Mineralogical and chemical properties of two acid sulfate soils as affected by lime and green manure application
1995
Sunthorn Poolpipatana | Hue, N.V. (King Mongkut's Inst. of Technology Lat krabang, Bangkok (Thailand). Faculty of Agricultural Technology. Dept. of Soil Science)
Incubation study was conducted to quantity the effects of lime or green manure on chemical and mineralogical properties of two acid sulfate soils (Bg series, Typic Sulfaquents; Ra series, Sulfic Tropaquepts) from Thailand. Lime rate was 6 Mg/ha CaCO3; and sesbania green manure, 40 Mg/ha. The temporal changes in chemical and mineralogical compositions of the soils were determined at 2, 42 and 90 days after incubation. In the unamended soils, strong and rapid acidification occurred, resulting in pH decreases to 2.8-3.2. By contract, liming and green manuring increased soil pH, EC, total exchangeable bases, and reduced Al+3 saturation percentage. The reduction in Al+3 was assumed to be the precipitation of soluble Al+3 from pH increases resulted from lime or green manure addition. X-ray diffraction analysis showed no detectable change in clay minerals by either amendment, in spite of strong acidification occurring in the control. Detailed mineralogical study of handpicked yellow particles from soils 90 days after incubation showed the presence of jarosite. Soil solution analysis suggested that jarosite might be formed by precipitation of K+, Fe+++ and SO4--. Based on the ion activity product (pIAP) and stability diagram, Al-hydroxy sulfate minerals existed in the acid sulfate soils. The Al+++ activities were pH dependent and apparently controlled by the solubility of and alunite-like mineral having a pKsp of 81.4.
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