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)