The completion of organic fertilizer fermentation requires a comprehensive evaluation across multiple dimensions. Below are the detailed criteria and methods:

I. Core Evaluation Criteria

1. Temperature Dynamics

• Composting:
  • Thermophilic Phase: Initial temperature rise to 50–65℃ (maintained for 5–6 days for sterilization).
  • Cooling Phase: Temperature gradually drops to ambient levels or slightly elevated (typically <40℃), indicating progression to the maturation phase.
• Biogas Fermentation:
  • Stable gas production with ignitable output, pH stabilization between 6.5–7.5, and absence of acidification (e.g., non-yellowing fermentation broth) indicate balanced activity between acidogenic and methanogenic bacteria.

2. Sensory Characteristics

• Color:
  • Raw materials transition from bright hues (e.g., golden straw) to dark brown or black, indicating thorough organic matter decomposition.
• Odor:
  • Initial strong odors (e.g., ammonia, hydrogen sulfide) diminish, replaced by earthy or mild alcoholic notes, with no offensive smells.
• Texture:
  • Material transforms from loose to soft and friable, with no coarse particles or agglomerates, achieving a fine, homogeneous consistency.

3. Chemical Parameters

• C/N Ratio:
  • Composting: Maturation occurs when the initial C/N ratio (25–35:1) decreases to 15–20:1, or the water-soluble organic carbon/organic nitrogen ratio reaches 5–6.
  • Biogas Fermentation: Optimal C/N ratio is 20–30:1.
• Organic Matter Content:
  • Volatile solids content decreases by 15–30%, indicating stabilized organic matter.
• Nitrogen Forms:
  • Ammonia nitrogen decreases, nitrate nitrogen increases, and total nitrogen content stabilizes.

4. Biological Indicators

• Seed Germination Test:
  • Organic fertilizer leachate, compared to control water, achieves ≥50% seed germination rate with normal seedling growth, indicating absence of phytotoxicity.
• Microbial Activity:
  • Reduced CO₂ production and oxygen consumption rates, dormant microorganisms, and decreased enzyme activity.

5. Laboratory Analysis

• pH Value:
  • Composting: Final pH stabilizes between 5.5–8.5.
  • Biogas Fermentation: pH stabilizes between 6.5–7.5.
• Hygiene Standards:
  • E. coli Count: ≤10–100 CFU/g of organic fertilizer.
  • Ascaris Egg Mortality: ≥95%.
  • Heavy Metal Content: Complies with national standards (e.g., GB 38400-2019).

6. Additional Criteria for Specific Fermentation Types

• Biogas Fermentation:
  • Stable gas production with ignitable output and stable pH (no acidification).
• Liquid Fermentation:
  • Rapid dissolution of material in water with no impurity precipitation.

II. Evaluation Workflow

1. Initial Observation:
   • Temperature drops to ambient levels, color darkens, odor becomes earthy, and texture softens.
2. Simple Tests:
   • Seed Germination Test: ≥50% germination rate in leachate vs. control.
   • Dissolution Test (liquid fermentation): Rapid dissolution with no sediment.
3. Laboratory Testing (if required):
   • pH, C/N ratio, heavy metals, E. coli count, and Ascaris egg mortality analysis.

III. Fermentation Duration Guidelines

• Composting:
  • Thermophilic Phase: 1–2 weeks (50–65℃).
  • Maturation Phase: Several weeks to months (depending on raw materials and conditions).
• Biogas Fermentation:
  • Start-Up Phase: Several days to two weeks (until stable gas production).

IV. Key Precautions

• Turning and Aeration: Regular turning during the thermophilic phase maintains aeration and prevents localized anaerobic conditions.
• Moisture Control: Maintain 50–60% moisture; excessive moisture promotes anaerobic bacteria, while insufficient dryness delays fermentation.
• Feedstock Ratio: Adjust C/N ratio to appropriate ranges (25–35:1 for composting, 20–30:1 for biogas).

By integrating these criteria, organic fertilizer fermentation completion can be accurately determined, ensuring safety and efficacy for agricultural use.