5.2C Synthetic fertilizers Notes

1. Intensive agricultural systems rely heavily on synthetic fertilizers because fast-growing crop varieties rapidly deplete soil nutrients.
2. Large monocultures demand continuous nutrient inputs, as they remove nitrogen, phosphorus, and potassium faster than natural processes can replace them.
3. Synthetic fertilizers provide an immediate and concentrated nutrient supply, enabling high yields for commercial farming.
4. Most synthetic fertilizers are manufactured using fossil fuels, especially natural gas, making production energy-intensive and contributing to climate change.
5. Dependence on synthetic fertilizers creates long-term unsustainability, as soils become biologically degraded and farmers must apply increasingly large amounts to maintain yields.

Environmental Problems Created by Synthetic Fertilizers

1. Pollution and Water Contamination

1. Most applied fertilizers do not stay in the soil because rainfall dissolves them, causing nutrient runoff into streams, rivers, and lakes.
2. Leached nitrates contaminate groundwater, posing health risks such as “blue baby syndrome.”
3. Runoff into surface waters leads to eutrophication, creating algal blooms, oxygen depletion, and aquatic biodiversity loss.

2. Soil Biological and Chemical Degradation

1. Synthetic fertilizers disrupt soil microbial communities, especially organisms involved in the nitrogen cycle.
2. Nitrogen-fixing bacteria decline, reducing the soil’s natural ability to replenish nitrogen.
3. Nitrifying and denitrifying bacteria increase, accelerating the conversion of nitrogen to gaseous forms.
4. This shift weakens natural soil fertility, creating increased dependence on synthetic inputs.

3. Greenhouse Gas Emissions from Soil

1. Nitrogenous fertilizers increase microbial production of nitrous oxide, a greenhouse gas nearly 300 times more powerful than carbon dioxide.
2. Enhanced nitrification and denitrification cause greater nitrogen losses from soil, reducing efficiency and increasing emissions.

Positive and Negative Feedback Loops in Fertilizer Use

1. Negative feedback occurs when added nitrogen triggers an increase in denitrifying bacteria, which convert nitrates back into atmospheric nitrogen and counteract added nutrients.
2. Positive feedback occurs when nitrogen-fixing bacteria decline due to repeated fertilizer use, making soils less fertile and forcing farmers to apply even more fertilizer.
3. The positive feedback results in a vicious cycle, where soil degradation increases dependence on synthetic fertilizers year after year.

Why Synthetic-Fertilizer-Based Agriculture Is Unsustainable

1. Soil fertility declines over time, leading to nutrient dependency.
2. Chemical runoff causes severe water pollution, affecting ecosystems and drinking supplies.
3. Fertilizer production requires fossil fuels, contributing to climate change.
4. Long-term environmental degradation may reduce yields, harming future food security.
5. Natural soil processes become disrupted, reducing resilience to shocks.

Methods for Improving Soil Fertility Sustainably

1. Fallowing

1. Fallowing involves leaving land uncultivated for a season or several years, allowing soils to naturally regenerate nutrients and restore microbial communities.
2. Organic matter accumulates in fallow periods, improving soil structure, moisture retention, and biological activity.
3. Longer fallow periods are required for poorer soils, while fertile soils can recover more quickly.
4. Population pressure reduces fallow lengths, as seen in The Gambia where fallow periods dropped from 30 years to 3 years between 1960 and 1990.
5. Fallow fields may attract weeds, but some weeds recycle deep nutrients and protect soil from erosion.

2. Organic Fertilizers (Manure, Humanure, Compost)

1. Organic fertilizers consist of decomposed plant matter, livestock manure, and human waste, providing a slow, steady release of nutrients.
2. These materials improve soil texture, aeration, and water retention, strengthening soil structure in ways synthetic fertilizers cannot.
3. Organic fertilizers encourage healthy microbial activity, including beneficial fungi and bacteria.
4. Nutrient release is gradual, preventing leaching and reducing pollution.
5. Organic fertilizers reduce soil acidity, unlike some synthetic nitrogen fertilizers.

3. Herbal Mixed Leys

1. Herbal mixed leys consist of grasses, legumes, and herbs planted together, usually in temperate climates.
2. These mixtures increase soil organic matter and improve soil texture and fertility.
3. Legumes in the mixture fix atmospheric nitrogen, naturally increasing soil nitrogen levels.
4. Many species produce flowers that attract pollinators, supporting biodiversity.
5. Herbal leys reduce overland flow and intercept rainfall, minimizing erosion and improving soil moisture.
6. Carbon is transferred from plants into the soil, increasing carbon sequestration.

4. Mycorrhizal Fungi

1. Mycorrhizae form symbiotic relationships with plant roots, increasing the surface area through which nutrients, especially phosphorus, can be absorbed.
2. Plants with mycorrhizae can access nutrients that roots alone cannot reach, improving growth and resilience.
3. Mycorrhizal fungi occur naturally in most soils, but can be added as spores to degraded or fumigated soils.
4. They play a major role in soil structure formation, promoting aggregation and reducing erosion.
5. Mycorrhizae are widely used in greenhouse crops, degraded land restoration, and reforestation projects.

5. Continuous Cover Crops

1. Continuous cover crops ensure that soil is never left bare, reducing wind and water erosion.
2. Leguminous cover crops (peas, beans, lentils) fix atmospheric nitrogen, replacing the need for synthetic nitrogen fertilizers.
3. Cover crops add organic matter as they decompose, improving soil fertility over time.
4. They suppress weeds, improve water infiltration, and enhance soil life.

6. Continuous Cover Forestry

1. Continuous cover forestry avoids clear-cutting and instead harvests trees individually or in small groups.
2. This maintains a stable forest canopy, protecting the soil from heavy rainfall and erosion.
3. Nutrient cycling remains continuous, as trees drop leaves, fruits, and branches that decompose into the soil.
4. Healthy forest soils maintain strong microbial communities, including mycorrhizae.
5. This approach improves soil fertility while allowing sustainable timber production.

7. Agroforestry

1. Agroforestry integrates trees with crops (silvo-arable) or livestock (silvo-pastoral) on the same land.
2. Tree roots access deep nutrients and redistribute them via leaf fall, enriching the topsoil.
3. Trees reduce wind and water erosion, stabilize slopes, and increase soil organic matter.
4. Agroforestry increases biodiversity, making the system more resilient to pests and diseases.
5. Farmers gain additional income through fruit, timber, fodder, resin, and fuelwood.
6. Diverse ecosystems tend to be more productive than monocultures due to complementary root systems and nutrient cycling.