MSc

An investigation was carried out during April 1996 to December 1997 at the
College of Forestry, Kerala Agricultural University, Vellanikkara to study the nodulation
characteristics of Acacia mangium Willd. plantation and to isolate and compare the
efficiency of rhizobia from different agroclimatic regions of Kerala. The efficiency of these
rhizobial isolates under varying conditions of soil fertility and acidity were also evaluated.
Survey conducted throughout the state showed rhizobial association and nodulation
in all mangium plantations. The soil of these plantations were acidic with pH values
ranging from 4.9 to 6.2.
Experiments conducted to study the rhizobial characteristics indicated that, the fast
growers (Rhizobium spp.) as well as the slow growers tBradyrhizobium spp.) are involved
in nodulation of A. mangium in Kerala.
The efficiencies of different rhizobial isolates were studied in polypots using
sterilized and unsterilized soil - sand mixture. Two other polypot experiments were
conducted to study the response of the rhizboial isolates to liming and Nand P application.
The experiments were laid out in C.R.D with three replications. Seedlings raised in
unsterilized soil performed better as compared to seedlings in sterilized soil. However, the
response to inoculation was more prominent in sterilized soil. In sterilized soil, seedlings
inoculated with Rhizobium isolates from mangium ;MRh-3 (Mananthavadi), MRh-
2(Perambra), MRh-13 (Trivandrum), MRh-6 (Vellanikkara) and MRh-lO (Mavelikkara)
and commercial mangium culture (" Agroforester - Group A") showed superiority in
nodulation and growth characteristics while the isolates from A. auriculijonnis and commercial
culture of A. auriculijonnis, ("Agroforester - Group C") were not much effective.



Seedling - response to inoculation varied with rhizobial isolates, and mangium showed its
specificity in Rhiozbium requirement for nitrogen fixation.
The application of lime adversely affected nodulation capacity of the rhizobial
isolates of mangium from Kerala, indicating the adaptations of these isolates to acidic soils.
The nodulation efficiency of commercial culture "Agroforester - Group A" was improved
by lime application, indicating that this strain is adapted to soils of higher pH.
The nodulation and nitrogen fixing efficiency of the isolates were improved by
supplementating fertilized N and P. The need for a starter dose of Nand P for effective
nodulation and efficient Ny-fixation in mangium seedlings. MRh-6 (Vellanikkara)
performed poorly in unfertilized soil, but showed superiority when applied with N and P
fertilizers, indicating the sensitivity of this isolate to site conditions. As the rhizobial
isolates from different agroclimatic regions of Kerala responded differently under varying
soil pH and doses of Nand P, inoculation of selected strains of rhizobia depending on site
conditions may be needed for successful establishment of mangium seedlings.