Fifty soil samples were collected from manured plots and unmanured plots of long term experimental farms at 25 Agricultural Experiment Stations and measured for saccharase activity by the method of Hofmann and Scheffer. When expressed on the basis of unit weight of dry soil, saccharase activity of volcanic ash soils is higher than that of non-volcanic ash soils. There is a significant correlation between saccharase activity and content of organic matter of the soil. However, the volcanic ash soils have lower saccharase activity value, when the value is expressed on the basis of unit weight of soil organic matter, than that of the non-volcanic ash soils. This suggests that quality as well as quantity of soil organic matter influences the saccharase activity level. Δlog k of NaOH-soluble soil organic matter is considered to be an appropriate index of humification-degree of soil organic matter. The soils are classified into two groups according to the value of Δlog k : 0.7 is regarded as a critical value of Δlog k. Correlation between saccharase activity and soil organic matter content - especially content of the easily decomposable organic matter - is evident for the soils with low Δlog k but not evident for the soils with high Δlog k. Increase in saccharase activity by manuring is highly correlated with increase in organic matter content by manuring for the soils with high Δlog k and low saccharase activity. On the contrary, this correlation is not recognized for soils with low Δlog k and high saccharase activity. A general feature of the changes in saccharase activity by manuring can be shown by a saccharase activity - Δlog k diagram. In this figure, a vector from an unmanured plot to manured plot at each Agricultural Experiment Station represents the change in saccharase activity by manuring in the field. Then, every vector points to two common small regions in this figure : the vectors belonging to the soil group with low Δlog k point to one small region and the vectors belonging to the soil group with high Δlog k point to the other small region.

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For the object of investigating the temporal changes of chemical properties in the new tephra erupted from Usu Volcano in 1977, Some experiments and field investigations were performed. Field investigations were continued for three years. The results obtained are summarized as follows : (1) Erupted new tephra deposit was supersaturated with exchangeable cations, such as Ca^<++>, Na^+, K^+ and Mg^<++>, and it was alkaline (pH=7〜8). Exchangeable sites were occupied with Ca^<++>, Na^+, K^+ ang Mg^<++>. Ca^<++> was predominant constituent of the bases. Na^+ content in the tephra was more than that in general soil. High content of Na^+ was considered to be one of the characteristic of the tephra. (2) From the result of the leaching model experiment and field investigation, the leaching of water-soluble bases from ash and pumice was rapid in the early stage, but in the late stage it was moderate. Especially the leaching of water-soluble bases from pumice in the late stage was more moderate. It seemed that the difference of interpore content between ash and pumice influenced the leaching pattern and rate. The content of exchangeable bases leached from ash and pumice was less than that of water-soluble bases. (3) Exchangeable Mg^<2+> content was increased slightly in the leaching model experiment. It seemed to suggest that alteration from non-exchangeable form as the unit of the mineral crystal to exchangeable form exceeded the leaching content. (4) pH of erupted tephra deposit increased at the early stage in experiment, but afterward decreased. It was assumed that the alteration of the ion component ratio was related to pH change. (5) Dealumination and deironation were recognized. It seemed that these changes were accompanied with the weathering of erupted tephra deposit.

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The methods currently in use for the determination of aluminum content in soil extracts are quite complicated and time-consuming. A new method using atomic absorption spectroscopy with the heated graphite atomizer furnace method (HGA method) was studied to develop a quick and easy method for the determination of aluminum content in soil extracts. 1. Experiment 1) Dried soils (see table 1) 2) Extraction of aluminum. Solutions for extraction: 1M KCl, 1M CH_3COO Na(pH4), (NH_4)_2C_2O_4(pH3.2), and a solution of 0.3M Na_3C_6H_5O_7, 1M Na HCO_3 and Na_2S_2O_4. 3) Dilution of extracted soil solutions with water to the proper volume. 4) Filtration through dry filter paper. 5) Preparation of sample solution : Al 0.1-2.5μg and H_3PO_4(1+1) 0.05 ml/ml. 6) 10-20μl of solution placed into graphite tube (P. E. 2100). 7) Measurement (wavelength 309.2nm) (see table 2). 2. Results Except for the 1M KC1 extract in which 3 out of 11 samples showed CV % of 12-16%, the CV% of all other extracts was less than 10%. (see table 9). When the HGA method was compared with the oxin method, there were no differences in regression coefficient, intersection, correlation coefficient, and passed the t-test. These results indicate that the HGA method is superior to currently-used method, being simpler and faster, yet equally accurate. Using this method, determination of aluminum content can be accomplished directly and rapidly, even if various solutions are used for aluminum extraction, (see table 10).

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The Fiji Group consists of over five hundred islands scattered in the South-west Pacific ocean. The Viti Levu island is the main among them, and the area is about 10,400 square kilometers, correspond to 57 percent of the total land area. In the center of Viti Levu island, backbone range of the 1,000 meters class in altitude arranges in the direction from south to north. And, tropical easterly wind (trade wind) dominates in whole year in Fiji. Therefore, the wet zone has appeared in the windward coast (east side), and the dry zone has appeared in the leeward coast (west side) of this island (Fig. 1). The tropical rainforest has usually distributed in the wet zone. The survey was carried on the Nukurua area which is situated in the south-eastern part of Viti Levu island. The area is formed of hilly and rolling lands with 100 to 150 meters above sea-level, consisting of the sediments in the Pliocene to Pleistocene. And, the humic latosols derived from basic sediments have widely distributed in this area. In order to establish the criteria for selecting the proper sites with the proper species originating the soil survey, the humic latosols were subclassified into four soil types, i. e. A-, B-, C- and D-types (Table 1, Fig. 2). The standard for the subclassification was put on the red horizon of over 2.SYR4/8 or 10R4/8, which is regarded as a diagnostic horizon. That is, the following aspects were considered ; whether this horizon exists, and its depth if it exists, the extent of fading of color, the influence of oxidation-reduction, etc. Judging from the experimental results, the soil acidities of surface horizons have genrally weakened with the moving from A-type to D-type. So soil fertility will be expected in order of A<B<C<D (Fig. 4, Table 2). The tendency like this has been also proved by the forest productivity survey. A typical stratigraphic sequence of the humic latosol in the Nukurua area is shown as in Fig. 5. From this survey, the humic latosol is derived from the weathered deposits which are accumulated unconformably on the mudstone of the Tertiary, and that is formed of three fundamental layers, i. e. the degraded layer due to forest humus, red latosolized layer and mottling layer caused by oxidation-reduction process. And, considering from the soil survey, the writer presumed that the oxidation-reduction layer sometimes has changed into the gley layer in depressions, and sometimes has developed into the lateritic soft plinthite layer in ridge of montane land. Since the humic latosols in Fiji are generally heavy clayey and compact, and high in credibility, the soil erosion and the landslide in forest lands will be easily accelerated for the rainy season. So, in Fiji, it is very important to maintain the soil fertility, and to conserve the forest lands from the soil erosion.

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