インドネシア•南スマトラのランポンにおいて,1996年から1999年の雨季の期間,異なった雑草管理下のコーヒー園からの土壌侵食量と地表流出量を観測した。試験区は,⑴地表面の雑草を完全除草したコーヒー園,⑵被覆植物として雑草種Paspalum conjugatumで地表面を被覆したコーヒー園,⑶自然植生の雑草で地表面を被覆したコーヒー園である。雑草の管理は2週間に1回の頻度で行った。試験区⑴では地表面の雑草を完全に除去し,残りの試験区 ⑵と(3)ではコーヒー樹周囲の直径lmの範囲を除草した。観測期間中における当地区の最大日雨量は82 mm,最大10分間雨量強度は120mm/hであった。しかし,全降雨のうち14.2%の最大降雨強度が,土壌侵食発生の限界となる降雨強度25mm/hを上回っていた。U S LE式で用いられる降雨係数R (m2 . t/ h a /h )と日雨量X (mm)の間には, R =1.624 (X-10.9)の関係が成り立った。完全除草区の流出率は7〜15%の範囲で変化し,コーヒーの樹冠の成長にともなって減少した。雑草によって地表面を被覆すると地表流出は著しく減少し,Paspalum試験区では3年目の雨季以降の流出率が0%になったのに対し,自然植生の雑草試験区では4年目から流出率が0%になった。土壌侵食量が最大となったのは2年目の雨季の完全除草区で,22.7 t /haであった。被覆植物を導入すると土壌侵食は著しく抑制され,Paspalum試験区では3年目以降から,自然植生の雑草試験区では4年目から土壌侵食が発生しなかった。完全除草区の土壌侵食量は土壌深にして1年間に1.24mmであり,インドネシアの土壌生成速度を下回っていた。
In acid sulfate soils, the material which causes heavy acidity is sulfate ; mainly pyrite (FeS2), produced from the sediments in the sea that contain a lot of organic matter and sulphuric acid and subjected to heavy reductive conditions. In the process of sulphuric acid production, particularly in the primary acidity stage, chemical acidity and bacterial acidity act together. These bacteria (Thiobacillus ferrooxidans and Thiobacillus thiooxidans) contribute a major role as catalysts. The purpose of this study is to investigate the relationship between the acidification process and the temperature and water content conditions through an incubation experimentation. Results are listed as follows :
(i) In 10-50°C temperature conditions, the soil was acidified more strongly at high temperature by chemical oxidation accompanied by high evaporation rate. The acidification was stopped when the quantity of evaporation became lower than the level of soil moisture.
(ii) When keeping the water condition with almost no evaporation, the speed of acidification is slow but continuous. At 30°C and at around —31.0 kPa, the acidity is extremely strong. The above results show that the speed of evaporation influences the acidification process and microstructure of the soil matrix. Drying is also an important factor that influences acidity due to bacterial activity.
Effects of different initial moisture conditions on the formation of wetted soil volume under micro-irrigation were investigated in a layered soil having a plow layer and a subsoil layer with macropores. Water was applied at a constant rate of 100 cm3 min1 for 720 minutes using a perforated tube covered with vinyl mulch. Water application efficiency was also evaluated from soil water storage and infiltration loss caused by bypass flow. In the case of initially dry condition, the wetting front was spread in an elliptical shape with the major axis in the downward direction. In the case of wet condition, however, the horizontal boundary between the plow layer having high saturated hydraulic conductivity and the subsoil layer having low one affected infiltration. Water reached to the boundary flowed horizontally and infiltrated into the plow layer from the bottom. The amount of water infiltrated vertically was more than that of horizontally infiltration in an imaginary main root zone. Infiltration loss beyond the root zone due to bypass flow was accounted for 89% in dry condition and 75% in wet condition for 720 min respectively. After 70 min irrigation, the rate of storage was unchanged in both the dry and the wet conditions. In view of water application efficiency, irrigation period exceeding about 70 minutes is inadequate.
In acid sulfate soils, the heavy acidity is caused by the oxidation of sulfate, mainly pyrite (FeS2). In the primal acidify stage chemical and bacterial acidify work together. These bacteria (Thiobacillus ferrooxidans, Thiobacillus thiooxidans) have a huge role as a catalyst. This studies were examined the effects of drying on bacterial acidification, mainly on Thiobacillus ferrooxidans, and the formation of iron in the soil by the incubate experimentation. Five water content stages were set (1.30〜0.25kg/kg) by drying process in the incubator at 30°C for water adjustment. After the treatment, these samples were contained in the each bottles and started to culture at 30°C. Thiobacillus ferrooxidans propagated rapidly in the water content of Liquid Limit (LL)〜Plastic Limit (PL). Near the water content of PL, the propagation of the bacteria became slow, and they became decline near by the water content of Shrinkage Limit (SL). Amorphous - free - iron increased at the primal of acidification and changed for crystallized - free - iron. Around SL, the water content that the soil pH was stable, the propagation of the bacteria became slow, and the formation of iron became stable. In the range of water content 1.00〜0.70kg/kg (LL〜PL), the samples acidified strongly.