Field experiments were conducted in a randomized complete block design with three replications in an experimental field of ICAR-Indian Institute of Horticultural Research (IIHR), Bangalore during the kharif season of 2015 and 2016. The experimental station is situated at an altitude of 890 m above mean sea level and 13o58" North latitude and 77o29" East longitudes. The nine treatments of the experiment consisted of different combinations of organic manure (FYM), bio-fertilizers and chemical fertilizers (NPK): T1- (FYM (10 t/ha) +100% recommended N through FYM), T2 - (FYM (10 t/ha) + 100% recommended N through FYM + Arka Microbial Consortium @ 5 kg/acre), T3 - (FYM (10 t/ha) + 75% recommended N through FYM), T4 - (FYM (10 t/ha) + 75% recommended N through FYM + Arka Microbial Consortium @ 5 kg/acre), T5 - (FYM (10 t/ha) + 50% recommended N through FYM), T6 - (FYM (10 t/ha) + 50% recommended N through FY+ Arka Microbial Consortium @ 5 kg/acre), T 7 - (recommended FYM (10 t/ha) only), T 8 - (recommended NPK(160:80:80 kg/ha) only), and T9- (recommended FYM (10 t/ha) + recommended NPK (160:80:80 kg/ha). Estimated N content of FYM used in this experiment was 0.64%. Arka Microbial Consortium (AMC) is a carrier-based product which contains N Fixing, P & Zn solubilizing and plant growth promoting microbes as a single formulation. After 15 days of transplanting, recommended dose of AMC @ 5 kg/acre was applied at 2 cm deep to individual plants in treatments T2, T4, T6 and immediately covered by soil. Similar method of application was followed for ratoon crop after harvest of main crop.

The land was brought to a fine tilth by ploughing and harrowing. The experimental site was divided into plots having dimensions of 4.8 m long and 4.0 m wide with the spacing of 40 cm between the plants and 60 cm between the rows. There was a space of 0.5 meter between plots and 0.5 meter between replications. Basil seeds were sown in two nursery beds of 6.0 m in length with 0.1 m in width and 10 cm height. Forty days old (40 days) healthy and uniformly rooted seedlings of sweet basil were transplanted to the field. Weeding was done manually and drip irrigation was given daily for half an hour during the early stages of the crop and subsequently irrigation was given depending on the soil moisture condition.

Five plants were randomly selected from each plot for recording the observations and the crop was harvested at full bloom stage before setting the seed. Fresh herbage harvested from each plot was weighed and converted to per hectare and expressed in tonnes (t).

Microbial population of the soil under different treatments was enumerated by standard plate count technique. Total bacterial count in soil was determined by serial dilution method. For the study, initial soil samples prior to the start of the experiment and after harvest were collected from the surface layer (0-15 cm) according to different treatments with three replications. Exactly 5 gm of soil sample was taken into 50 ml of sterile distilled water and shaken for 15 minutes. A series of 9 fold dilutions were prepared and 0.1 ml of each dilution was spread on media plates. To enumerate fungal, azotobacter, phosphate solubilising bacteria and actinomycetes population, potato dextrose agar (PDA), Jensen’s media, Pikovskaya Agar and Kenknight media were used, respectively. After 3- 5 days of incubation microbial population was counted following the spread plate technique and expressed as CFU ×103/ g of soil.

The organic carbon content of the soil was estimated by Walkley and Black wet oxidation method as described by Jackson (1973).

The data generated from the experiment were analyzed using SAS 9.3 version of the statistical package (SAS Institute Inc, 2011). Analysis of variance (ANOVA) was performed using SAS PROC ANOVA procedure. Means were separated using Fisher’s protected least significant difference (LSD) test at a probability level of p<0.01

The data in Table 1 indicated significant difference among the treatments with respect to population of heterotrophic free-living N2 fixers (CFU ×10 /g of soil). While, maximum population of the colonies in the soil after cropping (40.66 and 63.33 CFU×103/ g) was recorded in T with application of FYM (10 t/ha) +100% recommended N through FYM + Arka Microbial Consortium @ 5kg/acre during 2015 and 2016, respectively. Whereas, the treatment i.e. T7 recorded the lowest counts (26.13 and 34 CFU ×103/ g) during first and second year, respectively. The addition of organic manure greatly influences the microbial populations which expected to cause changes in the organic matter content of soil that directly influenced microbial dynamics of soil (Deforest et al . , 2012). Application of bio-fertilizer stimulates the native soil microorganisms and reactivates the biogeochemical cycles leading to increase in the organic material that significantly increases the bacterial populations. The results are on line with Watts et al., (2010), Krishnakumar et al. (2005) and Lalfakzuala et al., (2008).

The data on the population of phosphate solubilizing bacteria (CFU ×103/ g of soil) after cropping given in Table 1 indicated that there was no significant difference between the treatments during first year (2015). The application of FYM (10 t/ha) + 100% recommended N through FYM + Arka Microbial Consortium @ 5kg/acre i.e., T 2 recorded the highest population of PSB (5.6 CFU ×103/ g) while, the lowest PSB (3 CFU ×103/ g) was recorded in T8. However, there were significant differences among the treatments in respect to population of phosphate solubilizing bacteria in the soil after cropping was observed during second year (2016). Similar to first year, application of FYM (10 t/ha) + 100% recommended N through FYM + Arka Microbial Consortium @ 5kg/acre recorded the highest population of PSB (6.6 CFU ×103/ g) in soil while, the application of recommended dose of chemical fertilizer recorded the lowest population of PSB (3.9 CFU ×103/ g).Growth of P solubilizing microorganisms is generally accompanied by decrease in pH of the soil (Mishra, 1985). Reduction in pH due to application of FYM along with bio-fertilizer is a result of the production of organic acids which include citric, gluconic, fumaric, malic, oxalic, lactic, 2- ketogluconic, malonic acids etc. (Vassilev, 1996). Although chemical fertilization has resulted in increases in crop yield, this application was not sufficient in triggering a significant improvement in the soil microbial properties. Similar results were obtained by Wang et al., (2011). The addition of fertilizers enriched the soil microbial biomass and soil enzymes by enhancing the soil physico- chemical properties and soil organic matter, especially through the addition of FYM. Root exudates augmented the soil microbes in general by the crop growth and that could explain the increase of soil population at harvest t ime comparing with the initial soil.

Fungal population in the soil after cropping in two years of the experiment was affected significantly by the treatments involving different levels of organic manure with and without bio-fertilizers and inorganic fertilizers. As showed in Table 2 application of FYM (10 t/ha) +100% recommended N through FYM + Arka Microbial Consortium @ 5kg/ acre (T 2) recorded the maximum fungal population (6.4 and 5.33 CFU ×103/ g) in the soil while, the application of recommended dose of chemical fertilizer (T8) recorded the lowest fungal population (3.6 and 2.5 CFU ×103/ g) in the soil after cropping in 2015 and 2016, respectively.Microbial population size and community structure are sensitive to changes in chemical properties of the surrounding soil (Pansombat et al., 1997). Fungi constitute an essential component of biological characteristics in soil ecosystems. Organic carbon level in the soil and precipitation play pivotal role in fungal growth and sporulation. Greater microbial populations in FYM treated plots along with bio-fertilizer as compared to chemically amended plots due to enhancing the organic carbon in the soil. Similar kind of results was reported by Venkateswarlu and Srinivasa Rao, (2000). Application of farm yard manure can be viewed as an excellent way to recycle nutrients and provide a steady source of organic C to support the microbial community resulting in higher fungal populations compared to NPK- treated plots.

Lower fungal population in soil with application of chemical fertilizers alone is attributed to lack of organic amendment input. The microbial population dynamics is governed by interactions between plant type, climate, and management practice. So that, the low temperature that prevailed in the first season could have influenced the proliferation of fungi, which require low temperature for their growth; Song et al. (2007) indicated that difference in the establishment of field leads to alteration of microbial communities.

The data in Table 2 on actinomycetes population of the soil after cropping (CFU ×103/ g of soil) indicated significant differences between the treatments. The highest population of actinomycetes (29.93 and 44.56 CFU ×103/ g of soil) was recorded in T 9 with application of recommended NPK (160:80:80 kg/ha) + FYM (10 t/ha) during 2015 and 2016, respectively, while application of recommended dose of FYM (10 t/ ha) in T7 resulted in minimum population (20 and and 30.5 CFU ×103/ g of soil) during 2015 and 2016, respectively.

Actinomycetes are one of the predominant members of soil microbial communities and they have beneficial roles in soil nutrients cycling and agricultural productivity (Elliot and Lynch, 1995). Organic matter, salinit y, relative moisture, temperature, pH and vegetation are important factors which control abundance of actinomycetes in soil (Mcarthy and Williams, 1992). The density of actinomycetes is opposite to the hydrogen ion concentration, that could justify increasing its population with application of NPK along with FYM (Alexander, 1977) While, increasing the colony’s in the second season comparing to first one due to a relatively low level of moisture, this property of actinomycetes might be due to their sporulation capability under stress conditions (El- Tarabily and Sivasithamparam, 2006).

Table 1
Heterotrophic free-living N2 fixers and phosphate solubilizing bacteria population (CFU ×10 / g of soil) as influenced by different levels of N through FYM, bio-fertilizers and inorganic fertilizer

Table 2
Fungal and Actinomycetyes population (CFU ×103/ g of soil) as influenced by different levels of N through FYM, bio-fertilizers and inorganic fertilizer

The treatments effect on organic carbon per cent in the soil are presented in Table 3. Application of FYM (10 t/ha) +100% recommended N through FYM + Arka Microbial Consortium @ 5kg/acre i.e., T2 recorded the maximum organic carbon (0.63 and 0.66 %) in the post-harvest soil samples collected during 2015 and 2016, respectively. While, the minimum value (0.52 and 0.53%) was recorded in T8 with application the recommended dose of inorganic fertilizer (160:80:80 kg /ha)during 2015 and, 2016, respectively. Organic carbon per cent is fine indicators of soil quality which influence soil function in specific ways (e.g., immobilization–mineralization) and are much more sensitive to change in soil management practices (Saviozzi et al., 2001). The results showed the positive influence of higher level of N through FYM and bio-fertilizer in increasing the organic carbon content that could be because of the effect of FYM and bio-fertilizer in stimulation of soil microbial activity, therefore increasing the C output. Similar results were also found by Halvorson et al., (2002); Su et al., (2006) and Lou et al., (2011).

The fresh herbage yield of basil differed significantly between the treatments during two years of the experiment. It is evident from the Table 4 that the application of NPK (160:80:80 kg/ha) + FYM (10 t/ha) i.e., T9 recorded significantly the highest herbage yield in the main crop (41.59 and 38.31 t/ha) and ratoon (20.97 and 17.77 at/ha) during kharif 2015 and 2016, respectively. The lowest fresh herbage yield per hectare was recorded with recommended dose of FYM alone in the main crop (28.36 and 17.49 t/ha) and in ratoon (12.59 and 8.93 t/ha) during first and second year, respectively. Similar trend was also reflected in total herbage yield of basil. Application of NPK (160:80:80 kg /ha) + FYM (10 t/ha) i.e., T9 recorded significantly the highest total herbage yield (62.56 and 56.08) while, the lowest value (40.95 and 26.42 t/ha) was recorded in T7 during individual years.

Table 3
Organic carbon content (%) in the soil as influenced by different levels of N through FYM, bio-fertilizers and inorganic fertilizer

Table 4
Fresh herb yield (t/ha) of basil (Ocimum basilicum L.) as influenced by different levels of N through FYM, bio-fertilizers and inorganic fertilizer

Application of inorganic fertilizers are expected to release greater quantity of nutrients particularly N, P, K at a faster rate and higher level and there by greater uptake by the plants which resulted in higher growth and yield parameters. On the other hand application of FYM along with inorganic fertilizer release nutrients after mineralization. Such controlled but regulated supply of nutrients increased uptake N, P, K which in turn, brought about higher growth and yield. Increase in the yield parameters with combined use of organic and inorganic application reported in earlier reports of Joy et al. (2005) in black musli, Kothari et al. (2005) in Spilanthus acmella, Rajendran and Gnanavel (2008) in Aloe vera and Ravikumar et al. (2012) in coleus. Organic amendments show a slower nutrient release pattern than mineral fertilizer but facilitate an increased soil organic matter (SOM) content (Pinitpaitoon et al., 2011). Although Vanlauwe and Giller (2006) claim that organic resources are not sufficient enough to supply crops with the required nutrients, the increased SOM is enhancing productivity due to the improved soil properties (Watson et al., 2002). Similar results were obtained by Mohamad et al. (2014) and Asieh (2012).