillage-weeding method for the paddy cultivation described in agricultural books published in Edo-era was tried under present-day cultivating system. Seedlings were grown in the pool-floating system under open air. “Mini-culti” machine with “chain-weeder” was used to intertillage. The number of times of intertillage was 0–16. Almost the same yield to the customary practice was got for more than four times of intertillage. The quality of the rice was ranked as the first grade and “the taste value” was more than 85. These show that the method of agri-cultural books published in Edo-era was true. The essence of this method is the stirring of the surface soil. The stir-ring takes place to uniform the soil physically, to promote the decomposition chemically, to increase the nitrogen fix-ation biologically and to form the stable ecological system ecologically. This method is useful for the cultivation for the resource saving, low input and harmony with the envi-ronment.