Quality Assurance for the Growing Media and Substrate Industry

In recent years, climate protection efforts and the need to reduce CO2 emissions have led to increased efforts in the substrate industry to reduce peat as a raw material in substrates. Accordingly, peat alternatives are playing an increasingly important role.

Sphagnum peat is still an important component for the production of horticultural growing media and substrates, especially in the professional sector. Even with peat reduction already underway, its share in potting soils (Germany) was 41% in 2023 and in growing media (Germany) 71%.

Due to its chemical properties (low pH value and minimal nutrient content), it can be specifically limed and fertilized. This allows optimal parameters to be set for the respective applications and plant cultures. The physical parameters of sphagnum peat also prove advantageous for the root zone. Depending on the degree of humification, peat has good water retention capacity, and both white peat and frozen black peat positively influence air capacity.

Alternative raw materials will increasingly supplement and partially replace peat resources in the future. The substrate industry is actively seeking peat alternatives — there are certainly positive effects for special applications through the addition of proportions of bark humus, wood fibers or substrate composts.

Facts about Peat Extraction

• Peat extraction on intact bogs is prohibited in Germany
• Current peat extraction areas in Germany are for the most part former grassland or other agricultural land
• Extraction permits will expire in the coming years
• A renaturalization plan must be prepared for the period after peat extraction

Sphagnum peat is an essential component of plant production worldwide. The responsible management of peat extraction areas and the search for resource-conserving alternatives has a high priority in German horticulture.

Domestic Bark

Domestic bark comes from sawmills and undergoes a minimum four-week storage period during which growth-inhibiting substances break down. After shredding, different grain size classes are produced through sieving: fine (0-15 mm, 10-25 mm) or medium (10-40 mm). Used as ground cover against weed germination and erosion.

Mediterranean Pine Bark

The raw bark comes from France, Portugal and Spain. Pine bark is usually gently fractionated into different grain sizes without a shredding process and freed from fiber and bast parts through special sieving technology. Pine bark is visually appealing — however, significantly more expensive compared to products made from domestic bark.

Contrary to popular belief, the pH value of the soil is not significantly lowered by a layer of bark mulch.

Bark Humus

Bark humus is the fine bark fraction (0-15 mm) that undergoes fermentation (composting) in windrows. It is a valuable additive for growing media and substrates: in the substrate it can store nutrients and release them as needed (buffering), ensures stabilization of the pH value and is considered a structurally stable additive for improved drainage. Production requires oxygen supply and at least 6 months of maturation time.

Specifying Bark Products Correctly

Clear tender texts with precise performance descriptions make conscientious execution easier for all parties involved. RAL quality-assured bark mulch with 0-40 mm grain size does not exist in the market — the fine fraction under 10 mm is sieved out in certified products.

Recommended specification text: "Bark mulch (BM) with the RAL quality mark 'Substrates for Plants' RAL GZ 250"

Bark Mulch Quality — Unpleasant Surprises Can Be Avoided

Quality problems repeatedly occur with non-RAL quality-assured bark mulch products: foreign materials, inhomogeneous grain sizes, elevated heavy metal values. The causes lie in raw material scarcity and lack of term protection. Quality criteria include grain size classes (fine 0-20 mm, medium 10-40 mm, coarse 10-80 mm), wood content, foreign substances, heavy metals, insecticide residues and plant-damaging substances.

Bark Mulch — User Information

Origin: Bark mulch refers to shredded conifer bark without further additives.

Product Forms:

• Spruce mulch (Picea abies) — fine 0-15 mm, medium 10-25 mm, 10-40 mm
• Pine bark (Pinus sylvestris) — reddish, 0-8 mm, 7-15 mm, 15-25 mm, 10-40 mm
• Mediterranean pine bark (Pinus pinaster, P. halepensis) — "The premium choice among bark mulches"
• Colored bark mulch — actually colored wood chips, not bark

Functions: Suppression of seed weeds, balancing effect on temperature and water balance, erosion protection.

Application: Loosen the surface and remove weeds. For shallow-rooted shrubs: nitrogen compensating fertilization with horn meal (70-100 g/m²). Layer thickness 5-7 cm (fine grain sizes 3-5 cm). Apply only to the top soil layer, do not incorporate. Medium/coarser grain sizes suitable as path covering.

Maintenance: Top up to 5-7 cm every 2-3 years. Decomposition produces valuable humus. Slight mold growth in bags is tolerable. Fungi on mulched areas are natural (including yellow slime mold/Fuligo septica). The soil pH is not acidified by bark mulch.

→ PDF: User Information Bark Mulch

→ PDF: Unpleasant Surprises Can Be Avoided (DeGa 22/2008)

Texts and photos copyright horticon — reproduction and use only with permission.

Xylit is a by-product of lignite mining. The term comes from the Greek word Xylon = wood. Xylit is millions of years old, incompletely carbonized wood or plant material found as inclusions in lignite deposits.

When processed, it can be added to substrates instead of peat. Peat-free specialty products can be very well realized as mixtures with RAL quality-assured compost, bark humus and wood fibers using xylit from lignite.

Since 2006, horticon has been significantly involved in the development of xylit (Xylifibre®) for use in horticultural growing media and substrates.

Properties of Xylit

• pH value around 4.5 — ideal for acidic substrates
• Low salt content and low nutrient levels
• High exchange capacity and nitrogen stability
• Homogeneous quality — weather-independent availability
• Moderate water capacity with high proportion of organic matter

Data Sheets (PDF)

• Data Sheet Xylit 0-5 mm
• Data Sheet Xylit 0-15 mm
• Data Sheet Xylit 5-15 mm
• Data Sheet Xyligran

→ PDF: Xylit is Suitable as an Additive (DeGa 41/2005)

Green waste compost, or RAL quality-assured substrate compost, is one of the most commonly used peat substitutes in Germany. It is produced from green waste (leaves, grass clippings, shrub cuttings) in controlled composting facilities.

Properties

• High nutrient content — reduces fertilizer requirements
• Good structural stability and buffering capacity
• Positive effects on soil biology and water retention
• pH value typically in the neutral to slightly alkaline range

Quality Requirements

For use in growing media, only RAL quality-assured substrate compost (RAL GZ 250) should be used. The quality assurance ensures compliance with strict limits for heavy metals, foreign materials and plant-damaging substances.

In combination with other peat substitutes such as wood fibers, bark humus or xylit, peat-free substrate recipes of high quality can be realized.

Wood fibers for use as peat substitutes in growing media are generally produced exclusively from conifer wood chips. Various processes such as refiner or extruder technology are suitable for this purpose. Wood fibers are a sustainable alternative to peat as they are obtained from renewable resources.

Properties in Growing Media

Wood fibers have a number of properties in growing media that make them a valuable component:

• Good drainage properties and higher air capacity
• Very fine wood fibers can also achieve higher water storage potential
• Improved drying of the substrate surface reduces algae and moss formation

Points to Consider

• Nitrogen immobilization: If this is taken into account by the substrate manufacturer and in cultivation, consistent wood fiber quality allows this to be managed
• Settling: Due to decomposition in the substrate, a high proportion of wood fibers can lead to settling in the pot — consider this for long-standing cultures
• Buffering capacity: Wood fibers have only a low buffering capacity regarding pH value

When sourcing wood chips, there is competition with other applications (e.g. thermal use), but the required quantities can be increased in the future through the construction of appropriate production facilities.

The coconut palm (Cocos nucifera) is probably the most important plant for people in the tropics — every part of the plant is used. The mesocarp (the husk around the actual "nut") is separated into various products and is the part of interest for horticulture.

Three Product Types

• Coconut pith (Cocopith) — fine material between the fibers, high water capacity
• Coconut fiber (Cocofibre) — long and short, for geotextiles and as substrate component
• Coconut chips (Crushed/Chips) — structurally stable pieces for drainage and aeration

Processing

Proper processing is crucial for quality: aging (3-6 months), washing with fresh water, buffering with calcium nitrate solution, sun drying to 15-18% residual moisture, compression into 5 kg blocks (yielding approximately 60 liters when rehydrated).

Socioeconomic Aspects

SA 8000 certification, responsible water use and recycling practices play an increasingly important role in sourcing.

→ PDF: Peat Alternative Coconut — Fibers, Pith and Chips (DeGa 02/2014)

Flower pots and balcony planters do not exactly give plants optimal starting conditions for a flourishing life. The structure of the soil must provide high drainage and storage capacity so that plants neither drown nor dry out.

Good potting soil is usually brown, not black. A balanced nutrient ratio and an adapted pH value are important criteria.

Properties of Good Potting Soils

• Favorable pH value between 5.5 and 6.5
• Adapted nutrient level for the respective application
• High water storage capacity
• High air capacity at water saturation
• Good drainage behavior
• Structural stability throughout the entire cultivation period
• Good buffering capacity for pH value and nutrients
• Good rewetting properties
• Free from pests and weeds
• Free from plant-damaging substances

→ PDF: The Good Foundation (Landlust 01/2016)

Who does not know the problem? — a whitish or yellowish mold forms on the surface of potted houseplants. The causes usually lie in excessive moisture and insufficient ventilation.

In households, more plants certainly drown than dry out.

What to Do About Mold?

• Proper heating and ventilation — regular air exchange reduces humidity
• Keep the surface dry — water sparingly, not in advance
• Roughen the surface — loosen the top layer with a fork
• Apply a sand layer — thin layer of quartzite sand on the pot surface
• Water from below — add water to the saucer
• In case of severe infestation: completely replace the soil

Not Recommended

Vinegar water, dish soap, salt or chemical agents. Chamomile tea is harmless, but its effectiveness is questionable.

Small, annoying, black flies appear in increased numbers especially during the winter months. They live in flower pots, remnants of unused potting soil bags and preferably gather at windows.

These are fungus gnats (Sciaridae) — not true flies. The adult insects are harmless, but the larvae can damage fine root hairs of young plants.

Countermeasures

• Let it dry out — let the potting soil dry out completely when infested
• Keep top soil layer dry — When the top layer (1-2cm) is dry, don't water immediately. Use the finger test to check if the layer below is still moist!
• Quartzite sand — apply a thin layer of fine-grained sand on the surface
• Yellow sticky traps — yellow traps catch adult gnats
• Beneficial organisms — targeted use of parasitic SF nematodes (Steinernema feltiae)