Influence of long term application of fertilizers on soil organic matter content.

The soil samples were collected from the soils of Long Term Fertilizer Experiments that have been fertilized for the last 25 years from a) control plot (receiving no fertilizer/FYM); b) plot receiving 100% NPK fertilizer application and c) Plot receiving 100% N in the form of FYM, to study the changes in nature and amount of organic matter and these were compared with a similar adjacent soil under natural vegetation. The soil samples were collected from 0-15 cm (surface) and 15-30 cm (sub surface) layers. The humic substances were fractionated by Tyurins method. The total N and available N content was determined and C/N ratio was calculated. The humic acid was isolated, purified and analyzed for oxygen containing functional groups and spectral properties. It was observed that the soil under natural vegetation had a higher total N while the soil under continuous cultivation had higher available N. The C/N ratio of soil under natural vegetation was high compared to the soil under continuous cultivation. These values were high in surface layers compare to sub surface layers. The oxygen containing functional groups in Humic acid (HA) were high in soil under natural vegetation compared to the soil under continuous cultivation. The total acidity and COOH content of surface layers was high compared to sub surface layers in soils under both situations. Phenolic OH groups of subsurface layer was high in soil under natural vegetation and continuous cultivation indicating that this group in sub surface layers and high potential for interaction with clays and metal ions. The higher contents of total acidity and –COOH groups in soil under natural vegetation could be attributed to the difference in chemical composition and molecular weight of humic substances. The E4/E6 ratios of HA’S were less than 5.0 indicating high degree of condensation of aromatic humic acid. The potentiometric titrations of humic acids with standard 0.1 N NaOH indicated a gradual increase in pH with the addition of base thus indicating a high buffering capacity of humic acid. The potentiometric titration curves were sigmoid in nature indicating an apparent mono basic character and had a single break suggesting weak acidic character of humic acids. The conduct metric titrations of humic acids showed a slow increase in the initial stage followed by a steep increase at later stages. The apparent pKa values calculated from potentiometric titration data using Henderson-Hassebalch equation varied from 5.24 to 5.68.

Adsorption of atrazine on selected vertisols and alfisols.

Adsorption of atrazine was studied on two Vertisols and two Alfisols varying in their physicochemical properties. Soils were equilibrated with various concentrations of atrazine using batch techniques. Adsorption affinity for atrazine was approximated by Freundlich constant (Kf), which is a measure of the strength or degree of adsorption. Kd values were calculated for atrazine despite some non-linearity in adsorption on both Vertisols and Alfisols. The Kd values are greater for Vertisols than Alfisols and increased with organic carbon content. Correlations were worked out between extent of adsorption and soil properties and were positively correlated with organic carbon (r = 0.688), clay content ( r = 0.712) and clay + organic carbon (r = 0.708). KOC values were calculated taking into account the organic carbon content for both Vertisols and Alfisols.

Evaluation and selection of elite plant growth-promoting rhizobacteria for suppression of sheath blight of rice caused by rhizoctonia solani in a detached leaf bio-assay.

Sheath blight (ShB) of rice, caused by Rhizoctonia solani, is one of the most important rice diseases worldwide. The objective of this study was to screen selected plant growth-promoting rhizobacteria (PGPR) strains for suppression of ShB under controlled conditions. Sclerotia of R. solani were produced on PDA and immature sclerotia (< 5-day-old) were harvested. Leaves of 60-day-old rice plants grown under greenhouse conditions were used to screen PGPR strains by detached leaf assay. Leaf sections of 8 cm in length were cut and placed in Petri dishes, inoculated with immature sclerotia, and incubated in a growth chamber. Approximately 70 PGPR strains were screened. The disease was quantified by the Relative Lesion Height (RLH) method. Among 70 strains, 31 significantly suppressed the RLH of ShB lesions compared to the control. Among these, Bacillus subtilis strain MBI 600 resulted in greatest suppression of ShB disease severity under the conditions tested.

In-vitro antagonistic potential of pseudomonas fluorescens isolates and their metabolites against rice sheath blight pathogen, rhizoctonia solani.

Sheath blight of rice caused by Rhizoctonia solani is an economically important disease affecting rice production. Eight Pseudomonas fluorescens strains were isolated from rhizosphere of rice seedlings collected from Andhra Pradesh and Tamilnadu. These strains were characterized with PCR based RAPD technique and tested for their in-vitro antagonistic activity against R. solani. Crude metabolites from one particular isolate of P. fluorescens (P. f 003) were extracted with organic solvents such as ethyl acetate and petroleum ether and these were tested against R. solani. Commonly used fungicides in rice sheath blight disease management such as hexaconazole, carbendazim, copper oxy chloride and mancozeb at 150 ppm were screened against mycelial growth of R. solani using poisoned food technique. All the strains tested were exhibited antagonistic activity against R. solani. One isolate, P.f 003 gave 78% inhibition compared to control. All the fungicides and crude extracts of P.f.003 inhibited the mycelial growth of R. solani. Highest inhibition was recorded with hexaconazole and ethyl acetate crude metabolite extract. The results offer a scope for integrating P. fluorescens with chemical fungicides for control of sheath blight of rice.

Isolation of siderophore- producing strains of rhizobacterial fluorescent pseudomonads and their biocontrol against rice fungal pathogens.

Rice blast caused by Magnaporthe grisea and sheath blight caused by Rhizoctonia solani are the major diseases affecting the rice production. Application of beneficial bacteria as seed or seedling root dip to protect these diseases may be an alternative strategies to chemical control. In this study, fluorescent Pseudomonads isolated from rice seedlings were used to screen for their antagonistic ability and siderophore mediated antibiosis under in-vitro conditions against these pathogens. Among 10 isolates, strain P.f 003 gave significantly higher inhibition of mycelial growth of M. grisea and R. solani. Strains of P.f 001, P.f 003, P.f 005 and P.f 007 produced siderophores when grown on Fe deficient and Fe fortified King’s B medium. These strains again tested for their in-vitro antagonistic activity against M. grisea and R. solani on King’s B media with or without FeCl3. Our results showed that all these strains significantly reduced the growth of M. grisea and R. solani with FeCl3 in the media compared to without FeCl3. Strain P.f 003 activity was superior compared to other strains evaluated.