Effects of Humic Acid as Fertigation on Phosphate and Iron Fertilizers Efficiency and Some of Mineral Nutrients Concentration in Corn (Zea mays L.)

Application of natural organic matter derived components, i.e. humic acid, as fertilizer is a suitable way to improve soil fertility and increase yield and quality of agricultural products. Many researchers reported positive effects of humic acid on water holding capacity, soil aeration, root formation and development, microorganism activities, and availability of mineral nutrients in soil. Antagonistic interaction between soil phosphorus and some micronutrients, especially in calcareous soils, can cause micronutrients deficiency in plants. With regard to positive effects of organic compounds on bioavailability of mineral nutrients, it seems that humic acid can positively affect the phosphorus interaction with micronutrients. Therefore, investigation of the effects of humic acid incorporated into irrigation water, phosphate and iron fertilizers application, on nutrients concentration in plants and their interactions is considerable.

This study was carried out to investigate the effects of application of humic acid in irrigation water, and phosphate and iron fertilizers in soil, on corn growth and concentration of P, Fe, Mn, Zn, and Cu in corn tissues. To this aim, a factorial experiment was conducted based on completely randomized design, with three replications in greenhouse. The factors included humic acid in 0, 70, and 140 mg kg-1 levels, (7 times as fertigation during growth season; total use equal to 0, 490, and 980 mg kg-1 of soil, respectively), phosphorus (P, as monocalcium phosphate monohydrate) in 0 and 50 mg kg-1 levels, and Fe (as ferrous sulfate heptahydrate) in 0, 10, and 20 mg kg-1 levels. P and Fe treatments were mixed with 4 kg of air-dried soil (<2 mm in diameter) and filled to the pots. Six seeds of maize (Zea maye L. cv. Single cross 704) were seeded per pot, and three seedlings were finally kept and grown for two months. After harvest, fresh and dried weight of shoots were measured. The roots were accurately extracted from the soil, washed, dried at 65◦C, and weighed. Sample digestion and measuring concentration of P, Fe, Mn, Zn, and Cu were done according to conventional methods (P by a UV-Visible Spectrophotometer and metal elements by the GBS Savant Atomic Absorption Spectrometer). Statistical analyses were done by the IBM SPSS Statistics version 26 software.

According to this study results, the main effect of humic acid, on P concentration and dry matter of shoots and roots, was statistically significant. In presence of P (2nd P level), 490 and 980 mg kg-1 humic acid levels significantly increased the mean of dry matter compared to blank while humic acid had no significant effect on means of shoots and roots dry matter in 1st level of P (no P application). Increasing humic acid level from 490 to 980 mg kg-1, significantly decreased mean of shoots dry matter. The interaction effect between humic acid and the other two factors exhibited statistical significance concerning root dry matter. The treatment combination of 50 mg kg-1 of P, 490 mg kg-1 of humic acid, and 20 mg kg-1 of Fe yielded the highest mean root dry matter, which was 97% greater than that of the control. The 2nd level of P significantly increased the means of shoots P concentration in all levels of humic acid and Fe factors, compared to those of the 1st P factor level. There was no significant difference between means of shoots P concentration in different levels of humic acid and Fe factors, at the 1st level of P factor, separately. On the other hand, at the 2nd level of P factor, significant differences were observed between the means of P concentration for both other factors (significant interaction between P and humic acid, and between P and Fe Factors). Applying humic acid could significantly increase the means of shoots P concentration at the 2nd level of P factor, but there was no significant difference between those of 490 and 980 mg kg-1 levels. About the effect of Fe factor on shoots P concentration, only 10 mg kg-1 level of Fe significantly increased it. The main effect of the P and humic acid factors and interaction of the P and Fe factor on roots P concentration, were statistically significant. Roots P concentration increased significantly by 490 and 980 mg kg-1 humic acid levels. A significant increase of roots P concentration was observed in the 1st P factor level and 10 mg kg-1 level of Fe compared to the blank, and in 50 mg kg-1 level of P, Fe factor had no significant effect on it. The results showed that humic acid could not improve P uptake by corn from the soil with low available phosphorus (Olsen extractable P lower than 4 mg kg-1). The humic acid factor had no significant effect on Fe concentration of corn shoots, but its main effect and its triple interaction, with two other factors, on Fe concentration of the roots were statistically significant. There was no significant difference between the means of roots Fe concentration at the 1st level of P factor (9 treatments, various levels of humic acid and Fe factors). The highest mean of root's Fe concentration was found in treatment of the highest level of each factor, significantly more than those of the most of other treatments. About the Mn concentration in corn tissues, the Mn concentration in shoots was significantly increased by P fertilizer application, and Mn concentration in roots was significantly affected and increased by 490 and 980 mg kg-1 humic acid levels. The means of Mn concentration of roots in 490 and 980 mg kg-1 humic acid were not significantly different. The Zn concentration of corn shoots was significantly affected by interaction of the P and humic acid factors as the highest mean of it was in 0 mg kg-1 of P and 980 mg kg-1 humic acid levels, and there was no significant difference between those of other levels. The Zn concentration of corn roots was significantly increased by P applying and affected by the interaction of humic acid and Fe factors. When humic acid was at zero concentration level, Fe application of 20 mg kg-1 significantly decreased the Zn concentration of corn shoots while with humic acid application (490 and 980 mg kg-1) no significant difference was observed between the means. This result showed that humic acid can decrease the antagonistic effects of Fe and Zn in soil. The Cu concentration in shoots was significantly affected by the P and Fe factors. Usage of P fertilizer significantly increased the Cu concentration of corn shoots; on the contrary, the 2nd and 3rd levels of Fe factor (Fe applications) significantly decreased Cu concentration in shoots of corn. Moreover, using humic acid could significantly increase Cu concentration of corn roots without any significant interaction with the other two factors.

The findings suggest that in soils with very low available P, humic acid alone does not enhance the growth and dry matter yield of corn. However, the efficiency of phosphate fertilizer can be enhanced by applying humic acid fertilizer through irrigation water. Additionally, humic acid has been observed to mitigate antagonistic effects between P and certain micronutrients, as well as reduce antagonistic interactions among metal micronutrients. For the positive effect of humc acid on growth and adequate chemical composition of corn, concentration of 490 mg kg-1 humic acid is recommended.