Physiological growth indices of field bean (Lablab purpureus (L.) Sweet) as influenced by foliar application of nano DAP

Field bean (Lablab purpureus (L.) Sweet) is a multipurpose legume grown widely in Karnataka, yet its productivity is constrained by low phosphorus use efficiency in conventional fertilizers. Nano di-ammonium phosphate (DAP), with nanoscale phosphorus (16% P₂O₅) and nitrogen (8% N), offers enhanced solubility, absorption and translocation when applied as a foliar spray. A field experiment was conducted during late kharif 2024 at College of Agriculture, Shivamogga, to study the effect of foliar-applied nano DAP on physiological growth indices and yield of field bean under the Southern Transitional Zone of Karnataka (STZ). The experiment was laid out in randomized complete block design with ten treatments and three replications, comprising 100 per cent and 75 per cent recommended dose of fertilisers (RDF) (25:50:25 N:P₂O₅:K₂O kg ha⁻¹) combined with foliar sprays of nano DAP (2 and 4 ml L⁻¹) once at 30 DAS or twice at 30 and 60 DAS. Results revealed significant improvement in physiological growth indices under nano DAP treatments. The treatment with 100 per cent RDF + nano DAP @ 4 ml L⁻¹ at 30 and 60 DAS (T₈) recorded the highest leaf area index (2.30 at 60 DAS), absolute growth rate (0.50 g plant⁻¹ day⁻¹) and crop growth rate (7.42 g m⁻² day⁻¹). In contrast, relative growth rate (0.020 g g⁻¹ day⁻¹) and net assimilation rate (2.98 g m⁻² day⁻¹) were superior under 75 per cent RDF + nano DAP @ 4 ml L⁻¹ 30 and 60 DAS (T₁₀), reflecting the crop’s ability to maintain strong physiological performance even under reduced fertilizer input. Yield attributes followed a similar trend, with T₈ producing the highest seed yield (1234 kg ha⁻¹), with 25 per cent increase over RDF alone. Also recorded the maximum haulm yield (2035 kg ha⁻¹). The study highlights that foliar application of nano DAP, particularly at 4 ml L⁻¹ twice during the crop growth cycle, plays a crucial role in improving key physiological growth indices and enhancing yield of field bean under STZ conditions.