Red and yellow soils from tropical regions are generally more mature than their subtropical counterparts. Most of these soils contain kaolin as the dominant clay mineral. Exchangeable aluminium (Al) generally balances permanent negative charges and occupies strongly acidic exchange sites of the soil clay. The objective of this study was to identify those clay minerals that are most highly implicated in contributing exchangeable Al to red and yellow soils collected from the Islands of Okinawa and Java. All soils exhibited an acid reaction but varied in their exchangeable Al content and clay mineralogy. Clay content was high in all Javan soils but varied in those from Okinawa. Javan soils were dominated by kaolinite, and Okinawan soils by an association of illite and halloysite. However, 2 : 1-2 : 1 : 1 intergrades were significant components in both the Oku red soils (Okinawa Island) and the Pamagersari red soils (Java Island). Javan soils were characterised by a more mature (advanced) state of weathering than those from Okinawa. The source of exchangeable Al was halloysite in Okinawan soils and 2 : 1-2 : 1 : 1 intergrades in Javan soils.

The mangrove habitat is referred to as an advancing coast, where the land advances toward the sea due to soil sedimentation. In the present study, physico‐chemical and mineralogical characteristics of surface sediments in a mangrove forest were investigated to evaluate the mechanism of accumulation of surface sediments from the viewpoint of colloid chemistry. A mangrove forest in the Fukido River basin, Ishigaki Island was selected as the study site. Six locations along the river in the forest were selected, and samples of surface sediments (0.5 cm thick) were collected at 4 points (a point on the water side of the bank directly in the river water (hereafter referred to as “0 m from the river water”), a point on the bank at a distance of 0.5 m from the river water and points at distances of 5.5 m and 10.5 m into the forest from the river) of each location (total 24 points). The soil texture of the surface sediments considered of HC at nearly all the sampling points, and more clay tend to accumulate on the water side of the bank of about 200 m upstream from the downstream end of the study area. The silt fraction of the surface sediments contained quartz at a rate of about 70% along with feldspar and a small percentage of cristobalite. In the clay fraction, the contents of kaolinite and illite were slightly higher than the contents of vermiculite and vermiculite‐chlorite intergrade. The mineral composition was similar at all the points, suggesting that the minerals were carried from the upper reaches of the river and were deposited in the study area. Both the pH and EC values of the surface sediments tended to decrease toward the upper reaches of the river and to be higher on the bank than at the points farther into the forest. The total carbon content, organic carbon content, total nitrogen content and total phosphate content tended to be higher on the bank at about 200 m upstream from the low end of the study area, indicating a larger accumulation of organic matter at this site. High clay and high organic matter contents were observed on the bank at this location. From the above evidence, it was suggested that organo‐mineral complexes were likely to accumulate in mangrove surface sediments.

The ability of phosphate rock (PR) to dissolve rapidly in soil is a primary concern in its direct application as P fertiliser. The dissolution of 4 PR materials (Togo, South Africa, Florida, Morocco PRs) in 15 soil samples in Okinawa was investigated in a closed-incubation system for 7 days. The fertilisers were mixed with the soils at rates of addition of 600–1200 µg Ca/g soil. The extent and rate of dissolution of the PRs were determined by measuring the increase in extractable Ca of the fertilised soils compared with unfertilised soils, i.e. the delta Ca (ΔCa) technique. Generally, the amounts of dissolution of 3 of the PRs were very low (mean 6·7% for Togo PR, 13·6% for South Africa PR, and 20·8% for Florida PR). However, Morocco PR dissolved to an appreciable extent (mean 60·8%), suggesting that it can be an alternative P source, especially in the red and yellow soils of Okinawa. Soil properties identified as affecting dissolution were different for the different PRs. In order to predict the suitability of Morocco PR for Okinawan soils, the extent of its dissolution was related to soil properties in a multiple regression analysis. Results indicated that the best regression model for predicting the amount of dissolution was the combination of pH, Ca saturation, and Truog P (r2 = 0·55**). Measured values of percentage dissolution of Morocco PR were significantly correlated with calculated percentages (r = 0·844***), indicating that the equation obtained could offer a rapid estimation of amount of dissolution of Morocco PR in Okinawan soils.

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