Hydrophobic soil

Source Water droplets repelled by hydrophobic soil

Definition & Entomology

The term “hydrophobic” stems from the Greek prefix- hudōr, ‘water’ and the Latin and Greek suffix- -φοβία, ‘-phobía’. It is biologically defined as tending to repel or mix with water. Hydrophobicity is introduced as a property of soils when waxy, organic substances coat the surface area of soil particles, essentially making the soil impenetrable by any precipitation or other liquids. This water repellent layer is highly variable in effects depending on where the given site is.

Source Soil textural classification triangle

Distribution

Hydrophobic soils are expected to be more prevalent in hot, temperate climates (Rasa) in soils predominately composed of sand (<5% clay). The increased particulate surface area of sand, sandy loam, and loamy sand soils create the most attraction with organic substrates out of all soil classifications. The insoluble compounds that induce this phenomenon are allocated in the Vegetable Mould and Humus layers of the various Soil Horizons. Water repellent soils have been located in all western and southwestern parts of the United States, Canada, Finland (Rasa), Australia (Robichaud), Mexico (Alanis), Portugal (Doerr Thomas), Columbia (Jerez), Greece (ziogas) and is abundant in many parts of the world. Water repellent soils can be found on all continents excluding Antarctica.

Fertilizer use in both household and large-scale agricultural settings can accidentally induce water-repellency of soil regardless of temporal region. Treatments containing calcium or magnesium hydroxide especially have been shown to mix with the fatty acids of soils, creating insoluble molecules (Wander).

Consequences

 Wildfires

Landscapes with hydrophobic soil elements are vulnerable to catching on fire. The soil particles are deprived of water making the soil dense and extremely dry. Soil moisture is one of the most important factors when considering decomposition of organic matter, so with decreased soil moisture there is respectively less decomposition occurring. In forested biomes, the huge biomass stock of leaf litter and anthropogenically placed mastification (Busse) deposits on the forest floor acts as fuel for fire and responds quickly to ignition. Needle leaf trees such as those included in the pine (Pinus), spruce (Picea), and hemlock (Tsuga) genera often speed up the hydrophobic process as well when their needles fall and gather on the ground creating buildups of litter. When these areas are exposed to fire, the wax substrates on the needles melts off and creates a relatively thick hydrophobic wax layer on the surface of the soil.

 Erosion & Runoff

 Plant Establishment

Water repellency makes it extremely difficult for seeds to germinate and grow. Seeds get carried off of the site with the hydrophobically induced runoff, severely decreasing the chance of seed and plant establishment (debano). The seeds that end up settling into the soil must compete at greater levels with same and differing species for limited nutrients.

Treatment and Sampling

The most commonly documented treatment of hydrophobic soil is done by adding moisturizing clay agents in a process referred to simply as “claying”. Because the most vulnerable soils tend to be sand-based, clay mixtures aid the soil its being added to by allowing water and nutrient retention to occur. (unnamed)

To determine if a soil site must undergo treatment, a few things must be determined regarding the sample. The first and most straight-forward method of testing for general hydrophobicity is the Water Drop Penetration Time (WDPT) test- where 1-3 droplets of distilled water are placed directly onto a smooth soil surface and measured for rate of absorption. For general scale, a resulting time of 5 seconds or greater classifies the sample as "hydrophobic". Initial water intake of wettable soil occurs rapidly because of the strong attraction between the water molecules and dry soil aggregates (DeBano 5). However, the water infiltration rate of repellent soils is either very slow or non-existent. Therefore, repellent soil will cause naturally occurring precipitation and the experimental water droplet to bead on top of the sample, if or until it is completely absorbed. This test is best performed under dry climatic periods to attain the highest possible hydrophobic qualities that may occur in the field. The values obtained for this experiment are measured by absorption per parameter of time [cm/s] or percentage [%]. (ziogas)

Determining the classification of the soil sample is common for scientific and research purposes. Tools such as sieves and hydrometers are most typically used in Soil Particle Size Analysis Methods to determine what percent sand, silt, and clay are held in the soil.

Sources

“ Effects of Organic Compounds, Water Content and Clay on the Water Repellency of a Model Sandy Soil.” Soil Science and Plant Nutrition, vol. 53, no. 6, Dec. 2007.

Cesarano, Gaspare. “The Influence of Plant Litter on Soil Water Repellency: Insight from 13C NMR Spectroscopy.” PloS One, vol. 11, no. 3, Mar. 2016.

Busse, Matt D., et al. “Lethal Soil Temperatures during Burning of Masticated Forest Residues.” International Journal of Wildland Fire, vol. 14, 2005, pp. 267–276., doi:https://www.fs.fed.us/psw/publications/busse/psw_2005_busse001_ijwf.pdf.

DeBano, Leonard F. “Water Repellent Soils: a State-of-the-Art.” United States Department of Agriculture Forest Service Pacific Southwest Forest and Range Experiment Station: General Technical Report PSW-4, Mar. 1981, pp. 4–20.

Doerr, S.H., et al. “Soil Water Repellency: Its Causes, Characteristics and Hydro-Geomorphological Significance.” Earth Science Reviews, vol. 51, no. 1-4, Aug. 2000, pp. 33–65.

Jurez, Laura Daniela, et al. “Stabilization of Colombian Lateritic Soil with a Hydrophobic Compound (Organosilane).” International Journal of Pavement Research and Technology, vol. 11, no. 6, Nov. 2018, pp. 639–646., apps.webofknowledge.com.gate.lib.buffalo.edu/CitedFullRecord.do?product=WOS&colName=WOS&SID=8DQlpZIc13phMJI24Hm&search_mode=CitedFullRecord&isickref=WOS:000087736400012.

Rasa, Kimmo, et al. “Water Repellency of Clay, Sand and Organic Soils in Finland.” AGRICULTURAL AND FOOD SCIENCE, vol. 16, 2007, pp. 267–277.

Robichaud, P.R., et al. “A Probabilistic Approach to Modeling Postfire Erosion after the 2009 Australian Bushfires.” 18th World IMACS / MODSIM Congress, July 2009, pp. 1–7.

Wander, I. W. “An Interpretation of the Cause of Water-Repellent Sandy Soils Found in Citrus Groves of Central Florida.” Science, vol. 110, no. 2856, 1949, pp. 299–300., doi:10.1126/science.110.2856.299.

White, Carleton S. “Homogenization of the Soil Surface Following Fire in Semiarid Grasslands.” Rangeland Ecology and Management, vol. 64, no. 4, 2011, pp. 414–418.

Ziogas, Apostolos K., et al. "Soil water repellency in north-eastern Greece with adverse effects of drying on the persistence." Australian Journal of Soil Research, vol. 43, no. 3, 2005, p. 281+. Science In Context, http://link.galegroup.com.gate.lib.buffalo.edu/apps/doc/A133910175/SCIC?u=sunybuff_main&sid=SCIC&xid=2238eb00. Accessed 25 Apr. 2019.