"Peat is the classic — everything grows in it." It does grow — but with constant pH management battles if you don't understand its nature. Peat is acidic from the start: pH 3.5–4.5 without liming. Producers lime it — but different batches and different brands deliver different results. With overwatering, peat structure degrades, it compacts, and stops conducting air. When it dries out completely, it becomes hydrophobic and water no longer penetrates evenly. A "proven substrate" requires more attention than it appears.
Quick Glossary
- Peat — an organic substrate from partially decomposed plant material of bog origin; natural pH 3.5–4.5, CEC 100–200 meq/100g
- Peat substrate pH — the acidity of the mix after liming and preparation; normal working range after liming is 5.5–6.5
- Peat structure — the porous fibrous architecture that balances moisture retention with root aeration; degrades under excessive moisture and compaction
Why Peat Is Acidic and What to Do About It
Peat acidity is not a production defect — it is natural chemistry. Peat is composed of partially decomposed organic matter where organic acids are a product of decomposition. When wetted, these acids are released and acidify the solution even when you are supplying correctly adjusted pH.
Producers lime peat — adding CaCO₃ or dolomite lime to neutralise acidity and raise pH to 5.5–6.5. But the degree of liming varies between producers and batches. When buying a new batch or a new brand — always check the pH of the wetted substrate before planting. Dry peat pH and wetted peat pH can differ by 0.5–1.0 units.
Peat's high CEC (100–200 meq/100g) means that even after liming, it continues to "buffer" pH: if irrigation is too acidic, the substrate pulls pH up; if too alkaline, it pulls it down. This buffering is useful for stability — but it also makes a rapid pH shift in a peat-based system a slow and difficult process.
Peat Structure and How to Preserve It
Fresh, quality peat or a peat mix has a porous, fibrous structure — the balance between moisture retention and air space for the root. This structure is not permanent.
Overwatering — with constant waterlogging, peat fibres compact, pores close, and root aeration drops sharply. Without oxygen in the pore space, anaerobic conditions and pathogens develop. Peat does not tolerate a "permanently wet" regime — even for moisture-loving crops.
Drying out — when fully desiccated, peat becomes hydrophobic: the surface develops a waxy coating and water on the next irrigation does not penetrate evenly — it collects and runs down the sides of the pot or tray without wetting the core. Restoring even moisture distribution after severe drying is only possible by submersion.
Mechanical compaction — during transplanting, settling, or excessive pressure, fibres compact mechanically. Peat mixes should not be packed in — fill loosely and give a light tap so the structure settles naturally.
Peat Mixes: What Gets Added and Why
Pure peat is rarely used alone — it is typically a component of a mix. Additives serve specific purposes:
Perlite (10–30%) — improves aeration and drainage, prevents compaction. A standard component in most peat mixes for seedlings and container growing.
Vermiculite — retains moisture and nutrients, raises mix CEC. Useful for crops sensitive to drying out.
Coco (10–30%) — improves structure and adds longevity. Coco degrades more slowly than peat and maintains structure when the mix is reused.
Dolomite lime — for pH correction and Ca/Mg supplementation. Included in most commercial peat mix formulations.
Three Mistakes That Cost the Most
Not checking pH of a new batch before planting. Different batches of the same brand can have pH 5.8 or pH 6.5 — both "within spec." But at pH 6.5, microelements are already at the edge of availability; at 5.8 — optimal. Two measurements before planting prevent weeks of troubleshooting.
Watering peat "on schedule" regardless of actual substrate condition. Peat needs irrigation "by substrate" not by timetable: push a finger 2–3 cm in — if slightly moist, don't water; if dry, time to irrigate. Schedule-based or over-generous watering degrades structure within a few weeks.
Reusing peat without restoring structure. After one crop cycle, peat often compacts and clumps. Reuse without loosening and restoring aeration (adding fresh perlite) gives worse root conditions than fresh substrate.
Signs That Peat Is in Good Condition
- Wetted substrate pH is 5.8–6.3 when measured directly in the substrate or in the drainage
- Structure is porous — when a handful is squeezed, it springs back rather than clumping together
- After irrigation, drainage appears evenly across the whole surface rather than only at the pot or tray edges
- If drainage only appears at the edges — either the peat dried out and became hydrophobic, or the structure has degraded in the centre