The von Post scale: how peat decomposition is graded

Two peat samples can have the same screen size and behave differently in a pot. One feels fibrous and springy, while the other packs more closely and holds more water. The von Post scale helps describe part of that difference.

If you are comparing peat for a growing mix, it is worth knowing what an H grade tells you, how the squeeze test works and why it cannot replace a growing trial.

The von Post scale describes decomposition, not particle size

The von Post scale grades the visible degree of peat decomposition from H1 to H10. H1 is essentially undecomposed plant material, while H10 is so decomposed that the original plant structure is no longer recognisable. The H stands for humification, the breakdown of plant remains over time.

Decomposition is not the same thing as screening. A 0-20 mm fraction tells you the size range after screening, not how intact the plant fibres were beforehand. Both matter when choosing a substrate because fibre condition and particle size influence how closely the material packs.

An H grade is also a field description rather than a complete specification. It does not, on its own, give you pH, electrical conductivity, fertiliser content or a measured air-filled porosity. Use it alongside the product specification and, for a new application, a trial under your own watering and filling conditions.

How does the von Post squeeze test work?

Take a representative handful of moist peat and squeeze it firmly in your hand. Look at the water that comes out, note whether fine peat escapes between your fingers, then open your hand and examine what remains. The colour and clarity of the water, the amount of material squeezed out and the recognisable plant fibres all inform the grade.

At the less decomposed end, water tends to run relatively clear and the fibrous peat stays in your hand. As decomposition increases, the expressed water becomes darker and more turbid, and more fine material may pass between your fingers. At the most decomposed end, very little recognisable plant structure remains.

Moisture matters. A dry sample may give too little expressed water to judge, while an unevenly wetted sample can mislead you. Test several handfuls from different parts of a bale or batch, describe what you observe and treat a borderline H grade as an estimate rather than a precise measurement.

What do H1 to H10 mean in practice?

The scale is gradual, so adjacent grades can look similar. The following descriptions are practical signposts for interpreting a squeeze test, not laboratory limits or ratings for a particular product.

• H1 and H2. H1 is virtually undecomposed, with plant remains clearly identifiable. H2 shows only slight breakdown, and a squeeze generally leaves the material intact with relatively clear expressed water.
• H3 and H4. At H3, decomposition is more apparent but the fibrous structure remains easy to see. H4 shows further breakdown, and the expressed water tends to be cloudier or browner.
• H5 and H6. H5 sits near the middle of the scale, with noticeably decomposed material alongside remaining plant structure. At H6, the peat feels less fibrous and more fine material may be squeezed out.
• H7 and H8. These are strongly decomposed grades. Plant remains are harder to identify, expressed water is typically dark, and a substantial portion of the material may move between the fingers.
• H9 and H10. At H9, little recognisable structure remains. H10 represents the fully decomposed end of the scale, where the original plant remains cannot be distinguished by eye.

Do not read these grades as a guaranteed performance ladder. A screened fraction, a blend and the way a tray or container is filled can all alter the behaviour of a finished growing medium. The scale is most useful for describing the raw peat and making sensible comparisons between samples.

What does slightly decomposed white peat mean for air and structure?

White Sphagnum peat is slightly decomposed, which is why growers value its remaining fibre. White peat described in the H1 to H4 region generally retains more visible plant structure than strongly decomposed peat. In a suitable mix, that structure can help keep spaces for air around roots rather than allowing the medium to pack tightly.

There is still a meaningful difference between a fine and a coarser screening of white peat. Fine material is useful where close contact with small seeds or greater water holding is wanted. A coarser grade is often chosen when maintaining structure and air space in a larger container is the priority.

For example, SOLVIKA MIX 111 is fine white peat screened to 0-10 mm and intended for uses including seed germination and raising a mix’s water-holding capacity. SOLVIKA MIX 141 is medium-coarse white peat screened to 0-40 mm and is used as a base for planting two-year-old blueberries and other acid-loving crops. Those product descriptions give screenings and uses, not von Post ratings.

When comparing samples, handle both the peat and the finished mix. Check how the medium fills a tray, how it wets and drains, and whether it settles after repeated irrigation. An H grade can help explain what you see, but it cannot predict all of it.

Why does dark peat usually sit higher on the scale?

Dark-brown peat has undergone more decomposition than white peat. Its plant remains are less intact, so it would generally be placed higher on the von Post scale. That is a comparison between peat types, not a claim that every dark peat sample has one fixed H number.

The distinction matters in a recipe. White peat typically contributes more air and visible fibre, while dark peat holds more water and packs more densely. SOLVIKA’s published bulk weights reflect that difference: white peat is 80 to 200 kg per 1000 L, compared with 200 to 320 kg per 1000 L for dark-brown peat.

Neither type is automatically the better choice. A grower may want more fibrous white peat for structure, or use dark peat where water holding and a denser mix suit the crop and container. If you are evaluating a blend, ask for its peat types and fraction as well as any H-grade description, then test it in the intended production system.

Frequently asked questions

For a grade recommendation or a delivered price, send us your crop, container or tray size, preferred fraction, packaging, volume, destination and delivery term. A current substrate specification is helpful if you are looking to match an existing mix.