Residual Effect of Rice-fish Based Organic Nutrient Management Approaches on Rice Fallow Blackgram Productivity Enhancement in Cauvery Delta Zone of Tamil Nadu

Background: A field experiment was conducted during the rice fallow season of 2023 and 2024 (January to March) at Tamil Nadu Rice Research Institute (TRRI), Aduthurai, Tamil Nadu to study the residual effect of rice-fish-based organic nutrient management practices adopted in rice on succeeding blackgram grown as rice fallow. Methods: The experiment was carried out using randomized block design (RBD) with eleven treatments viz., T1- No Green manure-Rice crop alone (RDF), T2-Green manure- Rice crop alone (PSB+KSB+ZSB), T3-T2+Nanourea, T4-Green manure- Rice crop (PSB+KSB+ZSB) + Fish, T5-T4 + Nano urea,T6- Green manure- Rice crop (PSB+KSB+ZSB) + Fish + Azolla, T7-T6 + Nano urea, T8-Green manure-Rice crop (PSB+KSB+ZSB) + Fish + Duck, T9- T8+Nano urea, T10- Green manure- Rice crop (PSB+KSB+ZSB)+ Fish + Azolla + Duck, T11- T10+Nano urea. The treatments were replicated thrice. The blackgram variety ADT 6 was used as a test crop. Result: The results revealed that among the different organic nutrient management approaches followed in the rice-fish based farming system, the residual effect of Green manure- Rice (solubilizing bacteria such as PSB, KSB and ZSB) + Fish + Azolla + Duck (T10) resulted in higher plant height, DMP, LAI and SPAD value in blackgram. Similarly, the highest root length (19.43 cm), root dry weight (1.10 g), root volume (10.68 g/cc), nodule plant-1at 20 DAS (13.82) and nodule plant-1at 40 DAS (22.97) were also obtained with the same treatment. By virtue of higher growth and yield parameters, more number of pods plant-1 (26.88) and seeds pod-1 (7.73), grain yield (789 kg ha-1) and Halum yield (2136 kg ha-1) of blackgram were obtained in the residual treatment of the residual effect of Green manure- Rice (solubilizing bacteria such as PSB, KSB and ZSB) + Fish + Azolla + Duck (T10). The available soil nutrients at 20 DAS of blackgram were higher in Green manure- Rice (solubilizing bacteria such as PSB, KSB and ZSB) + Fish + Azolla + Duck (T10). Due to higher blackgram yield, the same treatment achieved higher gross return (Rs.41561 ha-1), net return (Rs.30156 ha-1) and BCR (3.64). The study also indicated that the trench (300 m3) dug in rice fields for fish rearing provided sufficient moisture for rice fallow blackgram and thereby evaded the terminal stress.