Peat moss fraction size: choosing a screening for each crop

Peat moss fraction size tells you the range of particle sizes in a screened grade. It is a useful starting point when choosing material for a seed tray, a potting mix or a larger container, but the largest number on the label does not tell you how the whole mix will behave.

The practical question is whether the peat will fill your cells evenly, hold enough water and leave suitable space for air around the roots. Start with the container and crop, then check whether you need unlimed peat for blending or a prepared substrate.

What does screening mean?

Screening separates peat into specified particle-size fractions. A grade marked 0-20 mm contains material within that size range, including finer particles. It does not mean that every particle is close to 20 mm, or that two grades with the same upper limit will feel identical.

Peat type matters alongside screening. Slightly decomposed white peat is relatively light and typically leaves more air space, while more decomposed dark peat holds more water and is heavier. The balance you see in a finished mix also depends on how the material is blended and how firmly it is filled into the container.

SOLVIKA supplies white and dark Sphagnum peat moss in several screenings. Its unlimed peat has a typical pH of 3.5 to 4.5 measured in water. If you are comparing fractions, keep peat type and pH treatment consistent so that particle size is the main change in your trial.

Treat a fraction label as a specification for selecting material, not as a prediction of irrigation frequency or crop performance. A small plug and a large nursery container place different demands on the same peat. Testing the grade in your own filling and watering system is more informative than judging a loose handful alone.

Fine fractions suit seed trays and small cells

In small cells, a fine grade usually fills corners and makes more even contact with the seed. The 0-5 mm screening is worth considering for particularly small cells or seeds where a level surface matters. 405LV is an example of a 0-5 mm white-peat seeding substrate with a wetting agent and a starter fertiliser charge.

A 0-10 mm grade gives a little more visible fibre while remaining fine enough for many seed trays and propagation uses. Cell size, sowing method and watering equipment should decide which of the two screenings you trial first. Watch for uneven filling, seed displacement during watering and how readily the cell drains after irrigation.

Fine material has more contact points and can retain water well, which is useful during germination. It can also become too dense for a particular cell if it is compressed during filling or kept wet for too long. Fill trays as you would in production, then assess the root zone after repeated watering rather than relying only on the appearance of the dry substrate.

Distinguish raw peat from a ready-to-use seeding mix. Unlimed peat is acidic and has no added fertiliser, so it is a blending ingredient rather than a substitute for a prepared mix in every crop. SOLVIKA's seeding and propagation substrates are pH-adjusted and include a starter charge.

A 0-20 mm fraction is a starting point for many pots

When plants move out of small cells, a medium 0-20 mm screening is often a useful starting point. It contains fine particles as well as larger material, giving a different balance from a uniformly fine grade. In a pot, that balance can help with both water retention and air space, although the result still needs checking under your irrigation regime.

120LV is a 0-20 mm unlimed white-peat grade for growers who make their own blends. It is not the same as a pH-adjusted potting substrate with fertiliser. If your team doses limestone and nutrients separately, specify the raw grade and check the finished mix, not just the peat before blending.

Consider pot depth and crop duration together. A shallow pot may dry differently from a deeper container even when both are filled with the same material. Compare how evenly the mix wets after a dry spell, how quickly excess water drains and whether roots occupy the full pot.

Do not assume that moving from 0-10 mm to 0-20 mm automatically solves a wet root zone. Filling pressure, pot design, added components and watering intervals can all change the outcome. A side-by-side trial using the same crop and pot makes the effect of fraction size easier to see.

A 0-40 mm grade can support larger containers

Larger nursery containers and longer growing periods often call for more lasting structure than a fine seeding grade can provide. A 0-40 mm screening includes coarse pieces but still contains smaller particles that help the mix fill around roots. It can be a practical base where the grower wants both water retention and air-filled space.

141 is a 0-40 mm unlimed white-peat grade described as a base for planting two-year-old blueberries and other acid-loving crops. Its acidity is relevant here: blueberry media are not selected on structure alone. For crops that need a higher substrate pH, choose an appropriate pH-adjusted mix or account for liming in your own recipe.

The 0-40 mm label does not mean a mix is made entirely of large pieces. Fines remain part of the fraction and affect how it packs and holds water. If structure is your main concern, examine the material after filling and irrigation, not just its maximum particle size.

Container height, irrigation method and growing time should guide the decision. In a trial, record how the substrate settles, whether water reaches the whole root ball and whether drainage remains suitable as the crop develops. These observations are more useful than selecting the coarsest available fraction by default.

What are 10-30 mm and 20-40 mm fractions for?

Unlike 0-20 mm or 0-40 mm grades, a 10-30 mm or 20-40 mm screening excludes much of the finest material. Growers may use these coarser fractions as blending components when they want to add structure to a recipe. On their own, they may not fill small cells evenly or provide the seed contact needed for reliable tray filling.

The two ranges serve slightly different blending decisions. A 10-30 mm fraction introduces coarse material across a broader middle range, while 20-40 mm places more emphasis on larger pieces. Neither is automatically better: the useful proportion depends on the finer ingredients already in the mix and the container it must fill.

For a meaningful comparison, change one component at a time. Keep the peat type, pot, crop and irrigation method the same while adjusting the coarse fraction. Check whether the blend feeds cleanly through your filling equipment, settles excessively or leaves gaps around transplants.

Finally, specify the complete job rather than ordering by millimetres alone. Tell your supplier whether the peat is for raw-material blending or direct planting, whether the crop needs acidic or adjusted pH, and what packaging suits your operation. That gives the fraction size a useful context.

Frequently asked questions

For a grade recommendation or delivered quotation, send us your crop, container or cell size, required pH treatment, volume, packaging, destination and delivery term. If you already have a substrate specification, include it so we can compare like with like.