🌱 Soil Fertility: The Plant's Supermarket
(IGCSE Agriculture 0600, Section 2.3)

Hello future farmers! This chapter is incredibly important. Think of soil as a supermarket for plants. Soil fertility is simply how well stocked and organised that supermarket is. If the soil has everything the crop needs in the right amounts, your plants will grow big, strong, and healthy!
We will look at the essential nutrients, how they cycle in nature, and how we can add them back to the soil, both naturally and through modern methods.

1. The Importance of Major Nutrients (N, P, K and more!)

Soil fertility depends heavily on the presence of essential mineral elements. Plants require many elements, but six are needed in large quantities—these are the Major Nutrients (or Macronutrients).

The Big Three: NPK

These three are always the most critical for plant growth. Use this analogy:
N-P-K = Shoot-Root-Fruit

  • N: Nitrogen (Compounds of Nitrogen)
    Role: Promotes strong, green, leafy growth (shoots). Essential component of chlorophyll (for photosynthesis) and proteins.
    Analogy: The booster fuel for rapid height and leaf development.
  • P: Phosphorus (Compounds of Phosphorus)
    Role: Essential for early root growth, flowering, fruiting, and energy transfer (ATP).
    Analogy: The foundation builder for strong roots and reproductive success.
  • K: Potassium (Compounds of Potassium)
    Role: Improves overall plant health, disease resistance, water regulation, and quality of fruits and grains.
    Analogy: The health tonic and quality controller.
The Secondary Major Nutrients

Plants need slightly less of these compared to NPK, but they are still essential major nutrients:

  • Ca: Calcium: Important for cell wall structure and root tip growth.
  • Mg: Magnesium: The central atom in the chlorophyll molecule. Absolutely necessary for photosynthesis.
  • S: Sulfur: Required for forming certain proteins and enzymes.
1.1 Identifying Nutrient Deficiencies

When a nutrient is missing or scarce, the plant shows physical symptoms. Recognising these signs helps the farmer know what treatment the soil needs.

  • Nitrogen Deficiency:
    Sign: Yellowing (chlorosis) of older leaves first, starting at the leaf tips, because the plant moves N to the new, growing parts. Stunted growth.
  • Phosphorus Deficiency:
    Sign: Leaves (especially older ones) turn a purplish or dark red colour. Poor root development and delayed maturity.
  • Potassium Deficiency:
    Sign: Yellowing or browning along the edges and tips of older leaves (called 'scorching' or 'leaf burn'). Weak stems.
  • Magnesium Deficiency:
    Sign: Yellowing between the veins of the older leaves (interveinal chlorosis), while the veins remain green.
  • Calcium Deficiency:
    Sign: Affects new growth (terminal buds and young leaves), causing them to become distorted or hooked.

Quick Review: Remember that deficiencies often show up in older leaves first (N, P, K, Mg) because these nutrients are mobile and can be moved to new growth. Calcium, however, is immobile and affects new leaves first.

2. The Nitrogen Cycle: Nature's Fertilizer Factory

Although air is 78% nitrogen, plants cannot use it directly in its gaseous form (\(N_2\)). It must be converted into soluble compounds (nitrates or ammonium) through a process called the Nitrogen Cycle. This cycle is vital for maintaining soil fertility.

Steps in the Nitrogen Cycle:
  1. Nitrogen Fixation:
    Gaseous nitrogen (\(N_2\)) is converted into usable forms (like ammonia) by specialised bacteria, often found in the root nodules of leguminous plants (like beans and peas). Lightning also fixes small amounts of N.
  2. Decomposition (Ammonification):
    When plants and animals die, or when manure is added, decomposers (bacteria and fungi) break down the organic matter, releasing nitrogen as ammonium compounds.
  3. Nitrification:
    In a two-step process, nitrifying bacteria convert the ammonium compounds into nitrites, and then other bacteria convert nitrites into nitrates (\(NO_3\)). Nitrates are the primary form of nitrogen absorbed by plants.
  4. Absorption:
    Plants absorb nitrates and ammonium from the soil water through their roots.
  5. Denitrification:
    Under waterlogged conditions (low oxygen), denitrifying bacteria convert nitrates back into gaseous nitrogen (\(N_2\)), which returns to the atmosphere. This causes a loss of fertility!

Did you know? Poor drainage is bad for nitrogen fertility because it encourages denitrification, causing valuable nitrate to be lost as harmless gas.

3. Maintaining Fertility: Organic Methods

While inorganic fertilisers give a quick boost, organic materials are crucial because they maintain good soil structure and long-term fertility.

3.1 The Importance of Legumes

Legumes (e.g., groundnuts, beans, clover) are essential tools in sustainable farming because they replenish nitrogen naturally.

  • Legumes form a symbiotic relationship with Rhizobium bacteria in their root nodules.
  • These bacteria perform nitrogen fixation, converting atmospheric nitrogen into compounds the plant can use.
  • When the legume crop is harvested or ploughed back into the soil, the nitrogen-rich roots and residues improve the soil fertility for the next crop (this is why they are great for crop rotation).
3.2 Organic Fertilisers (Manure and Compost)

Manure (animal waste) and compost (rotted plant material) are vital organic fertilisers.

  • Nutrient Supply: They contain all major and minor nutrients, although usually in smaller, slower-releasing amounts than inorganic fertilisers.
  • Improving Structure: They break down into humus (stable, dark organic matter). Humus acts like glue, binding soil particles together to form a stable crumb structure.
  • Water Holding: Humus significantly increases the soil’s capacity to hold water and nutrients, making them available to plants.
  • Aeration: Better structure means more air pockets, essential for root respiration and healthy soil organisms.
  • Danger of Oxidation: If organic matter is ploughed too deep or too often in hot, dry conditions, it oxidises (breaks down rapidly), releasing carbon dioxide and losing its long-term benefits to soil structure.

Key Takeaway: Organic matter is the long-term bank account for soil health; it improves structure, water retention, and provides slow-release nutrients.

4. Boosting Fertility: Inorganic Fertilisers

Inorganic fertilisers (also called mineral or chemical fertilisers) are synthetic or naturally mined chemical salts manufactured to provide specific nutrients quickly. They do not contribute significantly to soil structure.

4.1 Examples of Inorganic Fertilisers (Syllabus Requirements)

We need to know examples of fertilisers predominantly containing Phosphorus, Potassium, and a compound fertiliser:

  • Predominantly Phosphorus:
    Example: Triple Superphosphate (TSP)
    Use: High concentration of Phosphorus to boost root development, especially during planting.
  • Predominantly Potassium:
    Example: Muriate of Potash (MOP), which is mainly Potassium Chloride (\(KCl\))
    Use: Applied before flowering and fruiting to improve crop quality, disease resistance, and water efficiency.
  • Compound Fertiliser:
    Example: NPK 15:15:15 (or 17:17:17, etc.)
    Use: Contains a balance of Nitrogen (N), Phosphorus (P), and Potassium (K). Useful when the soil has a general deficiency across all three major elements. The numbers (e.g., 15:15:15) represent the percentage content of N, \(P_2O_5\), and \(K_2O\) respectively.

5. Measuring and Adjusting Soil pH

Soil fertility is also affected by soil pH (a measure of how acidic or alkaline the soil is). pH affects how well plants can absorb nutrients. Most crops prefer a slightly acidic to neutral pH (around 6.0 to 7.0).

5.1 Practical Soil Sampling and Testing

To get an accurate measure of soil fertility and pH, the farmer must collect a representative sample:

  1. Soil Sampling: Using a spade or auger, take small samples from 10–20 different spots across the field (in a zigzag pattern). Avoid boundaries or unusual spots (like old compost heaps).
  2. Mixing: Mix these small samples thoroughly to create one composite (combined) sample.
  3. Testing for pH: The pH of the sample can be tested using litmus paper, indicator liquids, or an electronic pH meter. The results indicate whether the soil is acidic (low pH, below 7) or alkaline (high pH, above 7).
5.2 How Fertilising and Liming Affect Soil pH

Farmers often have to adjust the pH to make nutrients available to plants.

A. Liming Practices

Liming involves adding compounds like agricultural lime (calcium carbonate).

  • Purpose: To increase (raise) the pH of acidic soils (make them less acidic/more alkaline).
  • Mechanism: The calcium carbonate neutralises the excess acid in the soil.
  • Benefit: Many nutrients (like phosphorus) become much more available to plants when the soil pH is raised towards neutral (pH 7).
B. Fertilising Effects on pH

Many inorganic fertilisers, especially those rich in Nitrogen (like ammonium sulfate), can cause the soil pH to drop over time (making the soil more acidic).

  • This happens because the chemical reactions required to convert the fertiliser into forms the plant can use release hydrogen ions, which increase acidity.
  • If a farmer continuously uses acidic fertilisers, they will eventually need to apply lime to correct the pH imbalance.

Don't worry if this seems tricky at first! Just remember:
Lime Lifts pH (up);
Nitrogen Lowers pH (down).

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Quick Review Box: Soil Fertility Key Concepts
  • Major Nutrients are N, P, K, Ca, Mg, S.
  • Nitrogen deficiency causes yellowing of older leaves (chlorosis).
  • Legumes fix atmospheric nitrogen using Rhizobium bacteria.
  • Organic fertilisers (manure/compost) create humus, improving soil structure and water retention.
  • We use soil sampling to test pH.
  • Liming is used to reduce soil acidity (raise pH).
  • Heavy use of nitrogen fertilisers can increase soil acidity (lower pH).