SHORT COMMUNICATION Natural compounds enhancing growth and survival of rhizobial inoculants in vermicompost-based formulations Alok Kalra & Mahesh Chandra & Ashutosh Awasthi & Anil K. Singh & Suman Preet S. Khanuja Received: 18 September 2009 / Revised: 17 January 2010 / Accepted: 19 January 2010 / Published online: 17 February 2010 # Springer-Verlag 2010 Abstract The present study demonstrates the usefulness of natural microbial growth-promoting compounds for im- proving the stability and life of vermicompost-based (both granular and its aqueous extract) bioformulations. Granular vermicompost maintained the number of cells of Rhizobium meliloti Rmd 201 up to 5.9×10 8 after 180 days at 28°C compared with 2.1×10 8 in charcoal (powdered), while aqueous extract of the vermicompost supported the 5.6× 10 7 rhizobia numbers even after 270 days. The addition of 25 μL/mL cow urine and 0.01 mM calliterpinone, a natural plant growth promoter, increased the rhizobia number significantly in granular vermicompost and its aqueous extract, respectively. Keywords Vermicompost . Rhizobium . Carrier . Calliterpenone . Cow urine Introduction Inoculation of legumes with Rhizobium is an important management for sustainable agriculture. An efficient inoc- ulation requires more than 1,000 rhizobia per gram of soil (Ben Rebah et al. 2002). To reach this number, it is important to use suitable carrier materials, whose most important characteristics are good water-holding capacity, good aeration, and capacity to support microbial growth and microbial survival. In addition, the carrier should be cheap, easily used, mixable, packageable, and available as powder or granules (Burton 1981; Smith 1992; Bashan 1998; Ben Rebah et al. 2002). Peat is widely used as a carrier for legume inoculants, wherever it is readily available. Other carriers include filter mud (Philpotts 1976), lignite (Kandasamy and Prasad 1971), coal (Crawford and Berryhill 1983), a coal–bentonite mixture (Deschodt and Strijdom 1976), cellulose (Pugashetti et al. 1976), bagasse, wheat straw, a compost of coir dust and soil, charcoal, manure, compost, powdered coconut shells, ground teak leaves, and also combinations of these substances (Tilak and Subba Rao 1978). Usually, peat is the favorite carrier but no large-scale deposits of peat exist in India and therefore any commercial exploitation of peat as a carrier is a remote possibility (Saha et al. 2001). The addition of vermicompost, including that produced from agrowastes, medicinal, and aromatic plants (Chandra et al. 2007), to soil can increase the population of beneficial microbes (Kumari and Ushakumari 2002). Studies con- ducted in our laboratory indicated that the addition of two natural compounds, calliterpenone (CT), isolated from a plant Callicarpa macrophylla and with substitute pattern similar to that of the ent-kaurenoid compound abbeokutone, the precursor of gibberellins, and cow urine (CU) increased the number of Rhizobium and Bacillus by 10-fold when supplemented at 12.5–25.0μL/mL (Kalra et al., unpublished data). The calliterpenone is a plant growth promoter in many mono- and di-cotyledons and herbaceous plant species, and its plant growth-promoting activities are better than GA 3 and many cytokinins tested in in vitro experiments (Singh et al. 2004; Goel et al. 2007). Considering the usefulness of organic manure in supporting the growth of beneficial microbes, the present study investigates the efficacy of granular vermicompost as a carrier material, or as a liquid carrier material, its water extract, on the inoculation of A. Kalra (*) : M. Chandra : A. Awasthi : A. K. Singh : S. P. S. Khanuja Division of Field Laboratories and Organic Farming, Central Institute of Medicinal and Aromatic Plants, (Council of Scientific and Industrial Research), Lucknow 226015 Uttar Pradesh, India e-mail: alok.kalra@hotmail.com Biol Fertil Soils (2010) 46:521–524 DOI 10.1007/s00374-010-0443-2