Evaluation of the Effectiveness of a Humic Substances-Based Product for Lettuce Growth and Nitrogen Use Efficiency under Low Nitrogen Conditions Atero Calvo, Santiago Magro, Francesco Masetti, Giacomo Navarro León, Eloy Ríos Ruiz, Juan José Ruiz Sáez, Juan Manuel Biostimulant Humic substances Lettuce Increasing crop yield with low-N supplies has become one of the main aims of current agriculture to reduce the excessive use of chemical fertilizers. A sustainable strategy to improve crop productivity, N assimilation, and N Use Efficiency (NUE) under limit-N growth conditions is the application of biostimulants, such as humic substances (HS). Here, we evaluated the effectiveness of an HS-based biostimulant, BLACKJAK®, in improving lettuce growth and NUE under N-deficit conditions. Thus, BLACKJAK® was applied radicularly (R) and foliarly (F) at the following doses: RHS 0.40 mL/L, R-HS 0.60 mL/L, F-HS 7.50 mL/L, and F-HS 10.00 mL/L. Three N levels were applied: optimal (7 mM) and N-deficit (3 mM and 1 mM). The results showed that shoot dry weight (DW) was reduced at 3 mM N (−32%) and 1 mM N (−42%). However, R and F BLACKJAK® enhanced plant growth at all three N levels, especially with F-HS at 10.00 mL/L, which showed an increase of 43% in shoot DW at 3 and 1 mM N, compared to plants not treated with HS. BLAKCJAK® also improved photosynthesis, NO3− and organic N accumulation, the activity of N assimilation enzymes, and the concentration of amino acids and proteins, regardless of the N level. In addition, HS enhanced NUE parameters under all N conditions, except for R-HS 0.60 mL/L at 1 mM N. Hence, our study suggests that the HS-based product BLACKJAK® could be a good candidate for reducing chemical fertilizer use and improving lettuce growth and NUE under low N conditions, although further research is required. 2024-09-16T08:28:46Z 2024-09-16T08:28:46Z 2024-09-01 journal article Atero-Calvo, S.; Magro, F.; Masetti, G.; Navarro-León, E.; Rios, J.J.; Ruiz, J.M. Evaluation of the Effectiveness of a Humic Substances-Based Product for Lettuce Growth and Nitrogen Use Efficiency under Low Nitrogen Conditions. Agriculture 2024, 14, 1492. https://doi.org/10.3390/agriculture14091492 https://hdl.handle.net/10481/94445 10.3390/agriculture14091492 eng http://creativecommons.org/licenses/by/4.0/ open access Atribución 4.0 Internacional MDPI