Fertilisers: Chemical compounds to increase crop growth
The Basic Science: What Plants Eat and Why
Just like humans need a balanced diet to grow strong, plants require specific nutrients from their environment. The most essential elements are drawn from the air and soil. Through photosynthesis, plants use carbon dioxide ($CO_2$) from the air and water ($H_2O$) to create sugars. However, for building proteins, DNA, and other critical molecules, they need additional nutrients from the soil. When these nutrients are naturally lacking, crop growth is stunted and yields are poor. This is where fertilisers come in.
Scientists have identified 17 essential nutrients for plants. They are divided into two main groups:
| Group & Source | Nutrients | Key Function |
|---|---|---|
| Macronutrients Needed in large amounts. Mainly from soil & fertiliser. | Primary: N, P, K Secondary: Ca, Mg, S | N (Nitrogen): For leaf growth (green color). P (Phosphorus): For root and flower development. K (Potassium): For overall plant health and disease resistance. |
| Micronutrients Needed in tiny amounts. Often sufficient in soil. | Fe, Mn, B, Zn, Cu, Mo, Cl, Ni | Act as helpers (cofactors) for enzymes in various plant processes. For example, Fe (Iron) is crucial for chlorophyll synthesis. |
A Tale of Two Fertilisers: Organic vs. Inorganic
Fertilisers are broadly classified into two categories based on their origin and production process.
Organic Fertilisers are derived from once-living materials. Examples include animal manure, compost from plant waste, bone meal, and seaweed. They release nutrients slowly as microorganisms in the soil break them down. This improves soil structure and increases its water-holding capacity. Imagine feeding your plants a slow-cooked, wholesome meal—it takes time to digest but builds long-term health.
Inorganic (or Synthetic/Chemical) Fertilisers are manufactured through industrial chemical processes. They are concentrated sources of specific nutrients. For instance, ammonium nitrate ($NH_4NO_3$) is a common nitrogen fertiliser. These fertilisers act quickly because their nutrients are readily available in a form plants can absorb immediately. Think of this as giving your plants an energy drink—a rapid boost of specific nutrients.
| Characteristic | Organic Fertilisers | Inorganic Fertilisers |
|---|---|---|
| Source | Plant/Animal residues | Chemically manufactured |
| Nutrient Release | Slow, over months or years | Fast, within days or weeks |
| Nutrient Concentration | Generally low | High and precise |
| Effect on Soil | Improves structure and biology | Little direct improvement |
From Factory to Field: How Fertilisers Are Made and Used
The production of synthetic fertilisers is a feat of modern chemistry. The most important process is the Haber-Bosch process1, invented in the early 20th century. It captures nitrogen ($N_2$) from the air—which is very stable and unreactive—and combines it with hydrogen ($H_2$, usually from natural gas) to make ammonia ($NH_3$). This ammonia is the starting point for almost all nitrogen fertilisers. The chemical reaction is:
$N_2 + 3H_2 \rightarrow 2NH_3$
Phosphate fertilisers come from mining phosphate rock and treating it with acid. Potassium fertilisers (Potash) are mined from ancient evaporated sea beds.
Applying fertiliser is a science in itself. The goal is to place the right amount at the right time in the right place. Methods include:
- Broadcasting: Spreading granules evenly over a field.
- Banding: Placing fertiliser in strips near the seed rows.
- Fertigation: Dissolving fertiliser in irrigation water.
- Foliar spraying: Applying liquid fertiliser directly to leaves for quick absorption.
A Practical Scenario: Growing a Champion Tomato Plant
Let's follow Maria, a student growing tomatoes for a school science fair. Her control plant gets only water and sunlight. For her test plant, she decides to use fertiliser wisely.
Step 1 – Soil Test: Maria learns that tomatoes need a lot of phosphorus for strong roots and fruits. A simple test kit shows her soil is low in phosphorus.
Step 2 – Choosing Fertiliser: She picks a fertiliser with a higher middle number, like 5-10-5, to boost phosphorus.
Step 3 – Application: At planting, she mixes a small amount of organic bone meal (rich in phosphorus) into the planting hole for slow release. A few weeks later, when the plant starts flowering, she applies a side-dressing of her 5-10-5 fertiliser and waters it in.
The Result: Her fertilised plant grows bushier, has darker green leaves, and produces nearly twice as many large, red tomatoes compared to the control plant. This hands-on experiment shows how targeted nutrient addition directly impacts growth and yield.
Balancing Act: Environmental Impacts and Sustainable Use
While fertilisers are powerful, their misuse can harm the environment. The main issue is nutrient runoff. When more fertiliser is applied than plants can use, rain can wash excess nutrients, especially nitrogen and phosphorus, into rivers and lakes. This causes eutrophication2—a process where algae grow uncontrollably, die, and decompose, using up oxygen in the water and creating "dead zones" where fish cannot live.
Another concern is the release of nitrous oxide ($N_2O$), a potent greenhouse gas, from fertilised soils. To farm sustainably, scientists and farmers are adopting Best Management Practices (BMPs)3:
- Precision Agriculture: Using GPS and sensors to apply fertiliser only where and when it is needed.
- Slow-Release Fertilisers: Coating granules so nutrients are released gradually.
- Cover Crops: Planting certain crops in the off-season to capture leftover nutrients and prevent runoff.
- The 4R Nutrient Stewardship framework: Applying the Right source, at the Right rate, at the Right time, and in the Right place.
Important Questions
Yes, absolutely. This is called "fertiliser burn." Applying too much, especially synthetic fertiliser, creates a high concentration of salts in the soil. This draws water out of the plant roots through osmosis, causing them to dehydrate and die. Symptoms include yellowing or browning of leaf edges, wilting, and stunted growth. Always follow package instructions.
Not always. While they improve soil health and release nutrients slowly, they can still cause pollution if mismanaged. For example, large piles of manure can also runoff into waterways, causing eutrophication. The key is responsible management and applying the correct amount based on plant needs, regardless of the fertiliser type.
They relied on natural methods like crop rotation (alternating crops that use different nutrients), planting legumes (like beans and peas) that host bacteria that "fix" nitrogen from the air into the soil, and applying animal manure and compost. These methods are still vital today and form the basis of organic farming.
Footnote
1 Haber-Bosch process: An industrial method for synthesizing ammonia ($NH_3$) from nitrogen gas ($N_2$) and hydrogen gas ($H_2$) under high pressure and temperature using a metal catalyst. It revolutionized the production of nitrogen fertilisers.
2 Eutrophication: The process by which a body of water becomes overly enriched with minerals and nutrients, inducing excessive growth of algae and plants. The subsequent decay of this biomass depletes oxygen in the water, harming aquatic life.
3 Best Management Practices (BMPs): Methods or techniques found to be the most effective and practical means for achieving an objective (like reducing pollution) while making optimum use of resources.