As a vital resource in agricultural production, organic fertilizers’ core value lies in providing comprehensive and balanced nutrients for crops, while also improving soil structure and promoting ecological circulation. Compared to chemical fertilizers, organic fertilizers release nutrients more slowly and over a longer period of time, and can significantly enhance soil’s ability to retain water and fertilizer. So, what nutrients do organic fertilizers contain that are essential for crop growth? How do these nutrients affect different stages of crop growth? The following will provide a detailed analysis of organic fertilizers’ sources, main nutrients, and their mechanisms of action.
A. Main Sources and Composition of Organic Fertilizers
Organic fertilizers are typically derived from animal and plant residues, excrement, or fermented organic waste. They are complex and rich in a variety of nutrients.
Common organic fertilizers, such as livestock and poultry manure, compost, and green manure, can be categorized into the following categories:
a. Macronutrients: Nitrogen (N), phosphorus (P), and potassium (K) are the three essential elements for crop growth.
- Nitrogen: Primarily promotes leaf and stem growth and is key to protein and chlorophyll synthesis. For example, the nitrogen content of composted pig manure is approximately 0.5%–1.5%, while leguminous green manures (such as milk vetch) can reach 3%–4%.
- Phosphorus: Participates in energy transfer and root development and is commonly found in bone meal (containing 11%–15% phosphorus) and poultry manure.
- Potassium: Enhances crop stress resistance and regulates water metabolism. Wood ash has a high potassium content, reaching 5%–10%.
b. Secondary Elements: Calcium (Ca), magnesium (Mg), and sulfur (S) are crucial for crop physiological functions.
- Calcium prevents fruit cracking (such as in tomato navel rot) and is commonly found in eggshell powder and lime-based organic fertilizers.
- Magnesium is a core component of chlorophyll and is found in high concentrations in straw-based fertilizers.
- Sulfur is involved in amino acid synthesis and is present in approximately 0.1%–0.5% of composted cow manure.
c. Trace Elements: Iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), and molybdenum (Mo), although required in small quantities, are essential.
For example, boron affects pollen development, and a boron deficiency can result in “flowers without seeds” in rapeseed. Wormcasting, high in zinc and iron, effectively prevents crop chlorosis.
d. Organic Matter and Microorganisms: Organic fertilizers comprise 30% to 60% organic matter, including humic acid and cellulose, which improve soil aggregate structure. Furthermore, the probiotics they carry (such as nitrogen-fixing and phosphate-solubilizing bacteria) activate inert nutrients in the soil.
B. Nutritional Characteristics and Applications of Different Organic Fertilizers
a. Livestock and Poultry Manure (e.g., chicken manure, cow manure):
- Chicken manure is high in nitrogen and phosphorus, but requires thorough decomposition to avoid root burn, making it suitable for leafy crops.
- Cow manure is rich in organic matter but low in nutrients, making it suitable for improving heavy clay soils.
b. Compost and Green Manure:
- Straw compost has a high carbon-to-nitrogen ratio (C/N) and is suitable for use in combination with high-nitrogen fertilizers;
- Green manures such as milk vetch can directly replenish soil nitrogen through biological nitrogen fixation, reducing reliance on chemical fertilizers.
c. Industrial Byproducts (such as sugarcane residue and mushroom residue):
- Sugarcane residue is rich in potassium and is suitable for fruit trees during the fruit-swelling period;
- Mushroom residue contains mycelial residue, which can significantly enhance soil microbial activity.
C. Mechanism of Action and Scientific Application of Organic Fertilizers
a. Slow-Release Effect: Organic fertilizer nutrients require microbial decomposition before they can be absorbed, so their effectiveness lasts for 2-3 months. For example, applying 2 tons of decomposed pig manure per mu can meet approximately 60% of the nitrogen requirements for a rice crop.
b. Soil Improvement: Long-term use of organic fertilizers can increase soil porosity by 20-30% and reduce heavy metal activity by over 50% (cited from the Ministry of Agriculture and Rural Affairs’ 2024 Soil Remediation Report).
c. Application Techniques:
- Deep Application of Base Fertilizer: Bury organic fertilizer 15-20 cm into the soil to prevent nutrient loss;
- Combination with Inorganic Fertilizer: Supplement with quick-acting chemical fertilizer during critical crop growth periods (such as heading);
- Controlled Dosage: The recommended dosage for fruits and vegetables is 3-5 tons per mu (approximately 1.5 to 2.5 tonnes) per mu (approximately 1.5 to 2.5 tonnes per acre); excessive dosage can lead to excessive growth.
D. Limitations and Development Prospects of Organic Fertilizers
Despite the significant advantages of organic fertilizers, their unstable nutrient content and high transportation costs remain challenges for their widespread adoption. Currently, “bio-organic fertilizers” produced through microbial fermentation technology (such as fertilizers containing phosphate-solubilizing bacteria) are becoming a research hotspot, potentially increasing nutrient utilization by over 30%. In the future, as the “integrated farming and animal husbandry” model becomes more widespread, organic fertilizers will play an even more central role in green agriculture.
Conclusion: Organic fertilizers serve as a link between natural cycles and modern agriculture, offering a nutritionally comprehensive profile far exceeding that of chemical fertilizers. Scientifically understanding its composition and action patterns is a key step in achieving the goal of “reducing fertilizers and improving quality.”
