Peat Substrates

Origin

Peat originates in a region where by local circumstances plant residues hardly break down. A lack of oxygen and very moist conditions allow for vegetable materials, often moss, to only partially decay. These circumstances can be climatological (cold) and/or hydrological (wet). Peat for Dutch horticulture comes from Germany, Ireland, Scandinavia, Baltic states and Canada. Potting composts are made by mixing peat products. Sometimes other products like perlite, clay and sand are mixed.

Characteristics

There many different qualities of peat on the market: from a low humidification degree (Sphagnum moss and white peat) to a high degree of humidification (black peat) and there many qualities concerning fraction distribution. Peat normally contains a lot of water. The air content is determined by the coarseness. As a substrate for long use in beds mostly a coarse peat with a moderate humidification degree is used because this combines a high air content with a reasonable structure stability. The choice depends mainly on the time the material is in use. The physical characteristics can also be altered by adding eg. Perlite.

If used as a substrate a basic fertilizer is necessary. If the material is used for many years an extra amount of cupper has to be added because this is partly fixated by the peat. By nature peat has a low pH (3.5-4.0) and therefore lime must be added before using the material.

Durability

In time coarse peat particles break down to finer particles. The level will depend on the mechanical forces that are used handling the material during and in between the cultivation periods. Also soil organisms (eg. Worms) can have a negative influence on the physical properties of peat. In Ireland tests are performed to check the influence of steaming. There appeared to be only minor influence on structure and fraction distribution of Irish peat.

Environmental Concerns

Peat is harvested from natural ecosystems. It is in areas called peat bogs, and is harvested by cutting out successive layers. Since it is a natural harvested product, it can be harvested irresponsibly and cause environmental damage. Peat is an organic product, and could be harvested sustainably. Heavy reliance on peat as a substrate may be a problem. It also requires shipping from northern latitudes. There is no known disposal problem since it is an organic product that is 1/2 composted.

Peat mosses include:

sphagnum peat - Composed mainly of partially decomposed Sphagnum moss species. Commonly called peat moss or sphagnum peat moss. Sphagnum peat moss must have a fiber content greater than 66%, all of which is derived from the genus Sphagnum. This is the most important type of peat for horticultural use.

hypnum peat - Composed mainly of stems and leaves from various Hypnum moss species. Hypnum peat moss must have a 33% minimum fiber content, of which 50% must be derived from the genus Hypnum.

reed peat - Composed mainly of reeds including Phragmites, Scirpus, Typa species.

sedge peat - Composed mainly of stalks, leaves, rhizomes, and roots of sedges and Carex species.

reed-sedge peat - Must have a 33% minimum fiber content with more than 50% reed-sedge and other nonmoss fibers.

carex peat - Peat in which significant amounts of sedges remain.

peat humus - Peat that is fully decomposed so that none of the original plants can be identified.

fibric peats - Relatively young mosses that are only partially decomposed. Usually have high water retention, low pH, low bulk density, and little ash.

hemic peats - An intermediate stage of decomposition. Older and more decomposed than fibric peats.

sapric peats - Oldest and most decomposed type of peat ranging in color from dark brown to (humic) black. Sapric peat is an excellent source of energy and used as fuel in many areas of the world.

World Peat Resources (4,13)

Global production of peat peaked in 1989 at 221 million tons, 85% of which was for agricultural and horticultural uses. Total peat production has declined about 26% because of reduced production in the U.S.S.R., mostly due to uncertain geopolitical events in the former Soviet Union. In 1992, total production of peat for agricultural and horticultural uses was 138.5 million tons.

Characteristics of Peat

Organic Matter of an acceptable quality peat should be at least 80% organic matter, but a high quality peat should contain 95% organic matter on a dry weight basis. The organic matter content is calculated after the ash and water content of the peat is determined using the following equation: 100 - (% ash + % water) = % organic matter.

pH is a measure of the acidity or alkalinity of a substance. The pH of different types of peat range from a very acid 3.6 to slightly alkaline 7.5. Two terms are used to describe the pH of peat. Low lime peats have a pH value below 5.0. These peats are usually calcium deficient. High lime peats have a pH above 5.0 and are usually calcium sufficient.,br>

Percent Water in peat is technically expressed on an oven-dry basis. When the weight of the water equals the weight of the peat, the water content is 100%. However, this method is not clearly understood by consumers, so the percent water is generally expressed on a moist weight basis, depending on the weight of the peat being sold.

Weed Seed Content is of concern because weed seeds are a soil contaminant. Sphagnum peat moss is virtually free of weed seeds, while reed-sedge and peat humus may contain weed seeds.

Water holding capacity in pure peat varies. Sphagnum peat can adsorb 20 to 30 times its dry weight in water, while hypnum peat and reed-sedge peat can adsorb 5 to 7 times their dry weight.

Nitrogen Content of peat is used to determine fertilizer needs of plants. Peats that are low in nitrogen will require supplemental fertilization for healthy plant growth. Much of the nitrogen in peat is tied up in the organic residue, and is slowly released over a long period of time as the organic matter decomposes.

Structure describes the physical properties of peat. Most baled peats look and feel fibrous or felty in structure, while packaged peat may be pulpy or spongy.

Characteristics of sphagnum peat and other horticultural peats (8)

Type of Peat Nitrogen (%) Water Absorbing Capacity (%) Ash Content (%) pH Vol. Weights (lbs./ft.3)

Sphagnum moss peat 0.6-1.4 1,500-3,000 1.0-5.0 3.0-4.0 4.5-7.0

Hypnum moss peat 2.0-3.5 1,200-1,800 4.0-10.0 5.0-7.0 5.0-10.0

Reed-sedge peat (low lime) 1.5-3.0 500-1,200 5.0-15.0 4.0-5.0 10.-15.0

Reed-sedge peat (high lime) 2.0-3.5 400-1,200 5.0-18.0 5.1-7.5 10.0-18.0

Decomposed peat 2.0-3.5 150-500 10.0-50.0 5.0-7.5 10.0-40.0

Grades of peat and particle size (2)

The American Society for Testing and Materials (ASTM) grades peat by particle size as follows:

Coarse - all particles > 2.38 mm

Medium - particles are between 2.38 mm and 0.84 mm

Fine - all particles < 0.84

Using Peat (2,3,11,14,15)

The advantages of peat have led to its dominance as a basic component in growing media and as a soil amendment. There is no single material currently available that provides all the advantages of peat, although there are many alternative products that are satisfactory in specific uses. Peat is usually sold on a volume basis in bales packed in cubit feet or yards. Baled peat is compressed when packaged so that it actually contains up to 2 times its volume when loosened.

There are several basic requirements of a good growing medium.

It must not change its characteristics during the cultivation period (structural stability).
It should have good water holding capacity and air porosity for good root development.
It should not be too heavy making transportation costs too high (bulk density).
Hazardous substances must not occur in the medium. A high salinity level also can be hazardous to many plant species.
Must be free of pathogens, pests, seeds and plant propagules that might cause disease or reduce plant growth.

Sphagnum peat is an ideal growing medium because it has:

a homogenous composition
high structural stability
high water and air capacity
low pH (3.5-4.5) which is easily adjustable
low salt content
low nutrient status which is easily adjustable
no pathogens, insect pests
free from weed seeds

Type of Peat to Use for Various Horticultural Applications (2)

Application Sphagnum Peat Hypnum Peat Reed-sedge peat Peat humus

Soil amendment excellent good good good

Top-dressing lawns excellent good good good

Mulch fair fair fair fair

Potting soil mix excellent good good fair

Rooting cuttings/Seed germination excellent good fair poor

Research Comparing Sphagnum Peat With Coir and Compost (11)

The Peat Research and Development Centre, Shippegan, New Brunswick, conducted research comparing performance of sphagnum peat moss, coir fiber and green compost as amendments in sandy and clay soils, and its effect on the growth of tomatoes and impatiens.

Research found that while compost and coir, in various blends with sandy or clay soil, performed nearly as well as peat moss under certain conditions, none worked as well as peat moss under all conditions.

Test Conditions Best Results Worst Results

Tomatoes--clay soils 50% soil, 35% peat moss, 15% compost 70% soil, 30% compost

Impatiens--clay soils 70% soil, 30% peat moss 70% soil, 30% compost

Tomatoes--sandy soils 50% soil, 35% peat moss, 15% compost 70% soil, 30% coir

Impatiens--sandy soils 50% soil, 35% peat moss, 15% compost 85% soil, 15% compost