Karakteristik Tanah Sawah dari Endapan Lakustrin di Sulawesi

Abstract

Abstrak. Dataran lakustrin di Sulawesi umumnya digunakan untuk lahan sawah, akan tetapi penelitian karakteristik mineral dan sifat-sifat fisik-kimianya masih sedikit dilakukan. Penelitian ini bertujuan untuk mempelajari komposisi mineralogi dan sifat-sifat kimia tanah sawah dari endapan lakustrin yang berasal dari beberapa sumber bahan induk. Sebanyak delapan profil tanah sawah dari endapan lakustrin dari daerah-daerah Tondano, Kotamobagu dan Dumoga (Sulawesi Utara), Limboto dan Paguyaman (Gorontalo), Napudan Bariri (Sulawesi Tengah), dan Sengkang (Sulawesi Selatan) telah diteliti sifat-sifat morfologinya di lapangan dan sebanyak 36 contoh tanah telah dianalisis komposisi mineral fraksi pasir dan liat, serta sifat-sifat fisik-kimianya di laboratorium. Hasil penelitian menunjukkan bahwa komposisi mineral dan sifat-sifat fisik kimia tanah sawah tersebut cukup bervariasi yang dipengaruhi oleh sumber bahan induknya. Tanah sawah Tondano seluruh lapisannya dipengaruhi bahan volkan intermedier, yang mengandung gelas volkan tinggi (54-80%), asosiasi mineral plagioklas-piroksen dan haloisit hidrat-alofan. Tekstur lempung berdebu, kandungan C organik dan KTK tanah tinggi (31-39 cmolc kg-1), dan bereaksi masam (pH 4,5-4,9). Tanah sawah Kotamobagu dan Dumoga juga dipengaruhi bahan volkan intermedier, dengan sedikit gelas volkan (2-5%), asosiasi plagioklas-amfibol-piroksen dan semektit-kaolinit. Meskipun mengandung semektit, KTK tanahnya rendah-sedang (11-23 cmolc kg-1). Berbeda dengan tanah sawah di atas, tanah sawah Limboto, Paguyaman dan Sengkang dipengaruhi oleh rombakan bahan volkan masam, sedimen dan metamorfik, yang didominasi oleh mineral kuarsa, asosiasi feldspar-plagioklas-amfibol, dan mineral liat semektit. Dominasi mineral semektit menyebabkan kandungan basa-basa dan KTK tanah tinggi (25-55 cmolc kg-1) dan bereaksi agak masam-alkalis (pH 5,7-8,0). Tanah sawah Napu dan Bariri didominasi oleh mineral kuarsa dan feldspar, serta asosiasi kaolinit-vermikulit, yang diduga berasal dari rombakan batuan granit-biotit dan sedimen. Hal ini ditandai dengan kandungan K2O total (eks. HCl 25%) sangat tinggi (187-752 mg 100g-1) sebagai hasil dari pelapukan biotit dan feldspar. Tekstur lempung berpasir, kandungan basa-basa dan KTK tanah rendah (2-10 cmolc kg-1) dan breaksi masam (pH 5,2-5,6). Pengaruh bahan induk, terutama bahan volkan telah memberikan dampak positif terhadap sifat-sifat tanah sawah terutama cadangan sumber hara tanah yang cukup tinggi, sehingga kesuburan tanah sawah dapat terpelihara dalam jangka panjang.

 

Abstract. Lacustrine plain in Sulawesi is generally used for paddy fields. However, study on their mineralogical and chemical properties is limited. The aim of the study was to characterize the mineralogical composition and chemical properties of paddy soils derived from lacustrine deposits with different sources of parent materials. Eight paddy soil profiles developed from lacustrine deposits in Tondano, Kotamobagu and Dumoga (North Sulawesi), Limboto and Paguyaman (Gorontalo), Napu and Bariri (Central Sulawesi), and Sengkang (South Sulawesi) have been studied their morphological characteristics in the field and 36 soil samples were collected and analysed for mineralagical composition and physico-chemical properties in the laboratory. Results showed that mineral composition and chemical properties of the paddy soils were varied, depending on the source of parent materials. All layers of paddy soil from Tondano were influenced by intermediary volcanic materials containing high volcanic glass (54-80%), association of plagioclase-pyroxene and hydrated halloysite-allophane. Soil texture was silt loam, high organic C and soil CEC (31-39 cmolc kg-1), and acid reaction (pH 4.5-4.9). Paddy soils from Kotamobagu and Dumoga were also influenced by intermediary volcanic material with little differences in mineralogical compositions, contained few volcanic glass (2-5%), association of plagioclase-amphibole-pyroxene and smectite-kaolinite. Although smectite was present, the soil CEC was low to moderate (11-23 cmolc kg-1). Paddy soils from Limboto, Paguyaman and Sengkang differed from those aforementioned soils. In the former, soils were influenced by weathered acid volcanic materials,sedimentary and metamorphic rocks, where minerlogical compositions were dominated by quartz, association of feldspar-plagioclase, and smectite clay mineral. The dominance of smectite promoted high content of base cations, high soil CEC (25-55 cmolc kg-1) and slightly acid to alkaline reaction (pH 5.7-8.0). Paddy soils from Napu and Bariri weredominated by quartz and feldspar, and association of vermiculite-kaolinite, which probably derived from weathered biotite-granite and sedimentary rocks.. These were indicated by very high K2O (HCl 25% extraction) (187-752 mg 100g-1) deriving from the weathering of biotite and feldspar. Soil texture was sandy loam, low base cations and CEC (2-10 cmolc kg-1), and acid reaction (pH 5.2-5.6). The influence of parent materials, especially volcanic materials have given a positive impact in providing high nutrient reserve for paddy soils to maintain a long term of soil fertility.