Peat wettability and hydrophobicity

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Peat is a material capable of storing extraordinary amounts of water. The sphagnums that make it up are capable of accumulating water droplets between the leaves, but they also have hollow cells known as hydrocytes where efficient capillary retention phenomena take place.

Not all sphagnum plants have the same capacity to absorb water. Morphological differences in the size, shape and arrangement of the leaves, as well as in the size of the hydrocytes, can make a difference. The species of record is probably Sphagnum imbricatum (L.), which is capable of storing water in quantities up to 44 times its dry weight.

On average, a sphagnum peat for horticultural use, which has gone through all the industrial stages of preparation for use, can absorb water up to 5-10 times its dry weight. The best performance is achieved with blond block peats and not with black peats, as is commonly believed.

The degradation of the absorption capacity of horticultural peats compared to sphagnum peats in the wild is due to the degradation of their botanical structure, which occurs during industrial processing and is most evident in “milled” peats, where the disturbance is most intense.

Nevertheless, peats remain unrivalled in their ability to absorb water and this is one of the fundamental reasons for their success.

When peats dehydrate, however, their behaviour changes significantly and they turn into water-repellent materials.

The phenomenon is also well known from an ecological point of view, because it afflicts tropical peat soils, where, due to frequent fires, huge areas dehydrate to the point of hydrophobic behaviour, with serious environmental damage.

Sphagnum hydrocytes (hyaline cells or hyalocysts) imaged under an electron microscope

The concept of hydrophobicity is strongly related to that of Wettability.

A material is defined as wettable when it possesses the ability to re-wet itself after dehydration. In physical terms, wettability refers to the displacement of one fluid surface by another (the system is always three-phase).

This is governed by Young's equation [γsv = γsl + γlv cosθ].

The degree of wettability is dependent on the surface tension of the liquid and the various interface energies. The contact angle is between 0° (complete or perfect wettability) and 180° (no wettability). A favourable wetting corresponds to a small contact angle, resulting in a situation where the fluid covers a large portion of the surface. Conversely, unfavourable wetting refers to the case where the fluid, having no particular chemical affinity with the solid interface, forms compact droplets on it, i.e. with a high contact angle.

In general, a material is considered hydrophilic (or wettable, or having good affinity for water) when the contact angle is less than 90°. Conversely, a material is hydrophobic (non-wettable or has poor affinity for water) when the angle is greater than 90°.

Wettability is often expressed in qualitative terms through parameters such as WDPT (Water Drop Penetration Time), which describes the speed at which a drop of water penetrates the surface of the material. 

Contact angle - Young's equation

Contrary to the common perception, black peats (the more humified and decomposed peats from the deep layers of peat bogs), when dehydrated, become more hydrophobic than the less decomposed oligotrophic peats (the so-called blond peats, which are more superficial).

To mitigate the effects of hydrophobicity in peat growing media, the use of wetting agents has become established.

Wetting agents are normally molecules of varying degrees of complexity (normally chains from C10 to C20) that have, as a common feature, lipophilic portions (generally hydrocarbons) combined with ionic portions (anionic or cationic head). Wetting agents succeed in reducing the solid-liquid interfacial tension by adsorbing their lipophilic chain onto the hydrophobic surface of the solid particles and orienting their ionic portion towards the aqueous phase.

By virtue of this property, they are also often referred to as surfactants.

For horticultural use, surfactants must not have phytotoxic effects on the vegetation, i.e. they must not impair the physiology and functionality of the root systems with which they come into contact.

For these applications, non-ionic (with no net charge) surfactants with no net charge in the hydrophilic head are preferred. This product family was predominantly in the 1990s. The application limitation was a rapid decay of the surfactant properties.

Modern wetting agents of the latest generation have overcome the durability limits and can be used at a very low dosage, in the order of 100 g per cubic metre of substrate (for substrates with a high content of black peat, the dosage should be increased to double).

They can be used both during the manufacture of substrates and during cultivation, combining them with irrigation and/or fertigation practices, as they do not interfere with the action of fertilisers. Some growers are conveniently accustomed to systematically adding them to their nutrient solutions at even lower doses than those indicated above.

It must be said, that the use of these surfactants can be avoided simply by keeping the moisture content of the peat within safe values. Peats with moisture levels in the range of 40-50% are never hydrophobic. Guaranteeing these moisture levels is, however, difficult, as these materials endure transportation of thousands of kilometres, which is all the more onerous and costly the wetter and heavier they are. In addition, several months may pass between the time of manufacture and the time the substrates are used for cultivation, during which time the moisture content is irreparably reduced.

If we want to order the main organic components of cultivation substrates according to their aptitude to develop hydrophobic phenomena, we could refer to the following sequence:

Black peat > Blond peat > wood fibre > coconut pith.

For example, percentages of coconut pith in the substrate formulation in the order of 20% by volume reduce the occurrence of hydrophobic phenomena.

Wood fibre, although to a lesser extent, can also make a contribution in this respect (it normally has a contact angle of around 90°).

Substrato di coltivazione con torba in miscela con midollo di cocco per la coltivazione vivaistica di fragola