Introduction
With the global population projected to reach 9.5 billion by 2050, the agriculture sector must significantly increase food production. Fertilizers remain essential for supplying vital nutrients to crops; however, improper fertilizer use—either excessive or insufficient—has led to declining soil health and environmental concerns, including nitrogen and phosphate imbalances and ammonia emissions contributing to air pollution.
Continuous overuse of mineral fertilizers reduces soil organic matter, hardens soil texture, depletes essential nutrients, and increases environmental risks through leaching and runoff. Therefore, knowledge-based fertilization strategies are essential to mitigate soil degradation, avoid secondary salinization, and promote sustainable agriculture.
Foliar and root fertilization techniques offer different advantages, with foliar applications emerging as an effective method to meet micronutrient requirements efficiently, particularly at critical growth stages. Water-soluble fertilizers (WSFs) provide faster nutrient availability, reduced application rates, and compatibility with modern irrigation systems. Nonetheless, the physicochemical properties of WSFs—such as moisture content, caking tendency, solubility, and compatibility—are crucial for their performance, handling, and storage stability.
This study compares the physicochemical properties of different WSF grades to identify optimal formulations for enhanced plant nutrition and ease of use.
MATERIALS AND METHODS
Fertilizer Samples
The following formulations were evaluated:
- 15-15-15 without micronutrients (commercial standard),
- 15-15-15 fortified with three micronutrients,
- 15-15-15 fortified with five micronutrients,
- 14-14-14 fortified with seven micronutrients.
Physicochemical Analyses
- Moisture Content: Determined by drying 5 g of fertilizer in a vacuum desiccator for 24 hours and measuring weight loss.
- Critical Relative Humidity (CRH): Measured using a variable-humidity chamber by exposing preheated samples and monitoring moisture absorption.
- Chloride Content: Quantified by silver nitrate volumetric titration after dissolving samples and acidifying the solution.
- pH Measurement: Determined using a calibrated pH meter on a 5% (w/v) solution of each sample.
- Solubility Test: Assessed by adding 5 g of fertilizer to 100 mL of water, stirring, and observing complete dissolution and sediment formation over an hour.
- Compatibility in Blends: Evaluated by storing blend samples at 30°C for 30 days and visually inspecting for signs of incompatibility.
- Caking Tendency: Assessed by packing samples under applied pressure and observing lump formation after 1–2 months.
RESULTS AND DISCUSSION
Moisture Content
Moisture directly affects fertilizer handling and stability. High moisture can cause caking and reduced flowability. Table 1 and Figure 1 illustrate the moisture levels of the samples, with the 14-14-14 grade exhibiting the lowest moisture content (0.29%), indicating superior stability.
Table 1: Moisture Content
| Grade | % Moisture |
|---|---|
| 15-15-15 (no micronutrients) | 0.80 |
| 15-15-15 (3 micronutrients) | 0.42 |
| 15-15-15 (5 micronutrients) | 0.39 |
| 14-14-14 (7 micronutrients) | 0.29 |
Figure 1. Comparison of moisture content among fertilizer grades.
Critical Relative Humidity (CRH)
Higher CRH reduces caking risk by minimizing moisture absorption from the atmosphere. Table 2 and Figure 2 show that the 14-14-14 grade had the highest CRH (77%), indicating better resistance to caking during storage.
Table 2: Critical Relative Humidity
| Grade | % CRH |
|---|---|
| 15-15-15 (no micronutrients) | 72 |
| 15-15-15 (3 micronutrients) | 74 |
| 15-15-15 (5 micronutrients) | 75 |
| 14-14-14 (7 micronutrients) | 77 |
Figure 2. CRH comparison.
Chloride Content
Excessive chlorides can damage crops through leaf burn and reduced water uptake. Table 3 and Figure 3 demonstrate that the 14-14-14 formulation had the lowest chloride content (0.23%), minimizing potential phytotoxicity.
Table 3: Chloride Content
| Grade | % Chloride |
|---|---|
| 15-15-15 (no micronutrients) | 0.76 |
| 15-15-15 (3 micronutrients) | 0.58 |
| 15-15-15 (5 micronutrients) | 0.28 |
| 14-14-14 (7 micronutrients) | 0.23 |
Figure 3. Chloride content comparison.
pH Measurement
Optimal pH supports nutrient availability; extreme pH levels can inhibit nutrient uptake. The 14-14-14 grade exhibited a near-neutral pH of 4.7 (Table 4, Figure 4), suitable for most crops.
Table 4: pH Values
| Grade | pH |
|---|---|
| 15-15-15 (no micronutrients) | 3.8 |
| 15-15-15 (3 micronutrients) | 4.1 |
| 15-15-15 (5 micronutrients) | 4.5 |
| 14-14-14 (7 micronutrients) | 4.7 |
Figure 4. pH comparison.
Solubility
All grades completely dissolved within one hour without precipitation, confirming their suitability for foliar and fertigation applications.
Compatibility in Blends
After 30 days of storage, all blends remained dry and free-flowing, with no signs of incompatibility such as wetting or gas release (Table 5).
Table 5: Compatibility Ratings
| Observation | Rating |
|---|---|
| Dry and free-flowing | Compatible |
| Damp but free-flowing | Predominantly compatible |
| Damp and non-flowing | Predominantly incompatible |
| Wet and non-flowing | Incompatible |
Caking Tendency
As shown in Table 6, the 14-14-14 grade with seven micronutrients exhibited no lump formation, indicating excellent handling properties.
Table 6: Caking Observations
| Grade | Observation |
|---|---|
| 15-15-15 (no micronutrients) | Medium lump formation |
| 15-15-15 (3 micronutrients) | Medium lump formation |
| 15-15-15 (5 micronutrients) | Light lump formation |
| 14-14-14 (7 micronutrients) | No lump formation |
CONCLUSION
Plants require a balanced supply of nutrients for optimal growth. Micronutrient deficiencies are widespread and can significantly reduce crop yields. This study demonstrates that fortifying N-P-K fertilizers with micronutrients—especially in the 14-14-14 formulation—enhances physicochemical properties, including reduced moisture content, higher CRH, lower chloride levels, and improved handling characteristics. Thus, micronutrients are as crucial as macronutrients for sustainable and efficient crop production.
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