Bioactivity of two different humic materials and their combination on plants growth as a function of their molecular properties

Background and aims Neutralization of adverse environmental effects of agriculture intensification to sustain population growth, requires ecologically sound alternatives for plant growth. We used as biostimulants towards germination of basil seeds and early growth of maize, two different humic materials: a potassium humate from leonardite (KH), and compost tea (CT) from a green compost made of coffee husks, and a 1:1 combination of the two (MIX). After their thorough chemical, molecular and conformational characterization, a relation between structure and bioactivity was investigated. Results CT showed the largest bioactivity on either seed germination or maize plantlets growth due to its great content of polar bioactive molecules including oxidized lignin fragment, saccharides and peptides. The more hydrophobic KH, rich of alkyl and aromatic moieties, also exerted a significant bioactivity on maize, though to a lesser extent. The application of MIX to hydroponically grown maize plantlets showed a smaller bioactivity of polar CT molecules due to their entrapment into new suprastructures stabilized by hydrogen bonds formed with complementary functions of KH hydrophobic components. However, while the KH hydrophobicity in MIX ensured adhesion to roots, its conformational flexibility was still sufficient to provide a greater bioactivity than control, by releasing bioactive CT components capable to enhance both biomass yield and root elongation. Conclusions Our study suggests that a combination of humic materials with diverse and well-characterized molecular properties may become a new tool to produce innovative and ecologically viable plant growth promoters, whose bioactivity may be modulated.

Automated summary

This study explores the use of different humic materials as biostimulants for plant growth. The results show that humic materials with polar bioactive molecules, such as oxidized lignin fragment, saccharides, and peptides, have a significant impact on seed germination and plant growth. The combination of humic materials with diverse molecular properties could serve as a new tool for producing environmentally friendly plant growth promoters. The bioactivity of these materials can be modulated to enhance biomass yield and root elongation.