Determination of physical characteristics of Ultisol was aimed to evaluate the dynamics of the soil properties as well as the impact on soil erosion and runoff (RO) during soybean cultivation in a wet tropical area. Soybean was planted within erosion plots (18 m 2 ) at 25% slope in Ultisols Limau Manis (having > 5000 mm annual rainfall). Soil samples for physical properties (soil texture, bulk density, total pore, permeability, aggregate stability,and organic carbon) as well as amount of RO and soil loss were analyzed at 5 different times (stages) during the cultivation. The results showed that there were fluctuations in physical properties of Ultisol during the cultivation. Likewise, the amount of runoff and soil loss also changed during the study. Among the physical properties analyzed, the aggregate stability index of the soil was highly correlated to the amount of RO (R 2 =0.73) and soil loss (R 2 =0.94). The amount of RO and soil loss was controlled by soybean development at the average rainfall intensity (≤ 36 mm/day), not at rainfall intensity (>36 mm/day). Thus, it is suggested not to open heavy clayey and low organic carbon (OC) soils for seasonal crop farming during rainy season in wet tropical areas.

Daerah rizosfer tanaman merupakan daerah yang kaya mikrob, salah satunya ialah kelompok cendawan. Penelitian ini bertujuan mengisolasi cendawan rizosfer dari tanaman krisan sehat dan menguji potensinya sebagai agens hayati untuk pengendalian Fusarium oxysporum penyebab penyakit layu. Isolasi cendawan rizosfer dilakukan dengan metode pengenceran berseri pada medium agar-agar dekstrosa kentang. Ada 6 isolat cendawan yang diperoleh dari rizosfer. Uji patogenisitas menunjukkan bahwa 6 isolat tersebut bukan patogen karena tidak menimbulkan gejala penyakit layu pada tanaman krisan. Hasil seleksi kemampuan antagonisnya menunjukkan bahwa 4 isolat mempunyai kemampuan daya hambat (46.42-51.61%) dan antibiosis terhadap F. oxysporum. Isolat yang memiliki potensi agens hayati ini diidentifikasi sebagai Trichoderma sp. (2 isolat), Penicillium sp., dan Paecilomyces sp.

7 PENGARUH PEMBERIAN BEBERAPA J ENIS BAHAN ORGANIK TERHADAP STABILITAS AGREGAT TANAH ULTISOL LIMAU MANIS Yulnafatmawita, Adr inal, dan Anita Febr iani Daulay Laboratorium Fisika Tanah Fakultas Pertanian Universitas Andalas Abstr actA field research about the effect of some sources of organic matter on aggregate stability of Ultisol was conducted in Limau Manis Padang. The research was aimed to investigate the contribution of different sources of organic matter to soil aggregate stability. Field research was arranged in Completely Randomized Block Design (RBD) consisting of 4 treatments (types of organic matter) and 3 replications (blocks). The treatments were : A = Tithonia diversifolia, B = Straw of Oryza sativa, C = Imperata cylindrica, and D = Leucaena glauca. The dosage for each treatment was 20 ton/ha based on dry weight. The result showed that Tithonia diversifolia gave the best contribution in improving soil aggregate stability of Ultisol Limau Manis for the first three (3) months.

Study about soil organic matter (SOM) leaching in soil profiles of tropical rain forest was conducted in Bukit Pinang-Pinang, the upper footslope of Mt. Gadut in West Sumatra, and in Soil Physical laboratory Andalas University Padang. Bukit Pinang-Pinang is located on 480-640 m asl and geographically is between 100°29'40" and 100°30'20 E" as well as between 0°54'55" and 0°55'45"S. This rain forest area is known as a super wet area because it receives high annual rainfall (>3500 mm). The objective of this research was to measure the amount of SOM leached from top soil into the deeper soil. Undisturbed soil samples were taken from three different land uses, forest, bush, and mixed garden, on the top, middle, and foot slopes. Undisturbed soil samples were leached with distilled water as the amount of rainfall accepted in natural condition. The results showed that, land use change from forest into ecosystems of mixed garden (for over 50 years with 100% soil surface coverage and without any cultivation) and bush land (for over 20 years left) did not cause the SOM content in the profiles decreased. Amount of SOM leached from the top 0-10 cm was found greater from mixed garden, then followed by forest on the upper slope, forest on the middle slope, and bush area.

A research aiming to improve soil aggregate stability of Ultisol in terms of creating sustainable agriculture and environmental development was conducted by applying fresh organic matter (OM)  into soil.  Three types of  OM sources used were Titonia diversifolia, Chromolaena odorata, dan Gliricidia sepium.   This research was conducted in Ultisol Limau Manis Sumbar having annual rainfall > 5000 mm, in 2008.   The fresh OM used was cut, mixed with soil up to 20 cm depth, and then incubated for approximately one month before corn seeds were planted.  Soil was sampled before being treated with OM and then after harvesting corn (4 moths after applying OM).  Soil OM content as well as soil aggregate stability (AS) and several other soil physical properties wer analysed in laboratory.  The results showed that soil OM content and AS increased after one planting period.  Besides that, soil physical properties were also improved.  As soil physical properties became better, corn production was also higher at plot with than without OM application.  Among 3 species of OM added, Tithonia diversifolia gave highest corn biomass.  Level of land slope (0-8% dan 15-30%) did not show significant difference either for biomass or for soil OM content and soil AS.Key Words: marginal soil, green manure, soil aggregate stability, soil organic matter