Abstract
The purpose of our studies is to investigate the process of the formation of principal paddy soil types in Japan by the following means, a) observation of soil profiles, b) estimation of general chemical and physical characteristics, c) identification of primary and secondary minerals, d) identification of the type of soil humus. In this paper the distributions of free oxides in several soil profiles are reported. These paddy soils are all evidently affected by acid humus derived from heavily dressed raw orgaic matters, their drainages are good and parent materials are cherty somewhat accompanied with shales and volcanic ashes. The characteristics of soil profiles are given in Table 1 and mechanical compositions in Table 2. Each soil is rich in gravels and porous throughout the profile. The structure is well developed. Much organic matter is found in every horizon and free Fe_2O_3 is broadly accumulated in subsoils. All the horizon presents brown, dark brown or blackish brown colour. Free oxides are determined by Mg-reduction method modified JEFFRIES' Mg-ribbon-potassium-oxalate method. The results are given in Table 3. As to profile A, free oxides intensely move from the surface soil to the lower part. There is a maximum accumulation of free Fe_2O_3 in the second horizon and free Mn_3O_4 in the third horizon and free Al_2O_3 in the fourth horizon. Free TiO_2 is equally found in all the horizons except the top horizon. Profile B and C are similar to A, but as to B the maximum accumulations of free Mn_3O_4 and Al_2O_3 are found in the same horizon. And as to C the maximum accumulations of free Fe_2O_3 and Mn_3O_4 are found in the third horizon. In all three profiles, the maximum accumulation of free Al_2O_3 are always found below that of free Fe_2O_3. The movement of free Al_2O_3 is characteristically different compared with that in the paddy soils, of which surface soils are not affected by acid humus. Results in these soils shall be reported in the latter part. The amounts of free Fe_2O_3 and Mn_3O_4 determined by different methods are compared. The results are given in Table 4 and 5. In the accumulation horizon, the ratio free Mn_3O_4/total Mn_3O_4 is higher than the ratio free Fe_2O_3/total Fe_2O_3. But in the surface soil the ratio free Mn_3O_4/total Mn_3O_4 is lower than that of iron. It seems to be because of rapid removal of free Mn_3O_4 from the surface soil to the subsoil. The amounts of free Al_2O_3 determined by Mg-reduction method are compared with the values of ascension of pH by NaF-treatment. The results are given in Table 6. The pH of the accumulation horizon of free Al_2O_3 is ascended to above 10 after 24 hours keeping at 24℃ by addition of N/2 neutral NaF. The value of ascension of pH in the maximum accumulation horizon of free Al_2O_3 is the highest in both A and B. The amounts of free TiO_2 are compared with those of conc. HCl soluble and total TiO_2. The results are given in Table 7. Total TiO_2 in both bulk samples and ignited samples are estimated. The results are given in Table 8. The movement of titanium in paddy soils is hardly found. There are the different horizons in total TiO_2 content which seem to be probably because of the difference of alluvial materials.
