Colloid-Surface Characteristics and Amelioration Problems of Some Volcanic Soils in West Sumatra, Indonesia

Andisols from elevational transects at Mt. Marapi and Mt. Pasaman in West Sumatra, Indonesia were studied to characterize their physico-chemical and mineralogical properties. These soils are developed under audic, isothermic and isohyperthermic climatic regime. They have dark epipedons with high contents of organic carbon and low bulk densities « 0.9 Mg m-3). All the nine pedons studied were found to meet the physical and chemical criteria of andic materials. Major minerals in the sand fraction are quartz, plagioclase, hornblende, augite, hypersthene, olivine and volcanic glass. Some of the volcanic glass is coated with amorphous materials. Allophane, cristobalite, feldspars and halloysite are maj or minerals in the clay fraction. Some soils contain imogolite. Halloysite exists as tubular crystals. Gibbsite is found in Mt. Pasaman soils, while opaline silica is present in the surface horizons of Mt. Marapi soils

MORFOLOGI DAN KLASIFIKASI TANAH

Pengetahuan tentang bentuk luar atau Morfologi tanah merupakan dasar untuk mengenal jenis-jenis tanah yang ada. Keragaman jenis tanah ini dipelajari dalam ilmu klasifikasi tanah. Ilmu ini merupakan ilmu dasar bagi setiap mahasiswa Jurusan Tanah. Dengan diketahuinya nama suatu tanah, berarti sifat dan ciri dasar dari tanah dapat dipahami agar tanah yang merupakan sumber daya alam yang sangat berharga ini dapat digunakan secara optimal baik untuk sektor pertanian maupun nonpertanian. Ilmu klasifikasi tanah merupakan ilmu yang bersifat dinamis dengan tingkat perkembangan yang cepat sejalan dengan kemajuan ilmu pengetahuan lainnya. Perkembangan ini harus diikuti dengan seksama terutama yang dilakukan oleh ahli ilmu tanah dari Amerika Serikat dan Eropah. Tim survey tanah (Soil Survey Staff) dari Amerika selalu memperbarui sistem taksonomi tanah setiap 2 tahun sekali. Sementara ahli ilmu tanah Eropah telah menyusun dan memodifikasi sistem FAOUNESCO menjadi World Reference Base for Soil Resources. Diharapkan buku ini dapat menjembatani mahasiswa dalam memahami buku teks klasifikasi tanah yang umumnya ditulis dalam bahasa Inggris

KURVA SORPSI FOSFAT MENURUT LANGMUIR DAN FREUNDLICH SEBAGAI PENDUGA KEBUTUHAN PUPUK FOSFAT PADA ANDISOLS SUMATERA BARAT

Andisols are among the most productive soils in the world, but these soils also strongly retained phosphate. The high capacity for phosphate sorption in Andisols is due to their high content of active Al and Fe compounds. The objectives of this study were to assess the effect of soil properties on P sorption, buffer capacities and requirements of some benchmark soil profiles in Mt. Marapi and Mt. Pasaman. The P sorption characteristics of Andisols from West Sumatra were described using Langmuir and Freundlich equations. The P sorption maximum in Mt. Pasaman soils was higher than in Mt. Marapi soils. The most important soil properties affecting P sorption and buffer capacities were oxalate Si and Fe, Al (Sio Feo and Alo), dithionite Al (Ald), pyrophosphate Al (Alp), which explained 89% of the variance in sorption maxima and 83% of P buffering indices. Allophane, Alo and Ald were positively correlated with P sorption maximum. Contrary to general assumption that organic matter had an inverse relation with P sorption maxima, in this study, however, there was no trend observed between P sorption maxima and organic carbon. Organic C has poor relationship with this parameter. Andisols from Mt. Marapi and Mt. Pasaman need P input between 320 to 7,800 mg P kg-1 to maintain the P level in soil solution at 0.2 mg kg-1