Comparative Assessment of Biostimulants in Enhancing Nitrogen Assimilation of Organic Spinach

In organic farming, matching nitrogen (N) availability to key developmental stages of plants with traditional compost or manure inputs can be challenging. Given its high solubility, limiting Chilean nitrate (CN; sodium nitrate) to 20% of the total N per crop cycle has been recommended because of potential environmental impacts on soil biology and nitrate (NO₃) leaching in organic farming. Although it effectively enhances N availability in the initial stages of growth, the impacts on N assimilation and its interactions with other organic amendments remain poorly understood. This study examined the effects of CN and its interactions with commercial biostimulants (iNvigorate®, B Sure®, Agrinos®, Manage™) on N assimilation in organic spinach production. We assessed their effects on yield, N utilization (total N, NO₃, ammonia, free amino acids), and mineral competition across four developmental stages in four spinach cultivars: Acadia, Corvair, Escalade, Shelby. The application of iNvigorate® led to increased total amino acid accumulation under CN, with the highest levels observed in ‘Escalade’ and ‘Shelby’. Total N showed incremental increases, peaking at harvest, whereas free nitrates decreased progressively across all cultivars during development, regardless of CN application. Agrinos® application enhanced potassium accumulation in ‘Acadia’ at harvest when CN was used. Interestingly, sodium accumulation was less with CN across all biostimulants than the control, except for Manage™ during the developmental stages. CN application at sowing did not improve yields significantly, but Agrinos® affected biomass production positively, indicating a beneficial interaction with supplemental nitrates from CN. Based on N utilization, we identified spinach cultivars and biostimulants suitable for commercial organic production.