Unit 1: Fundamentals of Organic Manures and Fertilizers - Subjective Questions

Organic manure is a natural material of plant, animal, or human origin that is added to soil to supply plant nutrients and improve soil fertility.

Its importance includes:

• Nutrient supply: It provides essential nutrients such as nitrogen, phosphorus, potassium, sulfur, and micronutrients.
• Improved soil structure: Organic matter promotes aggregation and makes the soil loose and porous.
• Higher water-holding capacity: It helps soil retain moisture for longer periods.
• Enhanced microbial activity: It supplies food and energy to beneficial soil microorganisms.
• Sustainable fertility: Nutrients are released gradually, supporting long-term soil productivity.

Thus, organic manure improves the physical, chemical, and biological properties of soil.

A fertilizer is a natural or manufactured material containing one or more essential plant nutrients in concentrated and readily available forms. It is applied to soil or plants to improve growth and crop yield.

Major characteristics of fertilizers are:

• They have a known and relatively high nutrient concentration.
• Their nutrients are generally available to plants quickly.
• They may supply one nutrient or several nutrients.
• They are required in smaller quantities than bulky organic manures.
• They do not usually add significant organic matter to soil.
• Excessive or improper application may cause nutrient loss, soil degradation, or environmental pollution.

Both inputs can complement each other when used according to crop and soil requirements.

Organic manures are broadly classified into two groups:

1. Bulky organic manures

These contain relatively low concentrations of nutrients and are applied in large quantities.

• Farmyard manure
• Compost
• Vermicompost
• Sewage and sludge
• Green manure

2. Concentrated organic manures

These contain higher concentrations of one or more plant nutrients and are applied in smaller quantities.

• Oil cakes such as neem cake and groundnut cake
• Bone meal
• Blood meal
• Fish meal
• Horn and hoof meal
• Poultry manure

The classification is mainly based on nutrient concentration and the quantity applied, although exact nutrient contents vary with source and processing.

Organic manures improve soil physical properties in several ways:

• Soil aggregation: Decomposed organic matter binds soil particles into stable aggregates.
• Bulk density: It generally reduces the bulk density of compact soils.
• Porosity and aeration: Better aggregation increases pore space and improves air movement.
• Water infiltration: Manured soil absorbs rainfall or irrigation water more effectively.
• Water-holding capacity: Organic matter retains water and makes it available to plants.
• Root penetration: A loose and well-aggregated soil allows roots to grow more easily.
• Erosion resistance: Stable aggregates reduce surface crusting and soil loss.
• Workability: Manure makes heavy soil easier to cultivate and improves the cohesion of sandy soil.

These effects create a favorable environment for seed germination and plant growth.

Organic manures affect soil chemistry through nutrient addition and organic matter decomposition.

• They supply macronutrients such as nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.
• They provide small quantities of micronutrients such as zinc, iron, copper, manganese, and boron.
• Decomposed organic matter increases the soil's cation exchange capacity, helping it retain positively charged nutrients.
• Organic compounds may form complexes with micronutrients and keep them available to plants.
• Manures improve the buffering capacity of soil and reduce sudden changes in soil reaction.
• Organic acids produced during decomposition can increase the availability of some forms of phosphorus.
• They reduce nutrient losses by holding nutrients and releasing them gradually.

However, their nutrient content is variable, so laboratory testing is useful for making accurate application decisions.

Organic manures are an important source of carbon and energy for soil organisms.

• They increase the population and activity of bacteria, fungi, actinomycetes, and earthworms.
• Microorganisms decompose organic residues and release mineral nutrients through mineralization.
• Biological activity promotes the formation of stable soil aggregates.
• Beneficial organisms may suppress certain soil-borne pathogens through competition or antagonism.
• Manures support nutrient transformations such as ammonification, nitrification, and sulfur oxidation.
• Earthworm activity improves soil mixing, aeration, and pore formation.
• A diverse soil biological community helps maintain nutrient cycling and ecological stability.

Fresh or contaminated manure must be handled properly because it may also introduce pathogens, weed seeds, or harmful substances.

Farmyard manure (FYM) is a decomposed mixture of cattle dung, urine, bedding material, litter, and leftover fodder collected from animal sheds.

Preparation:

• Dung, urine-soaked litter, and farm refuse are collected regularly.
• The materials are placed in a pit or heap in compact layers.
• Adequate moisture is maintained to support decomposition.
• The material is covered with soil, straw, or another suitable covering to reduce nutrient loss.
• After sufficient decomposition, the manure becomes dark, friable, and suitable for field application.

Agricultural value:

• FYM supplies nitrogen, phosphorus, potassium, and micronutrients.
• It adds organic matter and improves soil structure.
• It increases water retention and microbial activity.
• It releases nutrients slowly and has a residual effect.

Its quality depends on animal feed, bedding, storage conditions, decomposition, and protection from leaching and volatilization.

Composting is the controlled biological decomposition of organic wastes into a stable, humus-like product called compost.

The major stages are:

1. Preparation: Organic residues are collected, sorted, and reduced to a suitable particle size.
2. Mixing: Carbon-rich and nitrogen-rich materials are mixed to obtain an appropriate carbon-to-nitrogen ratio.
3. Mesophilic stage: Microorganisms begin decomposition at moderate temperatures.
4. Thermophilic stage: Rapid microbial activity raises the temperature, accelerates decomposition, and destroys many pathogens and weed seeds.
5. Cooling stage: Easily decomposable materials decline, and the temperature gradually falls.
6. Curing or maturation: Resistant organic compounds are transformed into stable humus-like material.

Successful composting requires suitable moisture, aeration, temperature, particle size, and carbon-to-nitrogen balance. Mature compost normally has an earthy smell, dark color, and stable temperature.

Green manuring is the practice of growing or collecting green plant material and incorporating it into the soil while it is still tender so that it decomposes and improves soil fertility.

Types:

• In-situ green manuring: A crop such as sunn hemp or dhaincha is grown in the same field and incorporated into the soil.
• Green-leaf manuring: Leaves and tender twigs are collected from plants grown elsewhere and incorporated into the field.

Benefits:

• Adds organic matter to soil.
• Leguminous green-manure crops contribute biologically fixed nitrogen.
• Improves aggregation, infiltration, and water retention.
• Encourages microbial activity and nutrient cycling.
• Reduces erosion and may suppress weeds.
• Helps recycle nutrients from deeper soil layers.

Limitations:

• Requires land, time, moisture, and labor.
• May compete with the main crop for a place in the cropping calendar.
• Decomposition can temporarily immobilize nutrients if the material is too mature or carbon-rich.

Vermicompost is a stabilized organic manure produced when suitable earthworms and microorganisms convert biodegradable wastes into fine, nutrient-rich castings.

Conditions required for production include:

• Suitable raw material: Partially decomposed crop residues, animal dung, and other non-toxic biodegradable wastes may be used.
• Earthworm species: Surface-feeding species adapted to organic waste are preferred.
• Moisture: The bed should remain moist but not waterlogged.
• Temperature: Moderate temperatures should be maintained because excessive heat can kill earthworms.
• Aeration: The material should remain porous and well aerated.
• Shade: Beds must be protected from direct sunlight and heavy rain.
• Suitable reaction: Extremely acidic or alkaline materials should be avoided.
• Protection: Earthworms must be protected from predators and toxic chemicals.

Mature vermicompost is dark, granular, and earthy-smelling. It supplies nutrients, beneficial microorganisms, and stable organic matter.

Fertilizers may be classified according to the nutrient or nutrients they supply.

1. Nitrogenous fertilizers:

• Urea
• Ammonium sulfate
• Calcium ammonium nitrate

2. Phosphatic fertilizers:

• Single superphosphate
• Triple superphosphate
• Rock phosphate

3. Potassic fertilizers:

• Muriate of potash
• Sulfate of potash

4. Secondary-nutrient fertilizers:

• Gypsum supplies calcium and sulfur.
• Magnesium sulfate supplies magnesium and sulfur.

5. Micronutrient fertilizers:

• Zinc sulfate
• Ferrous sulfate
• Borax

6. Compound or complex fertilizers:

These contain two or more primary nutrients in each granule, such as diammonium phosphate and various NPK complexes.

7. Mixed fertilizers:

These are physical mixtures of two or more fertilizer materials prepared to provide a required nutrient grade.

• Straight fertilizer: Primarily supplies one major plant nutrient. Urea is considered a straight nitrogenous fertilizer, while muriate of potash is a straight potassic fertilizer.
• Compound fertilizer: Supplies two or more plant nutrients through chemical combination or manufacture.
• Complex fertilizer: A type of compound fertilizer in which each granule contains two or more declared nutrients. Diammonium phosphate is a common example.
• Mixed fertilizer: Produced by physically blending two or more compatible fertilizer materials. Individual particles may have different compositions even though the mixture has a declared overall grade.

The major distinction is that nutrients in a complex fertilizer are chemically combined or incorporated within granules, whereas nutrients in a mixed fertilizer are supplied by physically blended fertilizer particles.

Fertilizer grade expresses the guaranteed percentages by mass of primary nutrients in the order nitrogen, phosphate, and potash, conventionally written as --.

For a 15-15-15 fertilizer:

• Nitrogen content
• Phosphate content as
• Potash content as

The nutrient quantity is calculated as:

Therefore:

Thus, 200 kg of 15-15-15 fertilizer supplies 30 kg of , 30 kg of , and 30 kg of . The remaining 110 kg consists of carriers, fillers, secondary nutrients, or other constituents.

The carbon-to-nitrogen ratio, written as , is the ratio of organic carbon to total nitrogen in an organic material. It influences the rate of decomposition and the availability of nitrogen.

• Carbon provides energy to decomposer microorganisms.
• Nitrogen is required for microbial proteins, enzymes, and cell growth.
• Materials with a very wide ratio decompose slowly and may cause nitrogen immobilization, during which microorganisms temporarily use soil nitrogen.
• Materials with a narrow ratio generally decompose rapidly and may release mineral nitrogen through mineralization.
• Extremely nitrogen-rich materials can lose nitrogen as ammonia under unfavorable handling conditions.
• During composting, carbon is lost mainly as carbon dioxide, causing the ratio to narrow.

A relatively stable and narrower ratio is one indicator of compost maturity, although it should be assessed together with temperature, odor, and other quality parameters.

Mineralization is the microbial conversion of nutrients in organic compounds into inorganic forms that plants can absorb. For example, organic nitrogen is converted first into ammonium and may later be transformed into nitrate.

Immobilization is the opposite temporary process in which microorganisms absorb inorganic nutrients from the soil and incorporate them into microbial biomass.

Their importance in manure management includes:

• Manures release much of their nutrient content only after mineralization.
• Temperature, moisture, aeration, soil reaction, and manure composition affect the mineralization rate.
• Carbon-rich residues with a wide ratio can promote immobilization and temporarily reduce nitrogen availability to crops.
• Well-decomposed manure generally creates more predictable nutrient release than fresh, resistant residues.
• Application timing should be coordinated with crop demand to improve nutrient-use efficiency.

Understanding both processes helps prevent early nutrient deficiency and later nutrient loss.

The quality of organic manure is affected by several interacting factors:

• Source material: Animal species, feed, crop type, and residue composition determine the original nutrient content.
• Carbon-to-nitrogen ratio: This affects decomposition and nitrogen availability.
• Moisture content: Excess water increases weight, dilutes nutrients, and may encourage anaerobic losses; insufficient moisture slows decomposition.
• Storage method: Exposure to rain can cause leaching, while uncovered storage may increase ammonia loss.
• Degree of decomposition: Immature manure may contain unstable compounds, pathogens, or viable weed seeds.
• Aeration and temperature: These regulate microbial activity and the rate of stabilization.
• Contamination: Soil, plastics, salts, heavy metals, pesticides, and other foreign materials reduce quality or create hazards.
• Sampling and analysis: Representative sampling is necessary because manure is often heterogeneous.

Important quality measurements include moisture, organic carbon, total nutrients, available nutrients, reaction, electrical conductivity, and indicators of maturity.

Testing is necessary to determine the identity, quality, safety, and agronomic value of manure and fertilizer.

• It measures the actual concentrations of important plant nutrients.
• It helps calculate an application rate that matches crop requirements.
• It identifies excessive moisture, inert material, or adulteration.
• It verifies whether a fertilizer satisfies its declared grade and relevant quality standards.
• It can detect undesirable characteristics such as excessive salinity, extreme reaction, heavy metals, or other contaminants.
• It helps estimate nutrient availability and avoid both deficiency and over-application.
• It supports economic decisions by determining the nutrient value of a product.
• It reduces environmental risks such as nitrate leaching, ammonia loss, eutrophication, and nutrient accumulation.

Reliable results require a representative sample, an appropriate analytical method, calibrated equipment, and proper quality-control procedures.

Possible environmental problems:

• Nitrate may leach into groundwater when nitrogen is applied in excess.
• Surface runoff can carry nitrogen and phosphorus into water bodies and cause eutrophication.
• Ammonia may volatilize from surface-applied manure or urea.
• Denitrification can produce nitrous oxide, a potent greenhouse gas.
• Repeated unbalanced fertilizer use may contribute to soil acidification, salinity, or nutrient imbalance.
• Poorly treated manure may contain pathogens, weed seeds, veterinary drug residues, or contaminants.
• Excessive manure application may lead to phosphorus, salts, or heavy-metal accumulation.

Preventive measures:

• Test soil and manure before deciding application rates.
• Match nutrient supply with crop demand.
• Apply nutrients at the correct time and by a suitable method.
• Incorporate manure when agronomically appropriate to reduce losses.
• Maintain buffer zones near water bodies.
• Store manure under protected conditions and control runoff.
• Use balanced fertilization and integrate organic and mineral nutrient sources.

Integrated nutrient management (INM) is the planned and combined use of organic materials, mineral fertilizers, biological sources, and native soil nutrients to maintain crop productivity and long-term soil fertility.

Organic manures and fertilizers complement each other in the following ways:

• Mineral fertilizers provide concentrated and readily available nutrients for immediate crop demand.
• Organic manures release nutrients more slowly and improve soil organic matter.
• Manures improve soil structure, water retention, cation exchange capacity, and microbial activity.
• Fertilizers can correct specific nutrient shortages that manures alone may not satisfy.
• Nutrients supplied by manure should be estimated and deducted from the total fertilizer requirement.
• Split fertilizer application can synchronize nutrient availability with different crop-growth stages.
• Soil testing, manure testing, realistic yield targets, and field history should guide the nutrient plan.

INM aims to achieve high nutrient-use efficiency, sustained yield, improved soil health, lower production costs, and reduced environmental loss.