Combined Application of Azolla and Inorganic Potassium Fertilizer Influence the Growth, Yield and Storability of Onion in Alluvial Soil

Azolla (Azolla pinnata R. Br.) is an aquatic fern that supplies potassium (K) and other plant nutrients with their use and organic matter after decomposition in the soil to improve soil quality. Alluvial soils are deficient in available K due to intensive cropping systems, the use of high-yield varieties (HYVs), and improper fertilizer application. With high demand for K fertilizers and complete dependence on imports in India have affected its availability and cost. Azolla can be an alternative option for a source of K nutrients. Keeping this fact in view, the present study was formulated to understand the effect of K applied through Azolla and inorganic K fertilizer on the growth, yield, and storability of onion (Allium cepa L.). An incubation study was carried out to examine the K dynamics as influenced by levels and sources of K. Results showed that the use of Azolla along with mineral K fertilizer (NPK1/2 + K-1/2 through mulching of Azolla and NP+ K-1/2 through green manuring (GM) of Azolla + K-1/2 through mulching of Azolla) has increased the water-soluble K (WS-K) and available K (Av-K) from 0 to 35 days of incubation (DI) and further decreased thereafter up to 105 DI. Azolla-based K application showed the availability of more K in the soil throughout the incubation period as compared to inorganic sources alone. The highest exchangeable K (Ex-K) was recorded in soil treated with Azolla similar to WS-K and Av-K. A field experiment was also conducted on the onion crop at the Bihar Agricultural College research farm, Sabour. The findings suggest that the maximum bulb weight (32.8 g bulb(-1)) and bulb yield (12.5 t ha(-1)) were recorded in the treatment NPK+ K-1/2 through mulching of Azolla in two splits increased by 33% and 43%, respectively. Treatments applied with the full dose of recommended inorganic K alone resulted in 14% less bulb yield than the treatment combined with the full dose of recommended mineral K through inorganic fertilizer and half dose of recommended K supplied through Azolla mulching. The results also indicated that Azolla-based K application resulted in higher nutrient uptake, soil available nutrient status, and better storability in onion than conventional mineral K fertilization. Overall, the study concludes that Azolla may maintain K availability by increasing the amount of Ex-K and ensuring constant availability of the nutrient during the entire growth period.