Evaluation of the effects of a blended seaweed extract of Gracilaria salicornia (Rhodophyta) and Sargassum swartzii (Heterokontophyta) on the growth and grain yield of rice (Oryza sativa) farmed in the lower Gangetic and Eastern Coastal Plains and Hills of India

Concentrated seaweed extract (CSE), prepared from Gracilaria salicornia (red) and Sargassum swartzii (brown), was characterized for its macro- and micronutrients and amino acids, evaluated for plant growth-regulating properties and tested as a biostimulant. Its effects on vegetative growth, grain and straw yield were assessed in rice varieties Satabdi (IET4786) and CO-51 (IET21605) in farmers’ fields in West Bengal (Lower Gangetic Plains) and Tamil Nadu (Eastern Coastal Plains and Hills), India. The control received the recommended NPK at 100% (150:60:60 kg ha⁻¹) applied as basal applications in three splits. In T1, basal nitrogen was reduced to 50% and a nitrogen-enriched nano formulation (NEN) was applied as a foliar spray at 2 mL L⁻¹ three times. In T2, the T1 regimen was supplemented with CSE at 2 mL L⁻¹ applied as a foliar spray three times. For Satabdi, grain yields were 4,391 kg ha⁻¹ (T2) and 4,208 kg ha⁻¹ (T1), representing 15.98% and 11.15% increases ( p = 0.012 by ANOVA with Tukey HSD) over the control (3,786 kg ha⁻¹); CO-51 showed comparable improvements in grain yield (T1: 10.58%, T2: 18.27%). The combined application of NEN and CSE increased grain yield relative to the control under the tested conditions, although the incremental benefit of adding CSE over NEN alone was modest, and its independent effect could not be isolated. Noted reductions in nitrogen input costs are specific to these study conditions and may vary regionally.