Organic and Inorganic Fertilizers: Pros and Cons

Fertilizers are substances added to soil to improve plant growth and yield by providing essential nutrients. They can be broadly categorized as either organic fertilizers derived from plant and animal sources or inorganic (chemical) fertilizers that are artificially synthesized.

Gardeners have used various organic materials, like compost, manure, and crop residues, in growing plants. But most use them interchangeably without understanding their distinct characteristics.

What are the key differences between organic and inorganic fertilizers?

Organic fertilizers are decomposed organic matter containing microorganisms that mineralize nutrients over time for gradual plant uptake. Inorganic fertilizers provide precise concentrations of nutrients in readily available mineral forms for immediate absorption by plants.

In this article, we will explore the differences between these two types of fertilizers and explain how to select the right one for optimal plant nutrition and soil fertility. Let’s start!

What are organic fertilizers?

Organic fertilizers are fertilizers derived from animal matter, animal excreta (manure), human excreta, and vegetable matter (e.g., compost and crop residues). Naturally occurring organic fertilizers include manure from cows, rabbits, sheep, horses, pigs, poultry, vermiculture, etc. Other examples of organic fertilizers include compost, bone meal, blood meal, seaweed, coconut fiber, grass clippings, and shellfish waste.

The organic matter in organic fertilizers breaks down over time through the action of microorganisms and releases nutrients for plant uptake. The nutrients in organic fertilizers are in organic forms that must be mineralized into plant-available forms before plants can use them. Mineralization involves the decomposition of organic compounds by soil microbes, which slowly converts nutrients into inorganic ions that can be absorbed by plant roots.

Organic fertilizers usually contain relatively low concentrations of nutrients compared to inorganic fertilizers. However, they perform many other valuable functions apart from just providing nutrients.

Benefits of Organic Fertilizers

• Improve soil structure: Organic matter in organic fertilizers binds soil particles into aggregates, improving soil structure. Well-structured soil with good aggregation has higher water holding capacity and air permeability for better root growth.
• Increase water holding capacity: Organic matter can hold up to 90% of its weight in moisture. This increases the water-holding capacity of sandy soils and reduces water loss from well-structured soils.
• Improve nutrient retention: Organic fertilizers enhance the ability of soils to retain and supply nutrients such as nitrogen, phosphorus, and sulfur in plant-available forms over a longer period of time. Nutrient losses from leaching are reduced.
• Promote soil biodiversity: The diverse microbial populations promoted by organic matter are beneficial for nutrient cycling and disease suppression. Mycorrhizal association is also enhanced.
• Reduce soil erosion: Organic matter promotes the formation of stable soil aggregates that are more resistant to erosion by wind and water.
• Replenish soil humus: Humus, the stable remnant of decomposed organic matter, acts as a soil conditioner, improves nutrient availability, and stimulates plant growth.
• Safe for plants: Organic fertilizers release nutrients slower and more steadily compared to chemical fertilizers, reducing the chances of nutrient toxicity.
• Environmentally friendly: Organic fertilizers do not pollute groundwater with nitrates, phosphates, etc. Their production has fewer detrimental impacts on the environment compared to chemical fertilizers.

Limitations of Organic Fertilizers

• Low nutrient content: The concentrations of essential nutrients, especially nitrogen, phosphorus, and potassium, are relatively low in organic fertilizers. Repeated heavy applications are needed to fulfill plant nutrient requirements.
• Slow nutrient release: Nutrients in organic fertilizers are released slowly through mineralization. They may not be able to meet the high nutrient demand of fast-growing crop plants. Additional inorganic fertilization may be required.
• Bulky: Large quantities need to be transported and applied due to the low nutrient content. This increases costs.
• Variable nutrient content: The nutrient content of organic fertilizers depends on the source material and decomposition rate, making standardization difficult.
• Potential contaminants: Improperly composted organic materials may contain pathogens, weed seeds, pharmaceutical residues, heavy metals, etc. that can be detrimental to crops.
• Potential odors: Raw animal manures and immature compost can produce unpleasant odors.
• Slow effects: The full benefits of organic fertilizers may only become evident after repeated applications over many cropping seasons.

Thus, while organic fertilizers enhance overall soil quality and provide a range of agronomic benefits beyond just supplying nutrients, they have limitations regarding their nutrient content and release characteristics.

What are inorganic fertilizers?

Inorganic fertilizers, also referred to as synthetic fertilizers or chemical fertilizers, are manufactured artificially through various industrial and chemical processes. They are formulated with specific amounts of essential plant nutrients such as nitrogen, phosphorus, and potassium, which are required in relatively large amounts. Other nutrients such as calcium, magnesium, sulfur, iron, manganese, copper, zinc, etc. can also be added in the required amounts.

The most commonly used inorganic fertilizers are:

• Nitrogen fertilizers: Urea, ammonium nitrate, ammonium sulfate, calcium ammonium nitrate, etc.
• Phosphorus fertilizers: Superphosphate, triple superphosphate, monoammonium phosphate (MAP), diammonium phosphate (DAP), etc.
• Potassium fertilizers: Potassium chloride (muriate of potash), potassium sulfate (sulfate of potash), etc.
• Micronutrient fertilizers: Zinc sulfate, ferrous sulfate, copper sulfate, manganese sulfate, sodium molybdate, etc.
• Multi-nutrient fertilizers: Diammonium phosphate (DAP), NPK complexes with different nutrient ratios, etc.

The nutrients in inorganic fertilizers are present in mineral forms that are readily soluble in water. Plants can directly and immediately take up the nutrients once the fertilizer dissolves in the soil.

Benefits of Inorganic Fertilizers

• Higher concentrations of nutrients are needed compared to organic fertilizers; hence, smaller quantities are needed.
• Nutrients are present in readily available mineral forms that are immediately accessible for plant uptake after application.
• Nutrient content is precisely known and consistent for a given fertilizer. This allows targeted application based on soil test recommendations.
• Release characteristics can be controlled to match crop demand. Slow, controlled-release versions are also available.
• The application provides an immediate boost to plant growth and productivity. Effects are seen within days or weeks.
• More cost-effective in the short term compared to organic fertilizers.
• Standardized products are available. Custom blended fertilizers can be prepared with the desired nutrient grades and ratios.
• Ease of storage, handling, and application Does not require bulk volumes.
• Lack of contaminants like heavy metals, pathogens, etc. that may be found in organic materials

Limitations of Inorganic Fertilizers

• Repeated use can lead to soil acidification over time for products like ammonium sulfate, urea, DAP, etc.
• Excessive use can increase soil salinity. Annual applications are needed to maintain fertility.
• Leaching and runoff losses of mobile nutrients like nitrate and phosphate contribute to groundwater and surface water pollution.
• Most nutrients remain readily soluble after application and are not retained against leaching. Exceptions are phosphorus that gets fixed in soils and potassium that gets held between layers of clay minerals.
• Does not contribute organic matter to improve soil structure, water retention, and microbiology.
• The salt index of some fertilizers can burn plant roots and foliage if not applied properly.
• Manufacturing requires non-renewable resources like natural gas, phosphate rock, petroleum, etc. Emits greenhouse gases.
• Exposure to some fertilizer components, like ammonia and superphosphate, may cause irritation to the eyes, skin, nose, and throat.

Thus, while chemical fertilizers provide nutrients in immediately usable forms, they do not provide long-term benefits to soil fertility or structure. Judicious integrated use of both organic and inorganic types is recommended for a holistic soil fertility management program.

Integrated Nutrient Management (INM)

Integrated nutrient management aims to maintain and improve soil fertility and plant nutrient supply to an optimal level for sustaining the desired productivity through optimization of all available organic and inorganic sources and approaches.

Some viable integrated approaches include:

• Combining chemical fertilizers with locally available organic manures, compost, crop residues, biofertilizers, etc. to provide a balance between meeting the immediate nutrient demand of crops and maintaining long-term soil health
• Using 50–100% of the recommended dose of inorganic fertilizers along with supplemental organic sources The quantity depends on the local availability of organics and crop, soil, and climate conditions.
• Adopting split applications: applying part organic and part chemical fertilizer at sowing, followed by side dressings of chemical fertilizer during later crop growth stages when nutrient demand is high.
• Applying inorganic fertilizers specifically targeted to address any identified nutrient deficiencies through soil testing instead of blanket applications Supplement with organics.
• Using precision agriculture approaches to apply inorganic nutrients site-specifically at variable rates tailored to intra-field soil fertility variations guided by sensing and soil maps
• Employing nitrification inhibitors along with nitrogen fertilizers to slow ammonium conversion to nitrate, thereby reducing nitrogen losses.
• Using sulfur-coated urea or polymer-coated fertilizers for the slow release of nutrients to synchronize with crop demand Reduces losses.
• Incorporating deep-rooted green manures that can capture nutrients from deeper layers or use insoluble forms reduces the need for chemical fertilizers.
• Adopting crop rotations with legumes that fix nitrogen from the air and reduce the need for nitrogenous fertilizers.
• Promoting root symbioses like arbuscular mycorrhizal fungi and rhizobia bacteria that enhance nutrient and water acquisition in crops, thereby potentially reducing inorganic fertilizer requirements,
• Integrating site-specific water management to optimize fertilizer use efficiency under rainfed conditions susceptible to droughts or flooding

Thus, the integrated use of organic and inorganic sources of plant nutrients can optimize crop nutrition while minimizing losses to the environment and sustaining soil health. The proportions, timing, and placement can be fine-tuned based on crop requirements, site conditions, and identified limiting factors for tailoring a location-specific INM program.

How Organic Fertilizers Work

Organic fertilizers increase soil fertility through:

1. Providing Plant Nutrients

Although concentrations are low, organic fertilizers do supply nitrogen, phosphorus, potassium, and other plant nutrients.

• Nitrogen is present in organic forms like proteins, amino acids, and amines, which are converted into absorbable ammonium and nitrate by soil microbes. Also slowly released from residual soil organic matter.
• Phosphorus: Partially soluble phosphates and organically bound phosphorus are mineralized into absorbable orthophosphate.
• Potassium is released through the decomposition of organic matter or the dissolution of potash deposits.
• Micronutrients: present as organic complexes and chelates that are broken down gradually to liberate nutrients.

2. Increasing Organic Matter

Organic residues stimulate the growth and activity of beneficial soil microorganisms. These microbes decompose and transform organic materials into humus, the stable organic fraction.

• Fresh residues → Partially decomposed residues → Active humus → Stable humus

Humus is a colloidal material that increases the soil’s cation exchange capacity (CEC) to store and supply nutrients, improves soil structure, enhances moisture retention, and provides energy for soil microbes.

3. Supporting Soil Microbial Activity

Diverse microbial populations are promoted, including fungi, bacteria, actinomycetes, protozoa, etc.

Key beneficial effects of enhanced soil biological activity include:

• Mineralization and cycling of nutrients
• Biological nitrogen fixation
• Solubilization of phosphorus and micronutrients
• Production of plant growth-promoting substances
• Suppression of plant diseases
• Elimination of toxins and waste
• Improved soil structure

Thus, both the direct nutrient contributions as well as the indirect benefits through improved physical, chemical, and biological properties of soil make organic fertilizers an integral component of soil fertility management programs.

How Inorganic Fertilizers Work

Inorganic fertilizers enhance plant growth primarily by providing essential nutrients in plant-available forms. The key nutrients supplied are:

1. Nitrogen

Nitrogen is required by plants in the largest amounts. It is a constituent of proteins, chlorophyll, nucleotides, alkaloids, enzymes, and growth hormones. Nitrogen promotes vegetative growth and leafy development.

Common nitrogenous fertilizers:

• Urea
• Ammonium nitrate
• Ammonium sulfate
• Calcium ammonium nitrate

Nitrogen is available to plants immediately after application in the form of ammonium or nitrate ions, which are absorbed directly by roots. Excess nitrate may, however, be lost by leaching.

2. Phosphorus

Phosphorus is a constituent of key molecules like nucleic acids, phospholipids, ATP, and phosphorylated intermediates in plants. It aids in photosynthesis, energy transfer, flowering, fruiting, maturation, root growth, and seed formation.

Common phosphatic fertilizers:

• Ordinary superphosphate
• Triple superphosphate
• Monoammonium phosphate (MAP)
• Diammonium phosphate (DAP)

Phosphates get adsorbed on soil colloids and react with cations like calcium after application. Small amounts get dissolved in soil solutions for immediate uptake by plants. Residual phosphates are temporarily retained to prevent leaching.

3. Potassium

Potassium does not form any structural components in plants; however, it has vital functional roles. It activates enzymes, maintains turgor pressure and electroneutrality, regulates stomatal movement, aids the translocation of metabolites and proteins, reduces water loss, and increases frost hardiness.

Common potassic fertilizers:

• Potassium chloride (Muriate of potash)
• Potassium sulfate (Sulfate of potash)

Potassium ions are held between the layers of some clay minerals in soils after fertilizer application and get slowly released into the soil solution. This retention prevents leaching losses.

4. Secondary Nutrients

Calcium, magnesium, and sulfur are required in smaller but still significant amounts. Calcium regulates enzymatic and hormonal processes. Magnesium is central to chlorophyll and activates enzymes. Sulfur is needed for protein synthesis.

5. Micronutrients

These include boron, chlorine, copper, iron, manganese, molybdenum, nickel, and zinc, which are essential in minute quantities. They activate various enzymes and are components of metabolites fundamental to plant growth.

Thus, inorganic fertilizers provide a concentrated, quick-release source of essential plant nutrients to correct deficiencies, balance the ratio of elements, and drive plant growth and productivity. However, long-term soil quality has not improved.

Effectiveness of Organic vs Chemical Fertilizers

While organic and inorganic fertilizers have their own unique advantages and limitations, a key aspect is their effectiveness for plant nutrition and productivity. Many comparative studies provide useful insights.

• Experiments on rice showed that combining 50% of the recommended nitrogen dose with organic manure and 50% with urea achieved similar yields as 100% inorganic nitrogen while improving soil health and saving costs.
• Maize yields with recommended NPK and farmyard manure were 20% higher than yields with only recommended NPK.
• Cotton yields were 18% higher when NPK was combined with poultry manure compared to only NPK application.
• Growth and yield parameters of tomatoes were enhanced most by vermicompost as an organic source, compared to recommended inorganic fertilizers alone.
• An overall meta-analysis of studies found that crop yields with organic sources were 16% lower than conventional mineral fertilizers. The yield gap reduced over time.

Thus, while inorganic fertilizers provide complete nutrition for immediate high yields, supplemental organic sources improve soil properties and enhance the long-term sustainability of production. Integrated nutrient management aims to get the best out of both approaches.

Organic vs Chemical Fertilizers: The Debate

There is an ongoing debate regarding the relative merits and demerits of organic and chemical fertilizers.

Arguments favoring organic fertilizers:

• Enhance soil physical properties and long-term fertility
• Release nutrients more slowly and steadily, minimizing losses
• Reduce reliance on synthetics derived from non-renewables
• More environmentally friendly; do not contaminate groundwater
• Produce nutritious food with more health-beneficial compounds

Arguments favoring chemical fertilizers

• Provide greater amounts of immediately plant-available nutrients
• Release characteristics can be customized to match crop demand
• Higher crop yields and productivity can support food security
• More cost-effective in the short term compared to organics
• Easier to transport, handle, store and apply
• Lack organic contaminants; composition is precisely known

Finding Middle Ground

Integrated nutrient management aims to find an optimal balance between these approaches by utilizing both organic and inorganic sources in a site-specific and need-based manner to maximize their benefits while minimizing their limitations.

With a growing world population and increasing pressure on land resources, neither organic nor conventional systems alone may be able to sustainably meet current and future food demands globally. An integrative combination that takes the best of both approaches may be the pragmatic solution.

Conclusion

In summary, while organic fertilizers alone may not be adequate to meet the high nutrient needs of intensive agriculture, their benefits for long-term soil fertility are invaluable. On the other hand, chemical fertilizers serve as an efficient source of nutrients to meet immediate crop demands, but at the cost of soil and environmental health when used indiscriminately.

Thus, neither can be labeled universally ‘good’ or ‘bad’. An integrative approach combining judicious amounts of organics and inorganics tailored to crop and soil requirements appears to be the most prudent and sustainable means of managing soil fertility, ensuring plant nutrition, and obtaining optimal productivity while also preserving the agricultural resource base for the future.

Further research and outreach are needed to develop regionally tailored INM techniques and promote their widespread adoption globally. Policy and economic measures aimed at incentivizing INM will also boost its expansion and success. Ultimately, a shift in how we view and manage soil—as a living ecosystem rather than an inert medium—is central to building truly sustainable food production systems.