Karbon (C) organik tanah yang merupakan bagian utama dari bahan organik tanah mengalami penurunan sebagai akibat perubahan-perubahan penggunaan lahan dari kondisi alami menjadi lahan-lahan pertanian. Penurunan C organik tanah menjadi semakin besar karena masukan bahan organik yang rendah dan bila penurunan ini. berlangsung terus menerus  maka pada akhirnya menyebabkan degradasi tanah. Artikel ini membahas peranan, dekomposisi dan struktur dari bahan organik tanah dalam hubungannya dengan dinamika fraksi C organik tanah pada pengelolaan lahan yang berbeda di daerah tropis basah. Perubahan penggunaan dan pengelolaan lahan menyebabkan terjadinya perubahan C organik total dan fraksi-fraksi C organik labil dan stabil tanah. Beberapa penelitian telah menunjukkan bahwa fraksi C organik labil seperti C organik partikulat lebih responsif terhadap perubahan-perubahan dari pengelolaan tanah dan merupakan suatu indikator yang sensitif dari kualitas tanah. Disamping itu fraksi C organik stabil seperti asam humat yang mengalami perubahan karena praktek-praktek  pengelolaan tanah dapat digunakan untuk menilai kapasitas potensial tanah sebagai penyimpan karbon. Perubahan-perubahan penggunaan lahan dan pengelolaan tanah mempunyai pengaruh negatif dan positif terhadap C organik total, fraksi C organik labil (C organik partikulat) dan fraksi C organik stabil (asam humat). Praktek-praktek pengelolaan yang dapat mempertahankan dan memperbaiki fraksi-fraksi C organik tanah  meliputi sistem agroforestri,  aplikasi pupuk organik, mulsa dan pengembalian sisa tanaman ke dalam tanah. Fraksi C organik partikulat dan asam humat menunjukkan perubahan-perubahan yang lebih besar dibandingkan C organik total akibat perubahan-perubahan penggunaan lahan dan praktek-praktek pengelolaan tanah di daerah tropis basah.Key words : Fraksi C organik, penggunaan lahan, tropis basah

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.

Optimalisasi pemanfaatan bahan organik akan dapat dilakukan dengan mengetahui kualitas bahan organik tersebut. Kualitas bahan organik tidak hanya ditentukan oleh kandungan hara, namun juga kecepatan pelapukannya yang dilihat dari kandungan lignin, nisbah C/N, C/P, dan C/S.  Pengujian terhadap 24 jenis sumber bahan organik di Kota Padang ini telah dilakukan pada bulan November 2016-April 2017 di laboratorium kimia tanah Fakultas Pertanian Universitas Andalas. Penelitian ini bertujuan untuk mengelompokkan bahan organik berdasarkan kandungan haranya, menentukan bahan organik yang dapat dimanfaatkan sebagai sumber biochar, dan mengkaji karakteristik dari biochar yang dihasilkan.  Parameter analisis bahan organik meliputi kadar air, C-total, N-total, P-total, K-total, C/N, C/S, C/P, dan kadar lignin.  Parameter analisis biochar meliputi kadar air,  kandungan N-total, C-total, P-total, K-total, S-total C/N, dan kadar abu. Dua puluh empat (24) sumber bahan organik yang tersedia, dapat disimpulkan bahwa berdasarkan kandungan haranya bahan organik  ini diklasifikasikan menjadi 2 yaitu : a)  bahan organik berkualitas tinggi, yang terdiri dari:  tithonia, krinyuh, gamal, kiambang, widelia, paku resam, azola, enceng gondok, alang-alang, jerami kacang tanah, jerami kedele, jerami jagung, kulit kakao, pupuk kandang ayam, pupuk kandang sapi, dan kulit jengkol, dan b) bahan organik berkualitas rendah yang dinyatakan berdasarkan kadar lignin, C/N, C/P dan C/S  yang tinggi, yang terdiri dari : jerami padi, sekam padi, pelepah kelapa sawit, tongkol jagung, tandan kosong kelapa sawit, ampas tebu, ampas kelapa sawit, dan serbuk gergaji. Bahan organik berkualitas rendah yang dioptimalkan untuk dimanfaatkan sebagai bahan biochar adalah jerami padi, sekam padi, dan tandan kosong kelapa sawit, dengan karakteristik kimia biochar yang dihasilkan memiliki C-total (28,86%), N-total (1,27%), P-total (0,28%), K-total (0,76%), S-total (0,21%), kadar abu (25,42%), C/N (22,72), C/P (103,07), dan C/S (137,43).Kata kunci : bahan organik, biochar.

This research was aimed to study soil physico-chemical properties at four slope positions under oil palm plantation in Dharmasraya, West Sumatra, Indonesia. Soils were sampled at 0-20 cm soil depth from 4 different slope positions (upper, middle, lower slope, and the bottom or flat area). The parameters analyzed were soil texture, SOM, bulk density, total pore, hydraulic conductivity, soil water potential (physical characteristics) as well as soil pH, CEC, Al-exchangeable, basic cations (Ca, Mg, K), N, and P (chemical characteristics). The results showed that the bottom area had better soil physicochemical properties than the others. SOM increased by 33%, total pore by 19%, void ratio by 47%, plant available water (PAW) by 28%, soil pH-H 2 O by 41%, CEC by 171 %, total-N by 170 %, and P-potential by 114 %, in contrast, soil BD and exchangeable-Al were lower (20 % and 96 %, respectively) in the bottom than in the sloping land. The middle slope had the poorest soil physico-chemical properties after 26 years of forest conversion into oil palm plantation. All sites had clay texture, the clay content increased (R 2 =0.93) by lowering slope position, and so did SOM content (R 2 =0.86), soil CEC (R 2 =0.93), and soil total-N values (R 2 =0.76).

A research entitled the effect of various sources of organic matter on improvement of N,P,K Nutrient of Ultisol was conducted in Limau Manis Padang. The research was aimed to investigate the contribution of different source of organic matter to improve N,P,K content in soil. Field research was arranged in Randomized Block Design (RBD)  consisting 5 treatments and 3 replications (blocks). Treatments are : A = no organic matter added, B = Tithonia, C = Paddy Straw, D = Imperata cylindrica, and E = Leucaena glauca. The dosage for each treatment was 20 ton/ha. The result showed that tithonia gave the best contribution in improving N,P,K content of Ultisol Limau Manis.Keywords :organic matter, N, P, K of Ultisols.