Manures

1. Introduction

• Manures are natural organic substances obtained from the decomposition of animal and plant residues. They are applied to the soil to improve fertility, structure, and biological activity, thereby promoting healthy plant growth.
• Manures act as a source of essential nutrients and help in maintaining the physical, chemical, and biological balance of the soil. Unlike chemical fertilizers, they release nutrients slowly and enhance soil organic matter content, which is crucial for sustainable agriculture.

2. Definition

• Manure is any organic material—solid or liquid—that is added to soil to supply nutrients and improve its physical, chemical, and biological characteristics.
• According to FAO (Food and Agriculture Organization): “Manures are materials containing plant nutrients in forms that are not readily available for plant use, but which after decomposition release nutrients slowly.”

3. Importance of Manures

• Supply all essential nutrients, though in small quantities.
• Improve soil structure and texture.
• Increase water-holding capacity of sandy soils.
• Improve aeration and permeability in clayey soils.
• Stimulate microbial activity and enhance nutrient cycling.
• Reduce soil erosion by improving aggregation.
• Provide humus which improves cation exchange capacity (CEC).
• Serve as a long-term soil fertility enhancer.

4. Classification of Manures

Manures are broadly classified into three categories:

5. Bulky organic manures

• These manures contain small amounts of nutrients but are applied in large quantities. They improve soil fertility mainly by increasing organic matter content.

5.1 Farmyard Manure (FYM)

Composition: Mixture of cow dung, urine, bedding materials, and leftover feed.

Nutrient Content (Average):

• Nitrogen (N): 0.5%
• Phosphorus (P₂O₅): 0.2%
• Potassium (K₂O): 0.5%

Preparation:

• Collected from cattle sheds and stored in pits for 4–5 months for decomposition.
• Decomposition carried out by microorganisms in moist, aerated conditions.

Application: 10–20 tonnes/ha applied 2–3 weeks before sowing and incorporated into soil.

Benefits:

• Improves soil structure, aeration, and moisture retention.
• Enhances microbial activity and nutrient availability.
• Reduces dependency on chemical fertilizers.

5.2 Compost

• Compost is the decomposed product of organic residues like crop waste, kitchen waste, and leaves under aerobic conditions.

Nutrient Content (Average):

• N: 0.5%
• P₂O₅: 0.3%
• K₂O: 0.5%

Types:

• Rural Compost: Made from farm and household waste (straw, dung, etc.).
• Urban Compost: Made from city refuse and garbage.
• Vermicompost: Prepared using earthworms.

Process of Composting:

• Waste is collected, moistened, and arranged in layers.
• Decomposition by bacteria, fungi, and actinomycetes.
• Compost is ready in 3–4 months.

5.3 Green Manure

• Green manuring involves growing and ploughing under green plants into the soil to improve fertility and organic matter.

Common Green Manure Crops:

• Dhaincha (Sesbania aculeata)
• Sunhemp (Crotalaria juncea)
• Guar (Cyamopsis tetragonoloba)
• Cowpea (Vigna unguiculata)

Nutrient Contribution:

• Adds 40–60 kg N/ha (depending on crop).
• Enhances soil organic carbon and microbial biomass.

Benefits:

• Improves soil structure and water infiltration.
• Reduces soil erosion and compaction.
• Adds organic matter and fixes atmospheric nitrogen.

5.4 Vermicompost

• Prepared by the decomposition of organic wastes with the help of earthworms (e.g., Eisenia foetida, Eudrilus eugeniae).

Nutrient Content (Average):

• N: 1.5%
• P₂O₅: 0.8%
• K₂O: 1.0%

Advantages:

• Enriched with plant growth hormones (IAA, GA).
• Provides enzymes and beneficial microbes.
• Enhances soil aeration and water-holding capacity.

5.5 Sewage and Sludge

By-products of municipal waste treatment plants.

• Sewage water: Used for irrigation (contains soluble nutrients).
• Sludge: Semi-solid residue used as manure after drying.

Nutrient Content:

• N: 3%
• P₂O₅: 2%
• K₂O: 1%

Caution: Should be tested for heavy metals before use.

5.6 Biogas Slurry

• Residue obtained after the anaerobic digestion of dung in biogas plants.
• Rich in nutrients and free from weed seeds and pathogens.

Nutrient Content:

• N: 1.5%
• P₂O₅: 1.0%
• K₂O: 1.0%

6. Concentrated organic manures

These are manures with higher nutrient content and applied in small quantities.

7. Biofertilizers

Biofertilizers are living microorganisms that enhance the availability of nutrients to plants by nitrogen fixation, phosphorus solubilization, or decomposition of organic matter.

8. Advantages of Using Manures

1. Improve soil physical properties (structure, porosity, aggregation).
2. Increase soil biological activity (microbes, earthworms).
3. Supply secondary and micronutrients.
4. Improve water-holding capacity and CEC.
5. Minimize fertilizer requirements.
6. Reduce nutrient losses through leaching.
7. Promote eco-friendly and sustainable farming.

9. Limitations of Manures

• Low nutrient concentration.
• Require large quantities and bulky handling.
• Slow nutrient release.
• High transportation cost.
• Nutrient composition not uniform.
• Decomposition depends on environmental conditions.

10. Role of Manures in Soil Fertility and Productivity

• Maintain long-term fertility by improving organic carbon levels.
• Promote microbial activity for nutrient cycling.
• Increase crop yield and soil health in the long run.
• Contribute to Integrated Nutrient Management (INM) practices for sustainable agriculture.

Summary Table