Humic acid has complex structure and diverse functions. Studying the direct and indirect effects of humic acid on the growth of maize roots and revealing the regulation mechanism of humic acid on maize roots can provide the theoretical support for further research on the development and application of humic acid fertilizer synergists.

A hydroponic experiment was carried out using Hoagland nutrition solution as the basic cultural liquid and maize cultivar of ‘Zheng Dan 958’ as material with a split-root design. The method of field plot split zone experiment was adopted and eight test treatments was set which was CK-control side (CK-C), CK-control side (CK-C)), HA-non-applied side (HA-C), HA-applied side (HA-T), OHA3-non-applied side (OHA3-C), OHA3-applied side (OHA3-T), OHA6-non-applied side (OHA6-C) and OHA6-applied side (OHA6-T) to study its effects on biomass, root vitality, root morphology and main chemical components of maize different organs.

(1) Single-sided application of humic acid to separate roots significantly increased the fresh root weight of maize on the applied and unapplied sides. Compared with the control, the fresh root weight of maize with the applied side was increased by 21.9%-78.6%, which with the unapplied side was increased by 27.9%-49.3%. (2) The addition of humic acid significantly increased the maize root activity and the total TTC reduction of maize on the applied side and the non-applied side. Compared with the control, the maize root activity and total root TTC reduction of the OHA6-applied side increased the most, which increased by 76.9% and 216.9%, respectively. Compared with the control, HA-applied side and OHA3-applied side treatments increased the maize root activity by 59.8% and 35.1%, respectively. Compared with the control, OHA6-non-applied side, HA-non-applied side and OHA3-non-applied side treatments increased the maize root activity by 62.2%, 53.6% and 25.5%, respectively. (3) Adding humic acid treatment significantly increased the root volume, root surface area, average root diameter, root length and root number on the non-applied and applied sides of maize. (4) Single root application of humic acid could effectively increase the content of maize root ester compounds, protein, amino acids, nucleic acid cellulose and polysaccharides. The application side of humic acid treatment was more conducive to maize root carbohydrates than the non-application side. However, the untreated side under humic acid treatment was more conducive to the accumulation of maize root nucleic acid. The carbohydrate content of the aboveground parts of maize treated with humic acid was significantly higher than that under the blank treatment.

Regardless of whether it was HA or OHA3 and OHA6, when humic acid was applied to separate roots on one side, the growth and activity of maize roots on the side where humic acid was applied were significantly higher than those on the side without humic acid, indicating that humic acid had a direct effect on root regulation. The root growth of the whole plant treated with humic acid on one side was better than that under the control treatment without humic acid on both sides, indicating that humic acid also had an indirect effect in regulating root growth. The root growth of the whole plant treated with humic acid on one side was better than that under the control treatment without humic acid on both sides, indicating that humic acid also had an indirect effect in regulating root growth.