Sorghum popularly known as Jowar is the most important food and fodder crop of dryland agriculture. The sorghum grain is used primarily as human food in various forms such as Roti/Bhakri (unleavened bread) or is cooked like rice or made into porridge with pulverized sorghum grown in southern India. The sorghum grain and fodder are used for the production of ethanol, starch and paper (Doifode 2021). Indian farmers are highly dependent on rainfall and all activities of agriculture starting from sowing to postharvest are affected either directly or indirectly by weather especially by rainfall.

Sorghum is a hardy crop that can withstand moisture stress and its potentiality can be augmented by biofertilizer application. Biofertilizers improve photosynthesis performance to confer plant tolerance to stress (Chi et al 2010). Microbial inoculants are one of the components in organic farming system which help to produce the plant growth hormones, vitamins and amino acids, fixation of atmospheric nitrogen, solubilization and mobilization of P and translocation of minor elements like zinc and copper. They are cheap, ecologically safe, non-toxic and easy to apply (Bhoraniya et al 2019).

The black soil of Aruppukottai, Virudhanagar district of Tamil Nadu is characterized by higher calcium content because of the Kankar nodules as indurate layers. In certain fields, one may be able to see numerous small bits of limestone on the surface soil itself. Such a high content of Ca will exert a reversible effect even if phosphobacteria solubilize the phosphate. Calcite or calcium carbonate (CaCO₃) is soluble only under an acidic environment. Rainwater is slightly acidic because of the carbon dioxide present in the environment that can be dissolved in it. Phosphorus is one of most essential nutrients for plant growth and development which contributes 0.2 to 0.8 per cent of plant dry matter yield (Sharma et al 2013). The availability of P is very low due to the calcium occurrence in neutral to alkaline condition and alminium (Al) and iron (Fe) in acidic environment ultimately that can lead to fixation of P. Therefore, it is worthwhile to use phosphobacteria and mycorrhiza together in calcareous or black cotton soil (Schutz et al 2018, Vidhya 2022, Johan et al 2021)

Hence, the individual bacterial biofertilizer and combination of phosphobacterial and mycorrhizal biofertilizers have been used in this experiment which can absorb, store and mobilize phosphorus to the crops.

The effect of biofertilizers on sorghum was investigated in pre-monsoon sowing at Regional Research Station at Aruppukottai, Virudhunagar, Tamil Nadu. A randomized replicated field trial was laid out in black cotton soil. Sorghum seeds of APK 1 variety were sown in plots of 5.2 m x 3.8 m size with a spacing of 45 cm x 15 cm. Three replications were maintained. The treatments used are given in Table 1.

The appearance of the soil and hotness prevailing in southern Tamil Nadu during summer gives an impression that this soil might not have any microbiological activity particularly in summer. In the present study, it was found that the soil here was dry and the ambient temperature ranged from 31-42ºC. Besides, the survival of inoculated organisms, it was likely that soil adhering to the seeds when removed for testing might also have contributed to the presence of Azospirillum and phosphobacteria as these were also observed in uninoculated seeds. Phosphobacteria-treated seeds showed a higher population of phosphobacteria. The phosphobacterial population was higher in the treatment receiving phosphobacteria + mycorrhiza in combination which recorded a population of 18 x 10³ cfu/g of seed followed by azophos treatment with 14 x 10³ cfu/g. Combined inoculation of azophos with mycorrhiza and phosphobacteria also recorded a relatively higher phosphobacterial population. In the uninoculated control also, phosphobacteria were present.

The Azospirillum population was higher in the treatment receiving Azospirillum with 14 x 10⁵ cfu/g of seed. The treatment of azophos + mycorrhiza, azophos also recorded a population of 9 x 10³ cfu/g+ 5 × 10⁷) of seed. It was negligible in the uninoculated seed. It is not only the bacterial inoculants adhered to seed when inoculated but also survived when the treated seeds were sown in a dry soil subjected to hot dry condition until the showers drenched the soil enabling the seeds to germinate. In the present study, under a simulated dry condition in pots containing the black cotton soil, the sorghum seeds treated with biofertilizers established survival of inoculated bacteria for 3 weeks.

Biofertilizers were found to induce early germination and augmented higher germination, seedling vigour and enhanced root proliferation conferring the drought tolerance (Nosheen et al 2021). In the present experiment, the adherence of the inoculated bacteria was seen in all the treatments.

In the phosphobacteria + mycorrhiza-treated combination, the VAM spores were comparatively higher (26/100 g soil) in 90 days after crop growth. In summer, activation of the bacterial cells was reported by Barnard et al (2013). Higher soil phosphorus content of 11.2 kg/ha was recorded by Ramalakshmi et al (2008) with azophos and mycorrhizal treatment in black cotton soil of Aruppukottai while 11.1 and 11.0 kg/ha was observed in the treatment of phosphobacteria and mycorrhiza in 90 days after crop growth. The results of the present study are in conformity with the above findings.

VAM-inoculated plants showed that the rate of photosynthesis in rice plants under drought and saline conditions which was reported by Ruiz-Sanchez et al (2010). Rani et al (2019) reported that in rabi sorghum, seed treatment of sorghum with liquid formulation of Azospirillum @ 4 ml + phosphobacteria @ 4 ml/kg seed, proved to be optimal for obtaining higher grain yield of 2.33 tons/ha with net return of Rs 51,217/ha and B-C ratio of 2.10. In pot and field conditions, combined inoculation of azophos and mycorrhiza increased the seed germination. Enhancement of the seed germination due to biofertilizer has already been reported in several crops like cardamom, coffee and rice (Sherpa et al 2021). The results of the current field study showed that VAM root infection was comparatively higher in azophos + mycorrhiza treatment combination of 76 per cent than the mycorhizal treatment alone, especially at 60 days after crop growth.

In dry land agriculture, any technology that enhances seed germination would be a boon to the farmer. The seeds have to germinate with meager moisture of the soil or before the soil gets dried up. Germination failure is the first constraint in an arid climate. Bareke (2018) observed that for germination to occur, seeds of each species must attain specific moisture content. In arid climates, inadequate moisture is a frequent cause of germination failure. Reddy et al (2020) reported that a poor seed-soil moisture contact reduced the rate of water uptake and thus caused delayed or poor germination. Abdelhameid (2020) reported that mycorrhizal inoculation and potassium fertilization in sweet sorghum cultivated under water stress in calcareous soil significantly increased leaves, stalks and grains nitrogen content by 9.49, 21.54 and 2.83 per cent compared to uninoculated treatment. Sorghum APK 1 raised in the present study also reported higher earhead yield of 13.00 kg per/plot in treatment azophos + mycorrhiza alongwith phosphobacteria + mycorrhiza with 12.8 kg per plant.

The increased germination observed due to biofertilizers might be due to elaboration of auxins and growth promoting substances. Patel and Saraf (2017) also reported production of gibberellic acid by Azospirillum. Plant growth promotion of plants at seedling stage and at subsequent growth stages could be mainly due to the plant growth promoting substances produced by Azospirillum brasilense (Backer et al 2018). Although auxin production by the biofertilizers tested has not been studied in the present investigations, the Azospirillum, phosphobacteria and azophos might have elaborated such substances that might have induced higher germination (Cortivo et al 2020). In the present experiment, azophos + mycorrhiza-treated plants showed a higher grain yield of 3,536 kg/ha followed by mycorrhizal treatment alone with yield of 3,094 kg/ha and phosphobacteria + mycorrhiza with yield of 3,000 kg/ha.

Conventional methods of agriculture play a significant role in meeting the food demands of a growing human population which has also led to an increasing dependence on chemical fertilizers and pesticides. Chemical fertilizers namely urea, DAP, SSP and potash are industrially-manipulated substances composed of known quantities of N, P and K and their exploitation causes air and groundwater pollution by eutrophication of water bodies. Hence, an appropriate method of biofertilizer (individual or in combination) application can reduce the recommended dose of fertilizers. The results of the present study established the beneficial effect of biofertilizers in pre-monsoon sowing of sorghum in black soil. The combined inoculation of azophos @ 3 packets (200 g/packet) with mycorrhiza @ 2 per cent (w/w) of seed treatment followed by soil application of azophos @ 10 packets (200 g/packet/ha). This treatment enhanced the seed germination, plant growth, root length and soil available N, P and K content in plants. The farmers can save 25 per cent of N and P by resorting to a low cost biofertilizer treatment without sacrificing the yield.

Each packet contained 200 g biofertilizer; Treatments T₁-T₆ received only 75% of N and P

DAS = Days after sowing; Moisture content of soil at 0 DAS = 8.4, at 7 DAS = 5.1

cfu 10⁷ on wet weight basis

DAG = Days after growth; Treatments 1 to 6 applied with 75% of the recommended N and P

Treatments 1 to 6 applied with 75% of the recommended N and P

References