Different degrees of peat moss
One of the most important ingredients of soilless substrates, peat moss, began in peat bogs shortly after the last ice age about 12,000 – 14,000 years ago. As the glaciers receded, depressions were left in the earth that collected precipitation from rain and snow.
Across Canada, there are different types of peat that formed, depending on the vegetation in the wetland area. However, sphagnum moss is predominant.
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How has peat moss formed?
The acid pH and low nutrient content of the water combined with the cool northern climate was conducive for the growth of sphagnum moss and the accumulation of peat moss. Over time, the environment of low evaporation, acidic pH and low oxygen limited the decomposition of the vegetation.
Different degrees of peat moss
Within the peat bog, there are different degrees of peat moss decomposition depending on the depth. If you were to take a cross section of a peat bog from top to bottom, the surface is composed of recently accumulated peat moss that is blonde in color and fibrous in texture compared to the lower sections of the bog that contain peat that is more humified, dark brown and less fibrous.
Raw sphagnum peat moss: Von Post scale
This scale was devised by Lennart von Post during his work on the 1926 Soil Survey of Sweden for measurement of degree of decomposition of dead plant matter such as Sphagnum moss. Using parameters such as fibre integrity, colour and viscosity of exudate, and presence of colloidal particles, it creates a descriptive framework across a wide range of organic soils, and assigns a numerical value from 1 (undecomposed) to 10 (colloidal). The USDA / FAO compressed von Post’s 10 steps into three levels (fibric, hemic, sapric) thereby reducing its diagnostic usefulness at field scale.
Raw sphagnum peat moss is graded using the Von Post scale to determine the level of decomposition and humification. The scale ranges from H1 to H10, with H1 being totally un-decomposed plant material and H10 being completely decomposed, based on peat moss color, fiber content and color of water squeezed from a sample.
How to Test
The von Post scale is simple to use: a handful of wet soil (small enough to cover with the fingers of one hand) is squeezed very hard, until as much as possible has extruded through the fingers. Colour and viscosity of exudate, proportion and condition of remaining fibre and other qualities are noted; a score is assigned according to the table above. Some experience is needed to use the method, as the actual instructions appear to exist only in Old Swedish; bowdlerised versions with the hard words left out were found on the Web.
This elegantly simple test clearly describes the continuum found in the research areas, from a sponge-like catotelm with nearly unlimited hydraulic conductivity to impermeable black peat. Although Sphagnum when left to itself passes from fibre to colloidal paste with no intermediate crumb stage, the von Post score correlates strongly with other characteristics of good soil functionality. It showed strong statistical relationships with soil depth, colour, K, pH, ash, hydrology, and smell, emphasizing its predictive value as a multi-variate indicator for field analysis. It may also indicate topographical or hydrological conditions that support or impair land restoration. In the research areas, desirable plants (forage grasses) grew most strongly in areas with von Post scores between 7 and 9.
The von Post scale replaces the soil textural triangle as a baseline for description of highly organic soils, and was shown to relate ‘humification’ to other measurements of soil functionality. Alone of descriptive techniques found in the literature, the von Post scale provided a rapid, sensitive and multi-variate method for characterisation of blackland soils in the field.
H1
Completely undecomposed peat which, when squeezed, releases almost clear water. Plant remains easily identifiable. No amorphous material present.
H2
Almost entirely undecomposed peat which, when squeezed, releases clear or yellowish water. Plant remains still easily identifiable. No amorphous material present.
H3
Very slightly decomposed peat which, when squeezed, releases muddy brown water, but from which no peat passes between the fingers. Plant remains still identifiable, and no amorphous material present.
H4
Slightly decomposed peat which, when squeezed, releases very muddy brown water. No peat is passed between the fingers but plant remains are slightly pasty and have lost some of their identifiable features.
H5
Moderately decomposed peat which, when squeezed, releases very muddy water with a very small amount of amorphous granular peat escaping between the fingers. The structure of the plant remains is quite indistinct although it is still possible to recognize certain features. The residue is very pasty.
H6
Moderately highly decomposed peat with a very indistinct plant structure. When squeezed, about one-third of the peat escapes between the fingers. The residue is very pasty but shows the plant structure more distinctly than before squeezing.
H7
Highly decomposed peat. Contains a lot of amorphous material with very faintly recognizable plant structure. When squeezed, about one-half of the peat escapes between the fingers. The water, if any is released, is very dark and almost pasty.
H8
Very highly decomposed peat with a large quantity of amorphous material and very indistinct plant structure. When squeezed, about two-thirds of the peat escapes between the fingers. A small quantity of pasty water may be released. The plant material remaining in the hand consists of residues such as roots and fibres that resist decomposition
H9
Practically fully decomposed peat in which there is hardly any recognizable plant structure. When squeezed, it is a fairly uniform paste.
H10
Completely decomposed peat with no discernible plant structure. When squeezed, all the wet peat escapes between the fingers.
When squeezed, all the wet peat escapes between the fingers. In North America, most peat moss harvested is in the range of H1-H5, with H1 being blonde and most fibrous and H5 being moderately decomposed peat. Different grades and colors of peat moss are a result of age, decomposition and depth within the peat bog.
Peat blending and grading
Once the peat moss is harvested, different grades are typically blended to achieve specific physical properties. This is because a specific Von Post grade peat moss may not be adequate for the market type.
For example, H1 peat moss is blond and very fibrous. However, when used alone for growing media, it will shrink in containers over time. H5 is dark brown and is very good for gardening, but it is often blended since alone it can be heavy.