Influence of pH on the Efficiency of Humic Fertilizers
In the previous article Some Methods to Enhance the Efficiency of Humic Fertilizers, we established that to produce a complete humic fertilizer like Totem Agro.Bio, the rate of ammonia water application should be increased compared to potassium humate.
We observed that the addition of potassium salt reduces the fertilizer's pH by 0.2–0.3 units. Therefore, it was hypothesized that, alongside increasing the nitrogen content in Totem Agro.Bio, the addition of ammonia water is necessary for another reason—to alkalize the fertilizer itself.
This hypothesis was tested in one of the vegetative experiments. To obtain fertilizers differing only in pH, they were prepared as follows. A sample of leonardite was treated with ammonia water, followed by orthophosphoric acid after 24 hours in an amount sufficient to achieve a humate pH of 7.5. The sample was then divided into two equal parts, and potassium fertilizer (K₂SO₄) was added to one of them. Subsequently, portions of fertilizers with pH values of 7.0 and 6.0 were prepared from both the humate and Totem Agro.Bio.
The experiment was conducted in soil culture with eight replications. Fertilizers were applied locally under the tuber at 25 ml of solution per container. One month after emergence, plants in all variants received additional feeding—1 g of nitrogen, phosphorus, and potassium per container.
Results of the Vegetative Experiment
| Fertilizer pH | Tuber Yield per Container, ml |
|---|---|
| Alkaline (pH 7.5) | 190 |
| Neutral (pH 7.0) | 207 |
| Acidic (pH 6.0) | 176 |
As shown in the table, acidification of both potassium humate and Totem Agro.Bio reduces their efficiency, which was also observed in previous experiments when Totem Agro.Bio was prepared from humate with the addition of a potassium component.
Analysis of Reasons for Reduced Efficiency
To explain this phenomenon, we proposed two hypotheses, which were subsequently tested experimentally:
- It is known that the solubility of humic acids decreases in the acidic range, so at pH 6.0, one would expect an absence or insufficiency of these physiologically active substances in the fertilizer.
- It was also possible that in an acidic environment, due to the interaction of mineral components with leonardite, these components could transform over time into forms less accessible to plants.
In both cases, the fertilizer quality would deteriorate, consequently negatively affecting the potato yield.
Below are the analytical data for potassium humate and Totem Agro.Bio, where the content of humic acids, nitrogen, phosphorus, and potassium was determined over a period roughly corresponding to the supply of nutrients to potatoes from pre-sowing fertilizers.
| Fertilizer pH | Content of Water-Soluble Humic Acids (mg per 100 ml of Fertilizer Solution) |
|---|---|
| Alkaline (pH 7.5) | 410 |
| Neutral (pH 7.0) | 190 |
| Acidic (pH 6.0) | 50 |
From a comparison of the analytical data and potato yield, it can be concluded that the dominant factor determining the efficiency of humic fertilizers is the mobility of humic and fulvic acids. Based on the yield, excessive solubility of humic acid in alkaline fertilizers appears to harm the plant, while in acidic fertilizers, there is a deficiency. The optimal amount of humic acids dissolves at a pH close to neutral, reaching 0.17–0.19% on the third day. This value can be used as a criterion for assessing the quality of humic fertilizers, provided the extract is infused for three days.
| Fertilizer pH | Initial Nitrogen Content | After 2 Weeks | After 4 Weeks |
|---|---|---|---|
| Potassium Humate (pH 7.0) | 0.90 | 2.03 | 1.44 |
| Potassium Humate (pH 6.0) | 1.55 | 1.82 | 1.76 |
| Totem Agro.Bio (pH 7.0) | 1.06 | 1.66 | 1.57 |
| Totem Agro.Bio (pH 6.0) | 1.43 | 1.58 | 3.06 |
| Fertilizer pH | Initial Phosphorus Content | After 2 Weeks | After 4 Weeks |
|---|---|---|---|
| Potassium Humate (pH 7.0) | 0.88 | 1.95 | 0.49 |
| Potassium Humate (pH 6.0) | 1.19 | 1.88 | 0.91 |
| Totem Agro.Bio (pH 7.0) | 0.58 | 1.76 | 0.30 |
| Totem Agro.Bio (pH 6.0) | 1.05 | 1.80 | 0.83 |
| Fertilizer pH | Initial Potassium Content | After 2 Weeks | After 4 Weeks |
|---|---|---|---|
| Totem Agro.Bio (pH 7.0) | 2.74 | 3.34 | 2.31 |
| Totem Agro.Bio (pH 6.0) | 3.36 | 3.68 | 2.87 |
Regarding mineral nutrition, as seen in Tables 15, 16, and 17, the content of readily available forms of elements in acidic fertilizers, both in potassium humate and Totem Agro.Bio, is generally higher than in the same fertilizers with a neutral pH. Therefore, the hypothesis that mineral nutrition conditions for potatoes deteriorate in acidic forms of humic fertilizers is ruled out.
The results of the fertilizer analysis are of independent interest, as they allow for identifying trends in element transformation during fertilizer storage. These data show that one month after production, the content of available nitrogen in fertilizers hardly decreases, whereas phosphorus retrogradation, mainly in the water-soluble form, becomes noticeable after two weeks.
It is noteworthy that potassium, in turn, influences the dynamics of nitrogen and phosphorus. Specifically, in acidic Totem Agro.Bio, the content of available forms of these elements is lower than in potassium humate. In the neutral form, nitrogen is also lower, but phosphorus is higher. Apparently, potassium significantly affects the strength of ammonia binding in humic fertilizers and, as a monovalent metal, displaces the ammonium ion, thereby contributing to its loss.
Conclusions
Since the issue of phosphorus transformation in interaction with humic substances is highly significant, further research is necessary.