Ultisols are a major group of marginal soils extensively found in the upland area of Indonesia. To better understand the potential of the Ultisols developed from claystone and sandstone in the Sasamba Integrated Economical Development Area in East Kalimantan, chemical and mineralogical characteristics of 27 Ultisols pedons consisting of 76 topsoil and 118 subsoil samples were investigated. Besides analysis and interpretation of data, relationships of several soil characteristics were constructed using simple regression. The results indicated that Ultisols showed acid to very acid reaction, had low content of organic matter and low base saturation. Soils generally exhibited net negative charge, and the point of zero charge was reached at pH 3.6. Both potential and available phosphates were low, and there was a trend that amorphous aluminum was responsible for phosphate fixation. The low content of exchangeable potassium in topsoil and subsoil indicated a positive correlation with potential potassium. Clay mineral was composed chiefly of kaolinite, with small amounts of illite, vermiculite, and quartz. The domination of kaolinite and low organic matter content causes the soils to have low cation exchange capacity. Soil management in this area should be focused on building up and maintaining soil fertility, and applying appropriate soil conservation techniques to minimize erosion. To obtain sustained productivity, various soil amendments including the use of farm and/or green manure, liming with agricultural lime, and application of rock phosphate and K fertilizers were highly recommended.
In Indonesia, peat soil in the highland area is only found in the plateau of Toba Highland, North Sumatra, and its morphologi-cal, physical, and chemical properties have been studied. Four profiles of peat soil were made in the field and eighteen samples were taken and analyzed in laboratory. Most of the properties of highland peat were similar with lowland peat. The differences were in their Al saturation, P retention, and andic soil properties. High Al saturation was resulted from weathering of primary minerals under the acid condition. Andic soil properties were caused by amorphous materials resulted from weathering of volcanic materials in the peat, and P retention was related with the presence of amorphous materials. Phosphate retention of the highland peat was classified high (>60%) and positively correlated with amorphous materials. The presence of amor-phous materials was confirmed by ammonium oxalate extract-able Alo+½Feo. The presence of andic soil properties in the highland peat created problem in their classification. To provide a place for andic soil properties that influence the P retention in the highland peat soil, the soil classification of peat soil should be modified by adding andic modifier at subgroup level. <br /><br />
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