Assessing Methods for Estimating Potentially Mineralisable Nitrogen Under Organic Production System in New Alluvial Soils of Lower Gangetic Plain

A dedicated soil testing method for estimating available nitrogen (N) in organic production system is currently lacking. In this backdrop, an innovative approach was initiated to develop a suitable method for assessing potentially mineralisable nitrogen (PMN) as N availability index in organically fertilised soils. To address the objective, a field experiment was conducted in an organically fertilised french bean crop after a 3-year crop cycle of aromatic rice-french bean-okra. Farmyard manure, vermicompost, mustard oil-cake, poultry manure and their different combinations equivalent to 120 kg N ha−1 were applied as sources of N. Chemically fertilised plots were also maintained as a check. PMN was derived using first-order kinetics, from the disappearance of organic-N at different growth stages of french bean using the methods involving 1/15 M phosphate buffer (PB), 0.01 M calcium chloride (CaCl2), 0.01 M sodium bi-carbonate (NaHCO3), and 0.1 M sodium hydroxide + 0.05 M ethylenediaminetetraacetic acid (Basic EDTA) extractants. The conventional alkaline permanganate method for N estimation was also used for comparison. PB derived significantly 22, 39 and 47% higher PMN than Basic EDTA, NaHCO3 and CaCl2 respectively. PMNs estimated by different methods were well correlated (r = 0.53** to 0.84**) among themselves while exhibited poor correlation with alkaline permanganate N (r = 0.30 to 0.41*). PB-derived PMN depicted the strongest linear relationship with pod yield (r = 0.89**, R2 = 0.80**) and N uptake (r = 0.81**, R2 = 0.66**). The reliability of PB as the most suitable method was further established by principal component analysis as PB explained the highest proportion (73%) of total PMN variation. Beyond its consequential quantitative evaluation, the ability of PB to extract organic-N compounds of similar chemical nature (uniform C:N ratio, 12–14) from diverse organic sources confirmed its superiority. Adopting PB to estimate PMN as an index of N availability will thus assist soil testing agencies to improve nutrient management advisory for organic farmers.