Why wool can be interesting for plants

Sheep wool consists largely of keratin, a protein rich in nitrogen. This nitrogen can be released as a nutrient for plants when the wool slowly breaks down. Additionally, wool can retain a lot of water: it can absorb up to about 3.5 times its own weight in moisture. As a result, it can help retain water longer in potting soil or the surrounding soil.

There are also other materials used to retain moisture in the soil, such as:

• biochar or coir (cocos fibre)
• walnut shells
• wood remnants
• superabsorbing polymers
• peat moss (sphagnum)
• compost
• minerals such as vermiculite, bentonite and zeolite

The special thing about wool is that it can fulfill multiple functions at the same time:

• it helps retain water in the soil
• it can gradually release nitrogen during decomposition.
• the slow release of nitrogen from wool waste, compared to the rapid release from mineral fertilizers, is beneficial from an environmental perspective. This is because it helps protect water sources from contamination with nitrogen compounds caused by rapid leaching.

In addition, wool has the ability to slightly lower the pH of alkaline soils.

Effect of wool on plant growth

In a planting experiment in which wool was mixed into the substrate, it was found that plants grew significantly better than without wool. For instance, a single addition of wool yielded up to four harvests of Swiss chard and five harvests of basil. The total basil yield was 1.6 to 5 times higher than in the control without wool (LINK). This effect is primarily due to the fact that wool slowly releases nitrogen during decomposition. As a result, plants receive long-lasting nutrition without the need to constantly add fertilisers.

Research into fertilisers based on wool keratin shows that these substances can further stimulate plant growth. In a greenhouse trial with maize, the application of a wool keratin product led to:

• larger plants
• larger leaf coverage
• more chlorophyll in the leaf
• approximately 32–34% more biomass than at the control

Researchers explain this stronger growth by stating that wool not only supplies nitrogen, but also amino acids that help the plant with stress resistance and growth (LINK).

In another trial where wool waste was mixed into the soil, various crops also grew significantly better. In field and greenhouse trials with basil, peppermint, sage, and jimsonweed, among others, the addition of wool led to higher yields and stronger plants.

After the addition of wool, the soil contained more available nitrogen, primarily in the form of:

• ammonium (NH₄-N)
• nitrate (NO3-N)

The total nitrogen content in the plant tissue also increased, which means that plants could absorb more nutrients (LINK).

In addition, a clear increase in microbial biomass in the soil was observed in these trials. This means that soil life became more active, which often has a positive effect on the availability of nutrients for plants.

In an experiment in Poland, wool was used as a soil conditioner in the growing substrate for tomatoes, peppers, and eggplants (LINK). Wool (10 g per liter of substrate) was added to a 5 cm layer of substrate and then covered with the rest of the substrate, so that roots had to grow through the wool layer.

The addition of wool led to a significant increase in fruit production (approximately 30%) in tomatoes and peppers, while no significant effect was found in eggplants. Furthermore, the plants were taller and had a higher fresh weight.

Because the plants were grown on these substrates for a relatively short period, the wool was only partially broken down. As a result, a large portion of the wool remained as a nutrient reserve for the next season.

During research in Wageningen (LINK) it was found that washed wool in the growing medium can improve the performance of strawberry plants, causing them to ripen faster. Additionally, both washed and unwashed wool appeared to increase the photosynthetic activity of garden cress and slow the spread of Pythium disease.

Better water management and less drought stress

Wool can retain a lot of water and therefore acts similarly to a moisture buffer or mulch layer in the soil.

In a study with bell peppers, it was found that wool mulch:

• made the water usage of plants more efficient
• kept soil moisture more stable
• increased the biological activity in the soil

The effect was particularly evident with less frequent irrigation. Due to the moisture buffer provided by wool, plants were better able to cope with periods of drought.

For growers, this means that wool can help to:

• to reduce water loss from the soil
• to keep the temperature in the root zone more stable
• to make the root environment more active

Sheep wool as a sustainable alternative to (mineral) fertilizer

In horticulture, commercial fertilizers are often used to grow plants. However, some of these fertilizers may contain small amounts of heavy metals, such as cadmium, mercury, and lead. Over time, these substances can accumulate in the soil and, in some cases, even end up in the crop.

Sheep wool works differently. When wool is incorporated into the soil, nitrogen is released slowly during the natural breakdown of the fibers. Research shows that this nitrogen is released primarily in forms that plants can readily absorb:

• nitrate (NO₃–N)
• ammonia (NH₄–N)

There is no evidence that wool forms harmful nitrogen compounds in the soil.

For growers, this means that wool can serve as a sustainable and natural food source for plants. When used wisely, wool can:

• improve the soil
• gradually supply nitrogen to the plants
• reduce the use of mineral fertilizer
• in some cases can completely replace mineral fertilizer

In this way, residual wool from sheep farming can be a practical and environmentally friendly addition to cultivation.

Effects on the soil life

Sheep wool can have a clear effect on soil life. In Wageningen (LINK) bacteria and fungi with plant biostimulating properties were found in substrates enriched with washed wool.

Other studies also show that wool is not harmful to soil organisms and in many cases even stimulates the activity of microorganisms.

When wool is incorporated into the soil, the composition of the bacterial community changes because microorganisms begin to break down the wool.

A study with olive trees (LINK) showed that:

• wool had no negative influence on total bacterial diversity
• some bacterial species actually became more active
• especially bacteria that can break down complex organic material occurred more frequently

This is logical, because wool consists of keratin, a sturdy protein that can only be broken down by specific microorganisms.

Other studies show that the addition of wool can lead to:

• more soil bacteria
• larger bacterial biomass
• a more active soil ecosystem

During decomposition, nutrients such as nitrogen, sulfur, and carbon are released, which in turn serve as food for soil organisms.

Effect on mycorrhizal fungi

Mycorrhiza are beneficial fungi that live in symbiosis with plant roots and help with the absorption of water and nutrients.

In a study (LINK) it was investigated whether wool influences these fungi. The results show that:

• wool had no negative influence on the activity of mycorrhiza
• the fungi retained their ability to colonise plant roots

Only at very high wool dosages (approximately 2% of the soil weight) was a slight decrease in bacterial diversity and mycorrhizal colonisation observed.

Dit suggereert dat matige hoeveelheden wol veilig zijn voor het bodemleven, maar dat extreme hoeveelheden beter vermeden kunnen worden. Meer onderzoek is nodig om dit verder te bevestigen.

From waste stream to a growing medium

In the European Union, more than 200,000 tonnes of sheep wool are produced annually. A sheep yields an average of 1.5 to 3 kilos of wool per year. However, the majority of this wool is coarse and of relatively low quality.

The quality of wool is primarily determined by:

• fiber thickness (diameter of the wool fiber)
• fibre length

Based on fiber thickness, wool is classified into five categories:

• extra fine – 18.5 micrometers or thinner
• fine – 18.6–20.5 micrometer
• medium – 20.6–22.5 micrometers
• wide – 22.6–25 micrometers
• coarse – 26–32 micrometers

Fine wool is particularly interesting to the textile industry. However, in Europe, the majority of wool consists of coarser fibers, which are more difficult to process into clothing.

Because the market price for this wool is often low, it is viewed as a by-product on many sheep farms. In practice, this wool is therefore regularly:

• discarded
• burned
• put into landfill

These are unsustainable solutions and can lead to environmental problems. Therefore, within the EU, there is an increasing focus on new applications for residual wool, for example in agriculture and horticulture. (LINK)

Practical applications: powder, pellets, or raw wool

Sheep wool can be used in potting soil or substrate in various ways: as powder, pellets, or raw wool. The form in which wool is used influences how easy it is to work with, how evenly it is distributed in the pot, and how quickly nutrients are released.

Wool powder

Wool powder (also known as wool grit) is ground wool that can be easily and evenly mixed into potting soil. This ensures a good distribution of nutrients throughout the substrate.

To produce wool powder, the wool is mechanically or chemically reduced into small fibers. It is possible to produce wool ranging from coarse granules (around 70 µm) to ultrafine granules, such as 2.6 µm. However, the production of ultrafine powders from wool requires either lengthy mechanical processing, such as ball mills and air jet mills, or intensive chemical treatment (with sodium silicate, which, incidentally, can also have a positive effect on plant growth). The production of coarse powder from wool, on the other hand, is relatively fast, sustainable, and less energy-intensive, while the powders remain suitable for processing and uniform mixing with the soil.

((It is unclear to us whether wool powder is also commercially available.))

Wool pellets (wool granules)

Wool pellets are compressed chunks of wool, similar to some organic fertilisers. They are easy to dose and fairly easy to mix into potting soil.

A disadvantage is that pellets remain locally in the substrate, which can lead to the formation of small zones with higher nutrient concentrations. As a result, the distribution is less homogeneous than with wool powder.

Unprocessed wool

A third possibility is the use of unprocessed wool. This is wool as it comes from the sheep, without further processing. The advantage is that this wool is often available cheaply or even for free.

The disadvantage is that unprocessed wool is difficult to mix homogeneously with potting soil. Therefore, it is often applied in layers, for example:

• substrate – layer of wool – substrate
• layer of wool – substrate – layer of wool – substrate
• layer wool – substrate

This principle has also been used in an experiment in Poland (LINK).

Please note that raw wool from the sheep farmer can contain up to approximately 40% impurities.LINK) A ton of raw wool contains on average:

• 640kg wool fibre
• 150 lkg lanoline
• 150 kg dirt (soil)
• 40 kg suint (wool fat and potassium salts)
• 20 kg of plant material

In addition, traces of insecticides may be present.

Ethical considerations regarding the use of sheep wool

The use of sheep's wool in potting soil can also raise ethical questions. Wool is an animal product, and the way sheep are kept and shorn is sometimes a subject of debate.

Both breeders and customers may have questions about this. A possible approach is to use wool from biodynamic or extensive sheep farmers and to regularly check whether the animal husbandry method aligns with one's own principles.

At the same time, it is good to realise that animal products are already frequently found in potting soil, for example in organic fertilizers. Many consumers are unaware of this.

Furthermore, many people are unaware that wool in the Netherlands is often a residual or waste product. This can create the impression that the wool could have been better used for clothing. In reality, it usually concerns lower-quality wool that is not suitable for textiles.

Therefore, clear communication with customers is important when wool is used in growing media.