Introduction

Soil is a renewable natural resource and there is a hidden microbial world of incredibly diverse nature below its surface. The underground environment of a plant is as important for the plant health as the above ground part which contains harmonious friendly microorganisms to normalize the soil health and put pathogenic organisms to stress conditions, thus extend the life span of a soil (Dar, 2010). Interactions among the rhizobacteria, present in rhizospheric soils and the roots of plants have been studied intensively (Kurkcuoglu et al., 2007; Ambrosini et al., 2012; Souza et al., 2013). Rhizobacteria colonize plant roots where they multiply and occupy all the ecological niches found on the roots at all the stages of plant growth (Antoun and Prevost, 2006). Some of the rhizobacteria play a key role in the natural nutrient cycles. Some species of rhizobacteria are capable of N₂ fixation, some mobilizing phosphorus, potassium, and sulphur in accessible forms in the soils. There is a considerable population of P and K solubilizing bacteria in soil, particularly in rhizosphere (Sperberg, 1958). Silicate bacteria were found to dissolve potassium, silicon and aluminum from insoluble minerals (Aleksandrov et al., 1967). The phosphorus and potassium are made available to plants when the minerals are slowly weathered or solubilized (Bertsch and Thomas, 1985). Bacteria, fungi and actinomycetes in the rhizospheric soils of apple trees may be taken as microbial indicators of their health status. Successful identification of an elite microbial strain capable of forming PGPS (Plant Growth Promoting Substances), solubilizing phosphorus, potassium, zinc and other essential minerals quickly in large quantity can conserve our existing resources and avoid environmental pollution hazards caused by heavy application of chemical fertilizers. The chemical indicators of soil health include its reaction (pH), salinity (EC) and the nutrient ion concentration. All the parameters have a significant bearing on physical and biological health of soil and hence on the plant growth.

Keeping in view the adverse effects of agro-chemicals on the soil health of apple orchards and their effect on growth and yield of apple crop, an attempt through the present study was made to assess the biological wealth status of the soils under apple in Kashmir valley.

Materials and Methods

The present investigation was carried out at the Regional Research Station and Faculty of Agriculture, Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir, Wadura Campus. Thirty rhizosphere soil samples of apple trees (var. delicious) were collected from thirty representative orchards of ten selected villages of Baramulla district of Jammu & Kashmir. The collected samples brought in sterilized zip-locked polythene bags to the laboratory were analyzed after due processing. Organic carbon was determined  by Walkley and Black’s wet oxidation method (Jackson, 1967), available nitrogen by Kjeldhal method (Subbaiah and Asija,1956), available phosphorus by Olsen’s method (Muhr et al., 1965), available potassium by flame photometer method (Stanford and English, 1949) and available sulphur by Chesnin and Yein method (1951). The electrical conductivity was determined by EC Bridge (Jackson, 1967) and the pH of the soil was measured by using a digital pH meter (Jackson, 1967). Bacterial, fungal and actinomycetes populations were also calculated by plate count method using colony counter.

Results and Discussion

Perusal of the data presented in Table 1 and  Table 2 revealed that organic carbon (OC) in the rhizospheric soil samples of apple trees ranged between 1.54% and 1.93% with the mean value of 1.80% and standard deviation of 0.10. Available nitrogen was 425.65 to 466.36 kg ha^-1 with mean value of 445.68 kg ha^-1 having standard deviation 10.67. Similarly available phosphorus ranged from 17.45 to 20.25 kg ha­^-1 with the mean of 18.86 kg ha^-1 having the standard deviation of 0.76. The available potassium ranged from 175.66 to 197.18 kg ha­^-1 with mean value of 187.74 kg ha^-1 with standard deviation of 5.38. The results are in conformation with the results found by Subash, and Tahir 2011. The upper limit for available sulphur was recorded as 28.17 kg ha^-1 and the lower limit was 23.11 kg ha^-1. The mean available sulphur content in the soil samples collected from Baramulla district was recorded as 26.09 kg ha^-1 having standard deviation of 1.45. The electric conductivity (EC) of the soil samples were from 0.15 to 0.39 dS m^-1 with mean value of 0.28 dS m^-1 and standard deviation 0.08. The pH recorded of the collected samples was slightly acidic narrowly ranged from 6.3 to 6.6. The viable bacterial population per gram ranged between 77.16 x 10⁶ and 86.38 x 10⁶ with the mean value of 81.96 x 10⁶. The viable fungal population per gram of soil sample ranged between 60.72 x10⁴ and 60.72×10⁴ with the mean value of 58.45×10⁴. The viable actinomycetes ranged between 32.98 x10⁵ and 29.58 x10⁵ with the mean value of 31.11 x10⁵.The results are in conformation with the results shown by Wani et al. 2015.

Table 1: Mean Characteristics of soil samples collected from district  Baramulla

Table 2: One-Sample Statistics of Baramulla District

With the increase in the pH the availability of organic carbon, available phosphorus, potassium and sulphur decreased significantly. At 95% to 99% confidence interval, Organic Carbon, available nitrogen, available phosphorus, available potassium and available sulphur showed positive significant correlation with each other except between available phosphorus and available sulphur (Table 3). These results are in conformity with the results of Sofi et al. 2012. The viable bacterial and fungal population in the rhizosphere of apple showed statistical positive significance with each other and with organic carbon, available nitrogen, phosphorus, potassium and sulphur.

Table 3: Correlations of soil characteristics of Baramulla district

**Correlation is significant at the 0.01 level,
*Correlation is significant at the 0.05 level.

Soil is a dynamic ecosystem that harbours many micro-organisms which are closely related to the plants. Microorganisms play their role in two ways one as pathogens causing diseases and the other as beneficial ones such as biological control agents and nutrient mobilizers and solubilizers. Numerous microorganisms, particularly those associated with roots, have the ability to increase plant growth and productivity (Chung et al., 2005). However, certain groups of microorganisms can directly or indirectly transform rocks and minerals in quantities large enough to influence the geological distributions. These transformations include enzymatic oxidation-reduction reactions, formation of chelates and complexes with protein, amino-acids, organic acids etc. (Henderson and Duff, 1963).

Conclusion

The following conclusion could be drawn from the present investigation. The distribution of microorganisms like bacteria and fungi in the rhizospheric soils of apple play a variety of roles in the growth and yield quality of apple by release of organic carbon; nitrogen fixation; solubilization of phosphorus, potassium, sulphur; plant growth promotion; and alleviation of stress which are statistically significantly correlated with essential macronutrients available in the rhizosphere.  So conserving the microbial activity in the soil is very important for release of essential nutrients in the rhizosphere for plant uptake and sustainable growth of the trees which are important indicators of soil health status.

Acknowledgements

The authors are thankful to the Hon’ble Vice Chancellor and Director Research, Sher-e-Kashmir University of Agricultural Sciences & Technology of Kashmir for providing research facilities.

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