Influence of long-term organic nutrient management on soil quality and crop productivity in rice (Oryza sativa)-potato (Solanum tuberosum)-okra (Abelmoschus esculentus) cropping system under irrigated condition

A long-term field experiment was conducted during 2006–07 to 2014–15 on organic nutrient management in rice (Oryza sativa L.)-potato (Solanum tuberosum L.)-okra (Abelmoschus esculentus L.) cropping system under irrigated conditions in the coastal alluvial sandy loam soil having pH 5.8 and organic carbon 0.62% at Bhubaneswar, Odisha. Eight treatments comprising of five organic, two integrated and one inorganic of nutrient management were tested with three replications in randomized block design. Organic sources of nutrient supply reduced the system yield by 7.6 to 9.7% over the inorganic source in the first year. However, the mean system yield of ‘conversion period’ (first three cropping system cycles) with organic nutrient management comprising of supply of one-third N each through farm yard manure (FYM), green manuring of dhaincha (Sesbania cannabina) or vermicompost and neem (Azadirachta indica) oilcake + Azospirillum or Azotobacter + phosphate solubilizing bacteria (PSB) was on par with inorganic approach. The mean system yield from fourth to ninth cropping system cycles (‘after conversion period’) with organic sources of nutrient supply, i.e. FYM + green manuring or vermicompost + neem oilcake + Azospirillum or Azotobacter + PSB was significantly higher (11.1%) than that of with inorganic source of nutrient supply i.e., 100% NPK + ZnSO4 @ 25 kg/ha to rice and 100% NPK + gypsum @ 110 kg/ha + borax @ 10 kg/ha + ammonium molybdate @ 0.8 kg/ha to both potato and okra. The organic nutrient management package increased the soil organic carbon and available N, P and K at the end of ninth cropping system cycle over the initial and the build-up was maximum in the soil applied with one third N each through FYM, dhaincha/vermicompost and neem oilcake + Azospirillum/Azotobacter + PSB. The microbial population in terms of colony forming units increased in a higher rate in soils with organic nutrient supply system (bacteria 36.5 to 39.4%, fungi 33.0 to 38.2% and actinomycetes 36.0 to 37.3%) compared to the inorganic source of nutrient supply system (bacteria 5.6%, fungi 10.3% and actinomycetes 12.7%) after ninth cropping system cycle over initial status. The microbial biomass carbon of the soils with organic sources of nutrient supply was enhanced considerably (57.7% to 66.8%) over the initial level (98.7 to 107.4 μg C/g). Application of one-third N each through FYM, green manuring of dhaincha and neem oilcake + Azospirillum + PSB to rice followed by similar combination of FYM, vermicompost, neem oilcake + Azotobacter + PSB to both potato and okra was the best organic nutrient management practice for rice-potato-okra cropping system for improving soil health and productivity. However, this system can be profitable under organic farming only when on-farm generated organic manures are used.