(8) Making compost - a natural organic fertiliser from organic waste

Compost is all about recycling organic kitchen or garden waste to make a natural organic fertiliser and return useful nutrients to the soil for crops or garden plants - we are usually talking about a relatively small scale operation.

The 'ideal conditions' for decomposers like bacteria and fungi to do their 'stuff'!

Some rates of reaction ideas from chemistry come in handy as well as the decay factors described above.

1. Its best if the compost material is shredded to increase the surface area for the decomposers to 'attack'.

2. The compost bin sides should have holes/mesh to allow air in (provides oxygen) and circulate - many microorganisms need oxygen for aerobic respiration.

Some microorganisms do not need oxygen for respiration (anaerobic), but they work more slowly.

For more details see Aerobic and anaerobic respiration

3. Warm conditions favour faster decay, hopefully the bulk of the composting material warms up from the heat released by the decomposition reactions to further speed up the decay.

Every 10^o rise in temperature roughly doubles the rate of decay - but not too high or the microbial enzymes are denatured above 50^oC and the organisms die.

Some compost bins are thermally insulated to keep heat in, but they must still be able to allow air to enter and circulate e.g. a mesh intake or some regular holes in the side of the compost bin.

4. The compost should contain some moisture, but not to dry or too damp - water is produced by the aerobic respiration of the decomposer microorganisms, but more is needed for the decomposer organisms to carry out their chemistry - water is both a reactant and a solvent for many biochemical processes.

5. You can add compost makers (decay accelerators) to speed up the process,

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Key points - Summary of ideas on making a good compost plus extra notes on compost composition

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Making Good Compost for Organic Fertiliser

Compost is a nutrient-rich organic fertiliser created by decomposing organic materials.

It improves soil health, enhances plant growth, and supports sustainable agriculture.

Below are key factors involved in making high-quality compost.

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1. Key Components of Compost

To create effective compost, you need a balance of green materials (nitrogen-rich) and brown materials (carbon-rich):

• Green Materials (Nitrogen Sources):

  • Fruit and vegetable scraps

  • Grass clippings

  • Coffee grounds

  • Manure (from herbivores)

• Brown Materials (Carbon Sources):

  • Dry leaves

  • Sawdust (untreated wood)

  • Cardboard and paper (shredded)

  • Straw

• Essential plant minerals (hopefully in the compost) e.g.

  • magnesium

  • iron

  • phosphorus

  • potassium

  • calcium

  • zinc

  • copper

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2. Factors Affecting Composting Process

Several conditions influence the rate and quality of compost production:

a. Temperature

• Optimal temperature range: 50-60°C

• Microbial activity generates heat; maintaining warmth speeds up decomposition.

• Excess heat (above 70°C) may kill beneficial microbes.

b. Moisture

• Ideal moisture content: 40-60%

• Too dry: slows decomposition.

• Too wet: causes anaerobic conditions (producing foul odours).

c. Oxygen & Aeration

• Composting is an aerobic process (requires oxygen).

• Regularly turning the pile every 1-2 weeks prevents anaerobic bacteria from producing methane.

d. Particle Size

• Smaller pieces decompose faster due to increased surface area.

• Shred materials (like leaves and vegetable scraps) before composting.

e. pH Level

• Initial pH: Slightly acidic (5-6).

• As decomposition occurs, pH stabilises around 6.5-8.

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3. Steps to Make Compost

1. Choose a compost bin or pile location – Ensure good drainage and aeration.

2. Layer organic materials – Alternate green and brown layers.

3. Maintain moisture – Water if needed but avoid excess moisture.

4. Turn the compost – Mix regularly to aerate and speed up decomposition.

5. Monitor temperature – Ensure microbial activity remains optimal.

6. Wait and harvest – Compost is ready in 2-6 months, depending on conditions.

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4. Benefits of Compost as Organic Fertiliser

• Improves soil structure – Enhances water retention and aeration.

• Provides essential nutrients – Supplies nitrogen, phosphorus, and potassium.

• Encourages beneficial microbes – Supports healthy plant growth.

• Reduces landfill waste – Minimises environmental impact.

• Increases crop yield – Enhances agricultural productivity.

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Summary of learning objectives and key words or phrases

Know how to make compost, a good natural organic fertiliser best.

Be able to describe and explain the best conditions to make compost from organic waste.

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