<p>The physical and chemical properties of the growing medium for <em>root trainer</em> of rubber are critical to support plant growth. This study aimed to obtain the best medium for growing rootstock rubber seedlings in the <em>root trainer</em>. Rubber clone PR 300 seeds were germinated for 21 days, then raised in <em>root trainer</em> pots for five months. The treatments evaluated were a mixture of <em>cocopeat</em>, Rawapening peat, and manure at different ratios. The experiment was designed in a Randomized Block Design, three replications, 30 plants per replicate, totaling 720 plants. The observed variables were N, P, K, Mg, C-organic content, pH, and cation exchange capacity of <em>cocopeat</em>, peat, and manure, as well as plant growth (height, stem diameter, wet weight, and dry weight). ANOVA used for analyzing data, followed with the <em>Duncan Multiple Range Test</em> at 5% level. The results showed that there were three best <em>root trainer</em> media for PR 300 rubber rootstock seedlings were a mixture of <em>cocopeat</em> + peat (1:1), <em>cocopeat</em> + peat + manure (1:2:1), and <em>cocopeat</em> + peat + manure (2:1:1). The growth and compactness of the roots are outstanding, so the seedlings are easily removed from the <em>root </em><em>trainer</em> pots without significant damage. Based on the initiation and cost, the best <em>root trainer</em> medium is a mixture of <em>cocopeat</em> + peat + manure (1: 2:1). It is necessary to study the use of other types of peat soils apart from Rawapening. Also, it needs to be observed further plant growth after inoculation with the stem and after planting in the field.</p><p><strong>Keywords </strong>: Planting medium, Rawapening peat, rubber clone PR 300.<em></em></p><p><em><br /></em></p><p><strong>Abstrak<br /></strong></p><p>Sifat fisik dan kimia media tanam di dalam wadah <em>root trainer</em> bibit karet sangat penting untuk mendukung pertumbuhan tanaman. Penelitian ini bertujuan mendapatkan media terbaik untuk pertumbuhan bibit batang bawah karet di dalam <em>root trainer</em>. Benih klon karet PR 300 dikecambahkan selama 21 hari pada bedeng pengecambahan kemudian dipindah-tanam dan dibesarkan selama 5 bulan dalam pot <em>root trainer</em>. Perlakuannya yaitu campuran <em>cocopeat</em>, gambut Rawapening, dan pupuk kandang (pukan) pada berbagai ratio, dan kontrol. Penelitian dirancang secara Acak Kelompok, tiga ulangan, masing-masing 30 tanaman, sehingga totalnya 720 tanaman. Variabel yang diamati yaitu kandungan N, P, K, Mg, C-organik, pH, dan kapasitas tukar kation dari <em>cocopeat</em>, gambut, dan pupuk kandang, serta pertumbuhan tanaman (tinggi, diameter batang, bobot segar, dan bobot kering). Data dianalisis dengan ANOVA, kemudian <em>Duncan Multiple Range Test</em> pada taraf 5%. Hasil penelitian menunjukkan bahwa ada tiga macam media <em>root trainer</em> terbaik untuk bibit batang bawah karet klon PR 300 yaitu campuran <em>cocopeat </em>+ gambut (1:1), <em>cocopeat </em>+ gambut + pukan (1:2:1), dan <em>cocopeat </em>+ gambut + pukan (2:1:1). Pertumbuhan dan kekompakan akar sangat baik sehingga bibit mudah dilepas dari <em>root tainer</em> tanpa kerusakan berarti. Berdasarkan kepraktisan dan biaya maka media <em>root trainer</em> terbaik adalah campuran media <em>cocopeat </em>+ gambut + pukan (1:2:1). Perlu dilakukan kajian mengenai pemanfaatan tanah gambut jenis lainnya selain dari Rawapening. Selain itu, juga perlu diamati lebih lanjut pertumbuhan tanaman setelah diokulasi dengan batang atas maupun setelah ditanam di lapangan.</p><p><strong>Kata kunci </strong>: Gambut Rawapening, klon karet PR 300, media tanam.</p>