Water repellant soils can be improved a number of ways so that they absorb and retain water more effectively, prolonging the benefits of watering and rainfall.
Well-composted, well dug soils that are appropriately mulched may completely dry out, but they’re rarely water-repellant. Healthy soil biology is an important and overlooked aspect. It’s important for soil and plant health to use a variety of types of mulch because these differences favour and feed different groups of soil microorganism. Ignore the good guys at your peril.
Soils repeatedly mulched with woodchip made from pine bark or eucalyptus woodchip gradually become water repellant. They stimulate a population boost of a narrow range of wood digesting fungi. As these fungi proliferate, they fuse individual particles of woodchip into solid ‘plates’. As feeding fungal hyphae build up over time, the increasingly expansive ‘plates’ prevent air and water from entering the soil. The result of repetitive mulching with woodchip may be the reverse of what it intended. Instead of conserving soil moisture and assisting plants through drought, soil can become anaerobic and hydrophobic.
Worse can happen. Sometimes these saprophytic fungi can change from being passive digesters of wood into active pathogens of live trees, shrubs and palms. This occurs when the plants have been weakened by anaerobic, water-repellent soil. Deep watering in this situation can help anaerobic bacteria make soil even less hospitable to plants.
One of the saddest garden consultations I have done was for a gardening couple in the suburb of Bunya. Their beautifully relandscaped and replanted garden had been thickly and repeatedly mulched with woodchip. They had heard that mulching was good, so they just piled it on. Eighty percent of their new plants died. So they replanted. And again their plants died.
Desperate, and significantly out of pocket, they asked for my advice. When I arrived I found half a hectare of malodorous, fungus-filled land, a paradise for anaerobic bacteria and other pathogens.
Should I use wetting agents?
When mulched plants start wilting in drought, the first thing people do is to water in the wetting agents. Is this always helpful?
Soil wetting agents and surfactants have become widely used products. Their purpose is to counteract water-repellant soils, but what are these compounds? How do they really work? Are they harmless to people, pets, plants and the broader environment?
How they work
Wetting agents either improve the absorption of water by a material that would otherwise repel water, such as dry soil, or to increase the adhesive properties of a spray in order to adhere to the surface of plants. These compounds are also known as surfactants and they work by reducing the surface tension of water, helping it to spread evenly.
Why not save money by using household soaps and detergents?
The Australian Standard for the biodegradability of detergents applies only to the active ingredient, not to any of the many additives they contain, like chemical fragrances. Many household soaps and detergents react with the soil and with fertilisers. The results can be fatal to plants.
There are a few artificial wetting agents that are certified organic. The majority are, however, unsuitable for organic gardens and their long term use may harm the soil.
Conventional wetting agents are usually alcohol or petroleum distillates, such as polyacrylamides. Polyacrylamides are commonly used on an industrial scale in irrigation water to (in theory) improve the absorption of water by soils and to reduce soil erosion.
Wetting agents are now almost universally included in commercial potting mixes which are not certified organic. Potting mixes are primarily bark based and water-repellant when dry. If you can find potting mixes using coir, these have a better long term performance because they don’t lose bulk as quickly as bark-based potting mix, and they don’t form a thick, gluey sludge that can block drainage holes, allowing pots to fill with water. Pots filled with water become anaerobic and this kills plant roots. But even coir-based potting mixes may contain wetting agents.
Polyacrylamides and other synthetic wetting agents are prohibited under organic standards. But natural wetting agents, including soaps, saponins and microbial wetting agents are allowed in organic gardens because they are beneficial and sustainable.
Organic-certified wetting agents are genuinely and readily biodegradable. I make my own agar agar-based wetting agent, and I also use ‘Eco-hydrate’ manufactured by Organic Crop Protectants. ‘Eco-hydrate’ contains organic humectants and organic surfactants and these are natural compounds.
New, equivalent products may exist, but I’m not commercial and I may not know of all that exist in the trade. Let me know if you find others.
Make your own natural soil wetting agent
You can make your own soil wetting agent using the food grade thickener, agar agar (algin), derived from seaweed. Buy powdered kelp from a health food store.
Slowly add boiling water to agar agar and stir to make a paste about the same consistency as wallpaper paste. Don’t know what that is like? Think thick custard.
Next, dilute 250 ml of this paste in 4.5 l of water. Water on to seed beds, seedlings, flower beds or containerised plants. This should cover 4 sq metres.
As with all kinds of wetting agent, this algin solution blends most effectively when applied to already moist soil.
Unlike chemical wetting agents, algin has no hidden, unexplained or harmful side effects. It is readily biodegradable, lasting for up to three weeks. In practical terms when transplanted seedlings have established in their new position it’s likely the algin has been completely digested by soil microbes, by which time it will have done its job.
History of chemical wetting agents
Invented in the 1950’s, there’s about 70 different types available, often pre-mixed into potting mixes and fertilisers.
Pre-mixing is advocated by agribusiness for home gardening. Unconfident gardeners are often reluctant to spend extra money on buying them separately because they are unable to discern whether they work at all. By adding them in the factory manufacturers ‘value add’ to existing products to gain a market edge. You still pay extra, but this is part of the sale price and no one questions this. The majority of potting mixes use artificial wetting agents and many golf courses routinely apply them in dry weather.
Research into and the evolution of wetting agents is poorly documented, resulting in confusion and a poor understanding of how to use them, in particular their effect on different soil types. Anecdotal evidence at the Royal Botanic Gardens, Sydney (1992 – 2003) indicates that wetting agents disrupt the mycorrhizal associations in the root systems of sensitive plants, including ferns, and the highly evolved roots of orchids, and plants in the Proteaceae family, plants where the association between mycorrhizal fungi and their root systems are particularly crucial.
Almost all plants have mycorrhizal fungal associations in their root systems and these fungi serve as accessories to the plant root system, greatly extending the capacity to absorb water. Mycorrhizae also help nourish their host plant, and they help defend their host from disease. In return, host plants supply these fungi with carbohydrates and nutrients they are less able to manufacture themselves. This vital aspect of soil and plant health is overlooked by most manufacturers of chemical wetting agents.
Levelled beds will always be easier to water and less likely to shed heavy rainfall and irrigation than sloping beds. I garden in Wynnum, QLD 4178, a dry suburb. Since 2003, the heaviest 24 hours of rain received was when ex-Cyclone Debbie passed by, delivering 219mm rain in 24 hours. Not one drop left my property; my soil soaked it all up.
Raising beds above ground level encourages drainage. Most moisture-loving plants prefer good or average drainage. The roots of the vast majority of land plants obtain their oxygen directly from the soil, so flooding and saturated or anaerobic soil can kill roots by suffocation. I advise all gardeners to guarantee good drainage, because all it takes is one very wet week in a poorly drained garden to wreak havoc. Drainage isn’t a fun topic, it’s just an essential aspect of successful horticulture.
When soil becomes saturated all the air pockets become water-filled and roots lose their source of oxygen. In warm, wet summers roots can suffocate in waterlogged soils in 24 hours or, at least, their defence systems against root rot disease become compromised. Consequently, roots will be attacked by pathogenic fungi, nematodes and bacteria. Waterlogged soil that has also been treated with wetting agents can become deadly to plants.
Compacted soils, especially clay and loamy soils, are always the hardest to water. Dig and loosen soil to alleviate compaction, annually adding gypsum (aka clay breaker) to clay or loamy soils to encourage what’s known as a ‘crumb structure’ which allows air and water to move more freely.
I garden on ancient acid sulphate soil. This contains clay, but in this instance adding gypsum makes acid sulphate clay more, not less, sticky. So I use lime or dolomite and plenty of compost and well-rotted manures to improve the crumb structure. It took five years, and the result is marvellous.
Soils affected by dryland salinity, such as parts of western Sydney, lose their natural crumb-like structure making it harder for air pockets to occur in the soil. Raised beds, compost-rich, well-dug soil that is annually conditioned with gypsum helps to make soil healthy and water absorbing. Gypsum also releases the salt so rain and irrigation water can rinse it down through the soil, making the soil more hospitable to horticulture.
Seaweed contains algin, so apart from being able to make a natural soil wetting agent, the use of seaweed solutions adds small amounts of this natural wetting agent to topsoil. Regular use helps combat water repellency in soil.
Vary the type of mulch used
Mulches should be varied. Wood-based mulches, like pine and teatree, favour fungi. Soft mulches, like fresh lawn clippings, and green manures favour bacteria. Mushroom compost and sugarcane favour both fungi and bacteria. Since 2003, I have varied the type of mulch used in my ornamental front garden in this order: mushroom compost, pine bark, lawn clippings, teatree, sugarcane.
It is important to mulch relatively thinly, which cools the soil, conserves moisture and – importantly – this allows small showers of rain to penetrate the soil and it allows oxygen to filter into topsoil. As a guide, apply chopped sugarcane at a thickness of up to 3cm around herbs and established vegetables (less for the onion family) and bark mulch around 5cm deep around trees, shrubs, bamboo, cycads and palms.
Unanswered concerns about the use of artificial wetting agents and water crystals
- Some of these compounds are derived from, or contain traces of, carcinogenic acrylamides or polyacrylamides. Water crystals, commonly used products, are composed of polyacrylamide;
- The interaction of some wetting agents with other compounds, such as pesticides (including Persistent Organic Pollutants or POPS) like DDT and its ‘sister’ pesticide Dicofol (recently banned from home gardens) and fertilisers in soil and waterways is not adequately understood for broad scale use;
- The breakdown products of some wetting agents:
- Are persistent in the environment, affecting river and marine ecology;
- Ultimately retard water absorption by soils;
- Enter the food chain directly and indirectly;
4. Field observations are that their breakdown products could:
- Cause adverse biological effects by interfering with the endocrine system and disrupting the physiological function of hormones in animals (like dugongs in Moreton Bay) and humans;
- React with salts or bauxite in soil;
- React with heat, such as from bushfires, resulting in the formation of toxic compounds;
- Have a plasticising effect on surrounding soils resulting in lower water absorption rates and encouraging soil erosion;
- Harm the roots of ferns, orchids and Proteaceae;
Meet the family of wetting agents and surfactants:
Wetting agents, which include surfactants, reduce the surface tension of water so that it spreads and is absorbed by dry soil and potting mix. They help water transfer from particle to particle. They are widely used to enhance the effects of pesticides.
There are four different types of surfactant:
Nonionic surfactants are the most commonly used wetting agents in horticulture. When used properly they do no direct harm to plants. Applied at too high a strength plants may be harmed. Research is investigating how to use their ability to harm zoospores (fungal spores that swim through soil moisture) as fungicides.
Anionic surfactants enhance foaming and spreading properties. Shampoo for hair contains sodium or ammonium laurel sulphate. In horticulture using an anionic surfactant can cause problems with sprayers that have an agitator, or where foam could disrupt water flow or the action of pumps.
Cationic surfactants are often very toxic to plants as they can disrupt membrane ion balance. They are not widely used for pest control.
Amphoteric surfactants are rarely used in horticulture and when they are used they are added to pesticides.
Wetting Agents are commonly added to potting mixes, which often contain a high proportion of pine bark. Potting mixes sometimes have unnecessarily high amounts of wetting agents. They work much like surfactants and include, or are based on, polyoxyethylene esters and ethoxy sulphates. Applied at too high a strength they become toxic to leaves and roots. Weather and temperature can affect plants similarly after their use, especially if they’re growing in bark or peat-rich media.
Other wetting agents
Penetrants. Penetrants dissolve or penetrate waxy layers on leaves and allow other chemicals to interact with plant or insect tissue, such as in pesticides.
Thickeners. Thickeners reduce spray drift. They may contain polyacrylamide, polyethylene polymers, polysaccharides or vegetable oils. Applied at too high a strength, and these products can burn plants, or block spray nozzles.
Emulsifiers. Emulsifier agents allow petroleum-based pesticides and water to blend.
Spreaders and stickers. Spreaders help pesticides to cover plants evenly, whereas stickers improve the adhesion of pesticides to foliage. Stickers are a diverse group and may contain fatty acids, latex, alcohols, plant oils or inorganic oils. They can harm rough-leaved and hairy-leaved plants, annuals and herbs.
Oils. There are two types of oil: plant and petroleum oils. These are generally used to suffocate sap-sucking and certain chewing pests.
Alkyl polyglucosides. These are environmentally-safe, being modified sugars used in genuinely biodegradable detergents and various pesticides, some of which are also acceptable for use in organic gardening.
References: The Evolution of Soil Wetting Agents for Managing Water Repellency in Soils.
D. Moore, S. J. Kostka, M. Franklin, L. L. Lennert, and R. A. Moore. Aquatrols, 843 East Parkway, Salt Lake City, UT 84106