Peat-free bulk constituents
What goes into a peat-free compost, what each ingredient does, why peat is being phased out, and how to turn that knowledge into full marks on an explain question.
Key points
Summary
| Constituent | What it is | Main job in the mix | Watch out for | Environmental footprint |
|---|---|---|---|---|
| Coir | Coconut husk fibre, a by-product | Holds water, re-wets easily | Low in nutrients; must be washed to remove salts | Shipped long distances; washing uses fresh water |
| Wood fibre | Forestry and sawmill by-product | Air and drainage; light | Nitrogen lock-up; low water-holding alone | Low when sourced locally |
| Composted bark | Bark from the timber industry, composted | Opens structure, adds air | Low in nutrients | Uses a by-product |
| Green compost | Recycled garden (green) waste | Adds nutrients and microbial life | Variable quality; can be alkaline or salty; possible contaminants | Recycles waste, diverts it from landfill |
| Loam | Sterilised topsoil (as in soil-based composts) | Weight, stability, holds nutrients | Heavy | Topsoil is a finite resource |
| Perlite | Volcanic rock expanded by heating | Air and drainage; sterile | No nutrients; dust (wet it, wear a mask) | Quarried and energy-intensive to heat |
| Grit | Crushed stone | Drainage and weight | Heavy | Quarried |
The other constituents in learning outcome 5.1 are covered in lesson 2.
Test yourself
Which bulk constituent is most likely to cause nitrogen lock-up as it breaks down?
- Grit
- Wood fibre
- Perlite
- Loam
Why is coir washed before it is used in growing media?
- To remove salts
- To add nutrients
- To raise the pH
- To make it heavier
Which statement correctly describes an environmental impact of peat extraction?
- It adds nutrients to nearby rivers
- It releases carbon that the bog had stored
- It increases rainfall in the area
- It makes the bog grow faster
Video transcript
Everything you grow in a pot depends on what's in it. In this lesson, we'll look at the bulk constituents of peat-free growing media: what they are, what each one does, and how to write about them in the exam.
First, an important distinction. Soil is the natural material in the ground. A growing medium is a mix made up to fill a container. Examiners regularly see candidates confuse the two, so keep them separate in your answers.
For decades, peat was the main ingredient. It holds plenty of water, yet still leaves room for air. It is light, consistent from bag to bag, and low in nutrients, so growers could add exactly what their plants needed.
But peat forms extremely slowly, around one millimetre a year, in waterlogged bogs. Those bogs lock away huge amounts of carbon. Extracting peat drains them, releasing carbon dioxide, and destroys specialist wildlife habitats. That is why horticulture is moving to peat-free mixes.
No single material replaces peat, so peat-free composts blend several bulk constituents. Coir, made from coconut husks, holds water well and re-wets easily. But it is shipped long distances, and it must be washed to remove salts. Wood fibre, a by-product of forestry, is light and very free-draining, though it can lock up nitrogen as it breaks down.
Composted bark opens up the structure and adds air. Green compost, made from recycled garden waste, adds nutrients, but its quality can vary. Loam gives weight and holds on to nutrients. And grit and perlite improve drainage.
So think of a compost as a recipe. Some ingredients hold water, some add air, some supply nutrients, and some add weight. The right balance depends on what you are growing.
Now, exam technique. When a question says explain, one mark is for the point, and one mark is for developing it. For example: coir holds water well. That's the point. Which means seedlings are less likely to dry out between waterings. That's the development, and your second mark. You'll practise this in the next step.
This is one of 33 lessons in Unit 1
The full course adds sorting drills, flashcards, a written question with a model answer, and a printable sheet for every lesson.