Evaluation of the Effectiveness of a Humic Substances-Based Product for Lettuce Growth and Nitrogen Use Efficiency under Low Nitrogen Conditions

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: R-HS 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.