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examBoard: Cambridge
examType: IGCSE
lessonTitle: Role of Organic Content in Soil
Soil is much more than just dirt! It's a complex living ecosystem that supports plant growth and life on Earth. One of the most crucial components of healthy soil is its organic content. Soil organic matter (SOM) consists of all the carbon-containing materials in soil that come from living organisms, including plant and animal remains in various stages of decomposition.
Key Definitions:
Organic matter is often called the "life-force" of soil. Even though it typically makes up only 2-10% of soil volume, it has an enormous impact on soil properties and plant growth. Healthy soils with good organic content are more productive, require fewer inputs and are more resilient to environmental stresses like drought.
Soil organic matter varies widely across different ecosystems. Forest soils and grasslands typically have higher organic content (4-10%) while agricultural soils often have lower levels (1-3%). Desert soils may contain less than 1% organic matter. In the UK, many agricultural soils have seen declining organic matter due to intensive farming practices.
Organic matter transforms soil in several important ways that directly benefit plant growth. Let's explore these benefits:
Organic matter acts like a natural glue that binds soil particles together into aggregates, creating a crumbly structure that's ideal for plant growth.
Organic matter can hold up to 20 times its weight in water! This means soils with good organic content can store more water during rainy periods and release it slowly during dry spells, reducing drought stress for plants.
The improved soil structure creates pore spaces that allow air to circulate. Plant roots need oxygen to respire and grow, while beneficial soil organisms also require good aeration to thrive.
The looser, more crumbly structure of soils with good organic content makes it easier for plant roots to grow and access water and nutrients throughout the soil profile.
Organic matter is a nutrient powerhouse that improves the chemical properties of soil in ways that directly benefit plant growth.
As organic matter decomposes, it releases essential nutrients like nitrogen, phosphorus and sulphur that plants need. This slow-release system provides nutrients throughout the growing season.
Organic matter increases the soil's ability to hold onto positively charged nutrients (like calcium, magnesium and potassium), preventing them from being washed away but keeping them available for plants.
Organic matter helps stabilise soil pH, preventing rapid changes that could stress plants. It can help neutralise both acidic and alkaline conditions, moving soil toward the neutral range most plants prefer.
Perhaps most importantly, organic matter supports the incredible diversity of life in soil - from microscopic bacteria to earthworms and insects.
Organic matter feeds the soil food web - the complex community of organisms living in soil. A teaspoon of healthy soil can contain billions of bacteria, metres of fungal hyphae, thousands of protozoa and dozens of nematodes. These organisms break down organic matter, cycle nutrients, create soil structure and even protect plants from disease.
Earthworms are indicators of healthy soil. They consume organic matter, mix it with mineral soil and create channels that improve drainage and aeration. Their castings (worm poo) are rich in nutrients and beneficial microbes. Charles Darwin called earthworms "nature's ploughs" because of their importance to soil health.
Soil organic matter plays a central role in two of Earth's most important nutrient cycles: the carbon cycle and the nitrogen cycle.
Plants capture carbon dioxide from the atmosphere through photosynthesis. When plant materials enter the soil as residues, soil organisms break them down. Some carbon is released back to the atmosphere as CO₂, but some is converted into stable humus that can remain in soil for hundreds or thousands of years. Soils are the largest terrestrial carbon store on Earth, containing more carbon than the atmosphere and all plants combined!
Nitrogen is essential for plant growth but most exists in forms plants can't use. Organic matter is the main source of nitrogen in soil. As microbes decompose organic matter, they convert organic nitrogen into ammonium (NH₄⁺) and then nitrate (NO₃⁻), forms that plants can absorb. Some bacteria can also "fix" nitrogen directly from the air, especially those living in nodules on the roots of legumes like peas, beans and clover.
The Rothamsted Research station in Hertfordshire, UK, hosts the world's longest-running agricultural experiments, some dating back to 1843. These experiments have provided invaluable data on soil organic matter. Plots that have received regular organic matter additions (like farmyard manure) for over 175 years have maintained higher crop yields, better soil structure and greater resilience to extreme weather compared to plots receiving only mineral fertilisers. The organically-managed plots contain about three times more carbon than mineral-fertilised plots and support more diverse and abundant soil life. This long-term study demonstrates that maintaining soil organic matter is essential for sustainable agriculture.
Maintaining and improving soil organic content is crucial for sustainable agriculture and ecosystem health. Here are some key management practices:
Regularly add compost, manure, green manures and crop residues to soil. These materials feed soil organisms and eventually become stable humus. Even urban gardeners can improve soil by adding compost from kitchen scraps and garden waste.
Excessive tillage (ploughing) accelerates organic matter decomposition by exposing it to oxygen. Reduced tillage or no-till farming helps preserve soil organic matter and protects soil structure built by soil organisms.
Use cover crops during fallow periods and mulch bare soil to protect it from erosion, suppress weeds and add organic matter. Living plant roots feed soil organisms through root exudates - sugars and other compounds that plants release into the soil.
When soil loses organic matter, it doesn't just affect crop yields - it has far-reaching environmental consequences:
Even if you don't farm or garden, your choices affect soil health. Supporting farmers who use sustainable practices like organic farming, agroforestry and regenerative agriculture helps maintain soil organic matter. Composting food waste rather than sending it to landfill allows organic matter to return to soil. Reducing food waste is also important - about one-third of all food produced globally is wasted, representing a huge loss of organic matter that could have returned to soil. By making informed choices, we can all contribute to healthier soils with better organic content.
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