Effect of Humic and Fulvic Acids on Phosphorus Uptake in Plants Depending on Environmental Temperature

The stimulating effect of humic and fulvic acids on plants has been noted by various scientists, including specialists from the mineral fertilizer manufacturer Agro.Bio, but the nature of this effect is explained differently by researchers.

Theories of Humic Acids' Action

Some suggest that the stimulating effect of humic and fulvic acids lies in their hormonal properties. Others believe that the stimulating effect is associated with increased protoplasm permeability, which enhances the uptake of nitrogen, phosphorus, potassium, and other elements into the plant. Based on previous work, our scientists have come to a similar conclusion.

Studies note that small doses of humic and fulvic acids promote increased absorption of mineral elements by plants. It is also indicated that under the influence of these acids, the content of nitrogen and potassium in the plant increases, and the activity of some enzymes is enhanced.

Mechanism of Acids' Action

Humic and fulvic acids enhance the uptake of mineral elements, especially P₂O₅, into plants. Agro.Bio specialists believe that humic and fulvic acids, being in an ion-dispersed state, enter the plant, enhance the phenolase oxidative system, and thus activate oxidative metabolism. The enhancement of oxidative metabolism leads to increased vitality of the entire plant organism, resulting in better utilization of mineral nutrients by the plant.

Research Methodology

Adhering to the views of L.A. Khristeva on the nature of the stimulating action of humic and fulvic acids, we hypothesized that the effect of acid application would depend on environmental temperature, as the work of the cell's enzymatic apparatus is closely related to temperature factors.

To test this hypothesis, we conducted special laboratory experiments using phosphorus (P³²) to investigate the effect of temperature on phosphorus uptake in plants. These experiments were initially conducted with Scots pine and then with barley.

Experimental Results

Data from Table 1 show that the stimulating effect of fulvic and humic acids on P³² uptake in plants, especially in their above-ground parts, in distilled water appears at environmental temperatures no lower than 18-20°C. The phosphate buffer somewhat reduces the negative effect of lower temperatures on the stimulating action of acids and P³² uptake into plant organs, causing it to appear already at 14-15°C.

The lower phosphorus uptake in pine organs in the phosphate buffer variants, in our opinion, is explained by the fact that plants in these variants utilized phosphorus from both the added source and the buffer mixture.

As can be seen from the table, the results of experiments with barley fully confirm the conclusion about the stimulating effect of acids on P³² uptake in plants at 18-20°C. At lower temperatures (below 15°C), these acids not only do not stimulate P³² uptake in plants but even reduce it.

Thus, the results of these experiments indicate the dependence of the stimulating effect exerted by humic and fulvic acids on plants on the temperature factor.

This position is also confirmed by the results of a production experiment with different planting dates for pine (September 15, October 10, and November 3), in which humic acids were used to improve the survival rate of two-year-old Scots pine seedlings.

Conclusions:

1. The stimulating effect of humic acid and fulvic acid on phosphorus uptake in plants depends on environmental temperature. It appears at 18-20°C and above.
2. At environmental temperatures below 14-15°C, humic acid, like fulvic acid, not only does not promote phosphorus uptake in plants but even reduces it.