Effect of Smart Fertilizer Application Rates and Irrigation Levels on Onion Sustainability in Semi-Arid Regions

Abstract

To optimize the productivity of water and nutrients in semi-arid environments with increasing climate stress, a timely shift towards lean precision management is needed. This study quantitatively evaluated the synergistic interaction between regulated deficit irrigation thresholds (35% [ ], 40% [ ], and 75% [ ] available soil moisture depletion) and smart foliar fertilization rates (0% [ ], 50% [ ], and 100% [ ] of the recommended dose) on a shallow-rooted onion (Allium cepa L.) crop in Kirkuk Governorate, Iraq. Hydro-physical analysis demonstrated that moderate deficit irrigation ( ) optimized the root-zone soil water characteristic curve, preventing non-productive deep percolation losses and maximizing Irrigation Water Productivity (IWP) to a peak mean of 25.69   compared to   (23.51  ) and   (16.98  ). Under severe soil matric stress ( ), exogenous application of full-dose smart fertilizer ( ) acted as a crucial physiological buffer; it physically bypassed drought-induced nutrient paralysis in the rhizosphere, restoring cellular turgor and recovering plant height to 52.00 cm compared to 29.33 cm in the unfertilized control ( ). Agronomic evaluation of the Partial Factor Productivity of fertilizer (PFP) strictly exhibited the Law of Diminishing Marginal Returns, where the half-dose ( ) achieved superior nutrient utilization efficiency (4791.67  ) over   (2732.64  ). Crucially, the strategic bio-economic interaction coupling moderate moisture depletion with full-dose foliar nutrition ( ) epitomized lean management by minimizing gross hydrological inputs while sustaining high bulb yields, achieving the absolute highest variable Benefit-Cost Ratio (BCR) of 14.68. The results showed that the use of calculated deficit irrigation combined with smart targeted foliar application is an effective strategy to reduce hydraulic losses and generate maximum economical productivity from every water drop used in sensitive dryland areas.