干拓地土壤に関する研究 (第18報)干拓新田における土壤の化学的性質の経時的変化について

It is the purpose of this report to study changes of chemical properties of the soil in a halogenetic young polder, developed along the seacoast of inner Kojima Bay, in relation to the length of time which has elapsed since these polder being diked and cultivated under paddy-field condition. Soil samples used in this study were collected from Kojima 7 Polder B Section, A Section and Kojima 3・5 Polder. 7 Polder B Section is located in a recently empoldered virgin region. Ages of 7 Polder A Section and 3・5 Polder under paddy cultivation are 5 years and about 10 years, respectively. The results obtained are to be summarized as follows: (1)Ground water contains a large amount of soluble salts and its table is very high in a recently empoldered region. As paddy cultivation proceeds, a continuous decrease in salinity and lowering of water table are observed in well drained fields. On the contrary, salinity and water table remain unchanged in poorly drained fields along the dike. (2)The content of soluble salt in recently empoldered soils is also very high in all the horizons. A considerable part of soluble salt has been washed cut from the upper layer during 5 year cultivation by rain or irrigation water, and most of surface soils show the electric conductivity of less than 7 millimhos/cm, but salinity of subsoils remains unchanged, having 7 millimhos/cm or more. After about 10 year cultivation, all of the first horizon and most of the second horizon show a marked reduction of salinity, having the electric conductivity of less than 4 millimhos/cm and 7 millimhos/cm, respectively. (3)The reaction of all the horizons of just empoldered soils lies within the range pH 7.0 to 8.1 but that of most surface soils becomes slightly acid, after 5 year cultivation, and considerably acid after about 10 year cultivation, showing the pH of 6.0-6.9 and below 5.9, respectively. (4)Of exchangeable bases, mono-valent cations and Mg are more prominent than others, and the base-exchange complex is nearly saturated with bases in just empoldered soils. It is shown that the V values and exchangeable mono-valent cations are decreased slightly in surface soils but remain unchanged in subsoils during 5 year cultivation. After about 10 year cultivation, mono-valent cations and Mg are replaced by hydrogen, and V values decrease greatly in surface soils and the composition of the exchangeable bases gradually approaches to that of normal acid soil. (5)A large amount of oxidizable sulfur is found in all the horizons of just empoldered soils and a considerable decrease of oxidizable sulfur in surface soils is found after 5 year cultivation, but a marked decrease of oxidizable sulfure in all the horizons is found after about ten year cultivation. It is shown that oxidizable sulfur is rapidly oxidized in a field condition and its product is easily leached out. A relatively close correlation is also found to exist between the degree of desalting and the decrease of oxidizable sulfur.