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Hydrophobic Soil

Hydrophobic soil refers to soil with water repellent properties. It prevents water infiltration when in contact with soil surfaces, and delays water penetration into the soil, sometimes for more than several hours. Soil hydrophobicity is a severe threat to soil health and consequently to production all around the world.

Overview

Hydrophobic Sand

Hydrophobic soil refers to soil with water repellency, and is an issue which occurs worldwide. Naturally induced hydrophobicityl is more common in the surface layer of sandy-textured soils where hydrophobic materials accumulate. It is reported that soil hydrophobicity is particularly severe in Mediterranean climate regions, such as Mediterranean Basin, and in parts of Western and South Australia. In addition, heat is the other inducement of soil hydrophobicity. The major expression of hydrophobic soil is a dry surface soil, even after a considerable amount of rainfall, and this has significant impacts on agricultural production due to water deficiencies.

Causes

Hydrophobic soil can be caused by natural causes such as organic matter, waxy substances, soil characteristics, the growth of particular vegetation species, the climate, fungal and microbial activity, as well as heat-induced causes such as wildfires and controlled burns in agriculture systems.

Natural causes

Soil organic matter originates largely from plants, and includes waxy materials that protect plants from dessication, and these materials tend to be hydrophobic. Soil hydrophobicity is naturally caused by waxy organic particles coating substantially all the particle surface area of sandy-textured soil. The lower particle surface area of sandy soil is more susceptible to be thoroughly covered by waxy organic compounds than a fine textured soil that has greater specific surface area. As the waxy materials are part of organic matter, soil hydrophobicity is commonly confined to the surface layer of soil where the accumulation of organic matter occurs. In addition, soil fungi also generate hydrophobic substances. It is reported that wetting and drying patterns have a considerable effect on soil hydrophobicity. When soils are exposed to hot and dry climatic conditions, the soil hydrophobicity tends to be the most severe as waxy compounds more readily become attached onto sand surfaces. Thus soil hydrophobicity can be particularly severe in Mediterranean climate regions, where the climate is characterized by dry hot summers and rainy winters. The hydrophobicity results from waxy coatings established during summers are then maximized by the cooler humid conditions in winters. Additionally, some root exudates produced when plant roots proliferate under wetting / drying cycles may contribute to soil hydrophobicity as well.   

Heat-induced hydrophobicity

Fire can alter soil hydrophobicity to a certain degree depending on the intensity and severity of the fire. Studies have shown that a hydrophobic layer can be formed below the soil surface during combustion. The heated hydrophobic organic substances are vaporized and translocated to lower layers where condensation occurs. Soil hydrophobicity will be increased when soil temperatures are between 175 °C to 205 °C. The steep temperature gradients produced by fires in the surface layers of the mineral soil fall into the above range. Other than redistribution and condensation of hydrophobic substances in the soil, heating also reduces vegetation cover, destroys surface litter (organic matter) and soil structure, raises the risk of soil erosion, and enhances the bonding of these hydrophobic substances to soil particle surfaces, which increase soil hydrophobicity.

Impacts

Soil hydrophobicity significantly affects the hydrological functions of soils, such as infiltration and water retention. When the soil moisture content is low, it will be difficult for plants to get enough water and consequently make it unable for plants to access the nutrients dissolved in water. Inadequate plant available water and nutrients slow down the plant growth rate and thus reduces plant productivity. The low water content of hydrophobic soils will slow down soil microbial activity, and reduce the amount of carbon released from the decomposition of soil organic matter. In addition, a hydrophobic layers on or below (heat-induced hydrophobicity) the soil surface will impede the infiltration of water, and excess water may lead to surface runoff that raises the risk of water erosion on sloping land or bare soil such as land where vegetation cover has been destroyed by fire.   

Management

Land management recommendations can be developed based on the causes and impacts of soil hydrophobicity, and the intended land use. In terms of management impact, removal strategies have bigger and more consistent impacts than mitigation strategies. While from an economic perspective, mitigation is more cost-effective than removal. For these reasons, the cheaper mitigating technics are more suitable for small patches of hydrophobic soil, and mitigating technics can be used in large-scale management.Global research efforts have identified the main causes of this problem and developed various strategies for management options. In order to ensure better effects, combinations of different management techniques can be used, such as the use of plant spices with strong root system with zero tillage to reinforce water infiltration. It is essential to understand the physical, chemical and biological mechanisms underlying the solutions, not only for achieving optimum results but also for having a sound basis on which to communicate research efforts to farmers and growers. 

Mitigation

Mitigation strategies aim to manage hydrophobic soils to allow desirable crop or pasture production.

Water harvesting

Ridge and furrow

Furrow sowing can generate furrows that allow water harvesting from the ridges down to the furrow. And this operation allows seeds landing deeper in the soil, where is moister. Water accumulated within the furrow provide larger hydraulic pressure, which facilitates infiltration of water. 

Soil wetting agents (surfactants)

Surfactants are surface-active agents that can assist water entry into the hydrophobic soil by lowering the surface tension of water on the surface of soil grains. In addition, some studies have shown that repeated leaching resulted in significantly reducing soil hydrophobicity as dissolved organic carbon is washed out through leachate. This finding suggests that using surfactants before planting may contribute to the leaching of hydrophobic compounds from the topsoil, with benefits through the rest of the growing season.

Zero tillage

Root systems can be considered as naturally formed networks of flow channels. Zero-tillage management preserves the biopores generated by plant roots and allow water movement through these channels.  

Inoculating microbial to improve decomposition of waxy substances

Researchers have found that certain types of bacteria, through decomposition, are able to reduce the content of waxy substances on the surfaces of soil particles, the main cause of soil hydrophobicity. Moreover, the bio-surfactants produced during the decomposition of waxes contribute to releasing hydrophobic coatings from soil surfaces, thus improving the wax decomposition by microbial. While the competition from natural microbial and undesirable environmental conditions are barriers to successful application in the field. 

Removal

The purpose of removal strategies is to change the properties of hydrophobic soils. The high cost and risk of soil erosion should be taken into consideration when applying removal strategies.

Addition of Clay

The much larger specific surface area of clay prevents it from being covered with hydrophobic compounds degraded from organic matter. Additionally, the surface charge of clay particles makes clay more hydrophilic. Clay can be applied by spreading onto the soil surface and then thoroughly incorporated with the original topsoil.   

Deep cultivation

Deep cultivation management brings non-hydrophobic subsoil to the surface, forming hydrophilic layers or channels for water infiltration. However, the mixing of topsoil and subsoil may cause a hydrophobic issue in the entire soil profile, but it will dilute both of the stability and severity of soil hydrophobicity. The effect of deep cultivation depends on how wide and deep the cultivation is, and how much subsoil is raised up to the soil surface.