Phosphate Buffer Extractable Organic Nitrogen as an Index of Soil Nitrogen Availability in Organically Fertilized New Alluvial Soils of Lower Gangetic Plain

Organic agriculture is bereft of a dedicated, easy to handle and real-time-computing soil testing method to correctly estimate available nitrogen (N) status in soil. The present study attempts to develop a suitable N availability index in organically fertilized French bean after a 3-year aromatic rice-French bean-okra cropping cycle. Farmyard manure, vermicompost, mustard oil-cake and poultry manure and their different combinations equivalent to 120 kg N ha−1 were applied as sources of N. We compared 1/15 M neutral phosphate buffer (PB) extractable organic-N (PEON) with the N estimated by existing conventional methods, i.e. 2 M potassium chloride (KCl-N) and potassium permanganate (KMnO4-N) for assessing the suitability of N availability indices. The methods differed significantly with respect to their estimated N and PB derived ~ 5 to 12 times more N over the other availability indices. PEON depicted the strongest linear relationship with the mineralized N (R2 = 0.61**), N uptake (R2 = 0.71**) and pod yield (R2 = 0.64**), followed by KCl-N (R2 = 0.56* to 0.60**) and KMnO4-N (R2 = 0.46* to 0.55*). The superiority of PEON was further established by principal component (PC) analysis where PEON secured the highest factor loading (0.398) in PC1 (explained 88% of total variation) and exhibited a strong association with plant parameters. Beyond its consequential quantitative assessment, the ability to retain similar chemical-natured protein-like compounds from diverse organic sources (uniform C:N ratio: 12–15:1) confirmed its reliability to use as a suitable N availability index. The inclusion of PEON in soil testing methods will support organic certifying agencies and soil testing laboratories to improve their nutrient advisory to organic farmers.