Coal fly ash amendment in acidic field: Effect on soil microbial activity and onion yield

• Soil microbial activity, quality and onion yield were studied with FA in acidic soil.
• FA at 50 Mg/ha improved density of PSB, AMF, actinomycetes and microbial activity.
• Optimal soil nutrients and less heavy metal content were at 50 Mg/ha FA.
• At 50 Mg/ha FA onion yield was 24% more than control.
• Soil microbes were highly correlated with soil quality and onion yield at 50 Mg/ha FA.

Beneficial soil microorganisms are directly or indirectly involved in promoting plant growth and yield. The ameliorating effect of fly ash (FA) with reference to the microbial density and activity is sparsely studied in acidic soil under field conditions. Hence, an experiment was conducted to assess the population dynamics of beneficial microorganisms at different levels of FA (25 Mg/ha, 50 Mg/ha, 100 Mg/ha and control with no fly ash) by using onion (Allium cepa L.) crop. It was observed that population density of phosphorus solubilising bacteria (PSB) (1.33 × 106 cfu/100 g soil), Actinomycetes (3.11 × 105 cfu/100 g soil) and arbuscular mycorrhizal fungi (AM fungi) were maximum at 50 Mg/ha FA. However, the density of Azotobacter (6.27 × 104 cfu/100 g soil) was highest at 25 Mg/ha. AM fungi root colonization and soil microbial activity were significantly highest at 50 Mg/ha dose of FA in comparison to control. FA amendment significantly improved the physico-chemical properties of soil. Cation exchange capacity (CEC), water holding capacity (WHC), available P and available K were optimum at 50 Mg/ha. Heavy metal content in FA amended soil was found to be reduced with respect to the control. Although, Hg and As were present in the experimental FA, they were found to be absent in FA amended soil. Highest onion yield was obtained at 50 Mg/ha dose of FA which showed an increase of 31.91% over control. Pearson correlation analysis exhibited significant correlation between pH, WHC, P and Ca with crop yield, microbial density and activity. Therefore, 50 Mg/ha dose of FA may be considered as a suitable dose for amendment in onion cultivation in terms of sustainable management of biological and physico-chemical health of acidic soil as well as safe FA utilization practice.